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Historical texts
Written accounts of leprosy date back thousands of years. Various skin diseases translated as leprosy appear in the ancient Indian text, the Atharava Veda, by 600 BC. Another Indian text, the Manusmriti (200 BC), prohibit contact with those infected with the disease and make marriage to a person infected with leprosy punishable.
The Hebraic root tsara or tsaraath (צָרַע, – tsaw-rah' – to be struck with leprosy, to be leprous) and the Greek (λεπρός – lepros), are of broader classification than the more narrow use of the term related to Hansen's Disease. Any progressive skin disease (a whitening or splotchy bleaching of the skin, raised manifestations of scales, scabs, infections, rashes, etc.) — as well as generalized molds and surface discoloration of any clothing, leather, or discoloration on walls or surfaces throughout homes — all came under the "law of leprosy" (Leviticus 14:54–57). Ancient sources such as the Talmud (Sifra 63) make clear that tzaraath refers to various types of lesions or stains associated with ritual impurity and occurring on cloth, leather, or houses, as well as skin. Traditional Judaism and Jewish rabbinical authorities, both historical and modern, emphasize that the tsaraath of Leviticus is a spiritual ailment with no direct relationship to Hansen's disease or physical contagions. The relation of tsaraath to "leprosy" comes from translations of Hebrew Biblical texts into Greek and ensuing misconceptions. | Leprosy | Wikipedia | 353 | 44700 | https://en.wikipedia.org/wiki/Leprosy | Biology and health sciences | Infectious disease | null |
All three Synoptic Gospels of the New Testament describe instances of Jesus healing people with leprosy (Matthew 8:1–4, Mark 1:40–45, and Luke 5:12–16). The Bible's description of leprosy is congruous (if lacking detail) with the symptoms of modern leprosy, but the relationship between this disease, tzaraath, and Hansen's disease has been disputed. The biblical perception that people with leprosy were unclean can be found in a passage from Leviticus 13:44–46. While this text defines the leper as impure, it does not explicitly make a moral judgement on those with leprosy. Some early Christians believed that those affected by leprosy were being punished by God for sinful behavior. Moral associations have persisted throughout history. In the 6th century Pope Gregory the Great and Isidore of Seville considered people with the disease to be heretics.
Middle Ages
The social perception of leprosy in the general population was mixed. On one hand, people feared getting infected with the disease and thought of people suspected of leprosy to be unclean, untrustworthy, and occasionally morally corrupt. On the other hand, Jesus' interaction with lepers, the writing of church leaders, and the Christian focus on charitable works led to viewing the lepers as "chosen by God" or seeing the disease as a means of obtaining access to heaven. | Leprosy | Wikipedia | 297 | 44700 | https://en.wikipedia.org/wiki/Leprosy | Biology and health sciences | Infectious disease | null |
Early medieval understanding of leprosy was influenced by early Christian writers such as Gregory of Nazianzus and John Chrysostom, whose writings were later embraced by Byzantine and Latin writers. Gregory, for example, composed sermons urging Christians to assist victims of the disease, and he condemned pagans or Christians who justified rejecting lepers on the allegation that God had sent them the disease to punish them. As cases of leprosy increased in the Eastern Roman Empire, becoming a major health issue, the ecclesiastic leaders discussed how to assist those affected as well as how to change the attitude of society towards them. They also tried this by using the name "holy disease" instead of the commonly used "elephant's disease" (elephantiasis), implying that God did not create this disease to punish people but to purify them for heaven. Although not always successful in persuading the public and a cure was never found by Greek medicians, they created an environment where victims could get palliative care and were never expressly banned from society, as sometimes happened in western Europe. Theodore Balsamon, a 12th-century jurist in Constantinople, noted that lepers were allowed to enter the same churches, cities, and assemblies that healthy people attended. | Leprosy | Wikipedia | 259 | 44700 | https://en.wikipedia.org/wiki/Leprosy | Biology and health sciences | Infectious disease | null |
As the disease became more prevalent in western Europe in the 5th century, efforts began to set up permanent institutions to house and feed lepers. These efforts were, inclusively, the work of bishops in France at the end of the sixth century, such as in Chalon-sur-Saône. The increase in hospitals or leprosaria (sing. leprosarium) that treated people with leprosy in the 12th and 13th century seems to indicate a rise in cases, possibly in connection with the increase in urbanisation as well as returning crusaders from the Middle East. France alone had nearly 2,000 leprosaria during this period. Additionally to the new leprosia, further steps were taken by secular and religious leaders to prevent further spread of the disease. The third Lateran Council of 1179 required lepers to have their own priests and churches and a 1346 edict by King Edward expelled lepers from city limits. Segregation from mainstream society became common, and people with leprosy were often required to wear clothing that identified them as such or carry a bell announcing their presence. As in the East, it was the Church who took care of the lepers due to the persisting moral stigma and who ran the leprosaria. Although the leprosaria in Western Europe removed the sick from society, they were never a place to quarantine them or from which they could not leave: lepers would go beg for alms for the upkeep of the leprosaria or meet with their families.
Multiple groups in Western Europe from the Middle Ages faced social ostracization and discrimination that was justified, in part, due to claims that they were the descendants of lepers. These groups included the Cagots and the Caquins.
19th century | Leprosy | Wikipedia | 363 | 44700 | https://en.wikipedia.org/wiki/Leprosy | Biology and health sciences | Infectious disease | null |
Norway was the location of a progressive stance on leprosy tracking and treatment and played an influential role in European understanding of the disease. In 1832, Dr. JJ Hjort conducted the first leprosy survey, thus establishing a basis for epidemiological surveys. Subsequent surveys resulted in the establishment of a national leprosy registry to study the causes of leprosy and to track the rate of infection. Early leprosy research throughout Europe was conducted by Norwegian scientists Daniel Cornelius Danielssen and Carl Wilhelm Boeck. Their work resulted in the establishment of the National Leprosy Research and Treatment Center. Danielssen and Boeck believed the cause of leprosy transmission was hereditary. This stance was influential in advocating for the isolation of those infected by sex to prevent reproduction.
Though leprosy rates were on the decline in the Western world by the 1860s, authorities frequently embraced isolation treatment due to a combination of reasons, including fears of the disease spreading from the Global South, efforts by Christian missionaries and a lack of understanding concerning bacteriology, medical diagnosis and how contagious the disease was. The rapid expansion of Western imperialism during the Victorian era resulted in westerners coming into increasing contact with regions where the disease was endemic, including British India. English surgeon Henry Vandyke Carter observed isolation treatment for leprosy patients first-hand while visiting Norway, applying these methods in British India with the financial and logistical assistance of Protestant missionaries. Colonialist and religious viewpoints of the disease continued to be a major factor in the treatment and public perception of the disease in the Global South until decolonization in the mid-20th century.
20th and 21st century
In 1898, the colonial government in British India enacted the Leprosy Act of 1898, which mandated the compulsory segregation of people with leprosy by authorities in newly established leper asylums, where they were segregated by sex to prevent sexual activity. The act, which proved difficult to enforce, was repealed in 1983 by the Indian government after MDT had become widely available in India. In 1983, the National Leprosy Elimination Programme, previously the National Leprosy Control Programme, changed its methods from surveillance to the treatment of people with leprosy. India still accounts for over half of the global disease burden. According to WHO, new cases in India during 2019 diminished to 114,451 patients (57% of the world's total new cases). Until 2019, Indians could justify a petition for divorce with their spouse's diagnosis of leprosy. | Leprosy | Wikipedia | 507 | 44700 | https://en.wikipedia.org/wiki/Leprosy | Biology and health sciences | Infectious disease | null |
The National Leprosarium at Carville, Louisiana, known in 1955 as the Louisiana Leper Home, was the only leprosy hospital in the mainland United States. Leprosy patients from all over the United States were sent to Carville to be kept in isolation away from the public, as not much about leprosy transmission was known at the time and stigma against those with leprosy was high. The Carville leprosarium was known for its innovations in reconstructive surgery for those with leprosy. In 1941, 22 patients at Carville underwent trials for a new drug called promin. The results were described as miraculous, and soon after the success of promin came dapsone, a medicine even more effective in the fight against leprosy. Leprosy incidence peaked in the United States in 1983, followed by a steep decline. However, case numbers have been slowly rising again since 2000. In 2020, 159 cases were reported in the country.
Stigma
Despite now effective treatment and education efforts, leprosy stigma continues to be problematic in developing countries where the disease is common. Leprosy is most common amongst impoverished populations where social stigma is likely to be compounded by poverty. Fears of ostracism, loss of employment, or expulsion from family and society may contribute to a delayed diagnosis and treatment.
Folk beliefs, lack of education, and religious connotations of the disease continue to influence social perceptions of those affected in many parts of the world. In Brazil, for example, folklore holds that leprosy is a disease transmitted by dogs, or that it is associated with sexual promiscuity, or that it is a punishment for sins or moral transgressions (distinct from other diseases and misfortunes, which are in general thought of as being according to the will of God). Socioeconomic factors also have a direct impact. Lower-class domestic workers who are often employed by those in a higher socioeconomic class may find their employment in jeopardy as physical manifestations of the disease become apparent. Skin discoloration and darker pigmentation resulting from the disease also have social repercussions.
In extreme cases in northern India, leprosy is equated with an "untouchable" status that "often persists long after individuals with leprosy have been cured of the disease, creating lifelong prospects of divorce, eviction, loss of employment, and ostracism from family and social networks."
Public policy | Leprosy | Wikipedia | 505 | 44700 | https://en.wikipedia.org/wiki/Leprosy | Biology and health sciences | Infectious disease | null |
A goal of the WHO is to "eliminate leprosy," and in 2016 the organization launched "Global Leprosy Strategy 2016–2020: Accelerating towards a leprosy-free world". Elimination of leprosy is defined as "reducing the proportion of (people with) leprosy in the community to very low levels, specifically to below one case per 10,000 population". Diagnosis and treatment with multidrug therapy are effective, and a 45% decline in disease burden has occurred since MDT has become more widely available. The organization emphasizes the importance of fully integrating leprosy treatment into public health services, effective diagnosis and treatment, and access to information. The approach includes supporting an increase in health care professionals who understand the disease, and a coordinated and renewed political commitment that includes coordination between countries and improvements in the methodology for collecting and analysing data.
Interventions include:
Early detection of cases focusing on children to reduce transmission and disabilities.
Enhanced healthcare services and improved access for people who may be marginalized.
For countries where leprosy is endemic, further interventions include an improved screening of close contacts, improved treatment regimens, and interventions to reduce stigma and discrimination against people who have leprosy.
In some instances in India, community-based rehabilitation is embraced by local governments and NGOs alike. Often, the identity cultivated by a community environment is preferable to reintegration, and models of self-management and collective agency independent of NGOs and government support have been desirable and successful.
Notable cases | Leprosy | Wikipedia | 306 | 44700 | https://en.wikipedia.org/wiki/Leprosy | Biology and health sciences | Infectious disease | null |
Josephine Cafrine of Seychelles (1877–1907) had leprosy from age 12 and kept a journal that documented her struggles and suffering. It was published as an autobiography in 1923.
Saint Damien De Veuster (1840–1889), a Catholic priest from Belgium, contracted leprosy and ministered to lepers who had been placed under a government-sanctioned medical quarantine on Molokai in the Kingdom of Hawaii.
Baldwin IV of Jerusalem (1161–1185), Catholic king of Latin Jerusalem, had leprosy.
Ingeborg Grytten ( – after 1705), Norwegian writer whose leprosy is thought to have influenced her poetry, which is characterized by a strong religious faith in God's salvation.
Josefina Guerrero (1917–1996), Filipino World War II spy who used the Japanese fear of her leprosy to listen to their battle plans and deliver the information to the American forces under Douglas MacArthur.
King Henry IV of England () possibly had leprosy.
Vietnamese poet Hàn Mặc Tử (1912–1940)
Ōtani Yoshitsugu (1558 or 1565–1600), a Japanese daimyō (feudal lord).
British writer Peter Greave (1910–1977).
In media | Leprosy | Wikipedia | 255 | 44700 | https://en.wikipedia.org/wiki/Leprosy | Biology and health sciences | Infectious disease | null |
— the short story collection Life's Handicap by Rudyard Kipling has a story "The Mark of the Beast" in which a traveller on horseback literally stumbles into a leper colony in India.
— Jack London published "Koolau the Leper" in his Tales of Hawai'i about Molokai and people consigned to it circa 1893.
— James Michener's novel Hawaii dramatizes Molokai's leper settlement, including Father Damien.
— Ben-Hur depicts the title character's mother, Miriam, and younger sister, Tirzah, are imprisoned by the Roman Empire. When they are freed years later, they have leprosy and leave town for the Valley of the Lepers, rather than stay and reunite with Ben-Hur. They leave the colony, and when Jesus dies on the cross, they are miraculously cured.
— English author Graham Greene's novel A Burnt-Out Case is set in a leper colony in Belgian Congo. The story is also predominantly about a disillusioned architect working with a doctor on devising new cures and amenities for mutilated victims of lepers; the title, too, refers to the condition of mutilation and disfigurement in the disease.
— Forugh Farrokhzad made a 22-minute documentary about a leprosy colony in Iran titled The House Is Black. The film humanizes the people affected and opens by saying that "there is no shortage of ugliness in the world, but by closing our eyes on ugliness, we will intensify it."
— The lead character in The Chronicles of Thomas Covenant by Stephen R. Donaldson suffers from leprosy. His condition seems to be cured by the magic of the fantasy land he finds himself in, but he resists believing in its reality, for example, by continuing to perform a regular visual surveillance of extremities as a safety check. Donaldson gained experience with the disease as a young man in India, where his father worked in a missionary for people with leprosy.
— Moloka'i is a novel by Alan Brennert about a leper colony in Hawaii. This novel follows the story of a seven-year-old girl taken from her family and put on Molokai's leper settlement.
— Squint: My Journey with Leprosy is a memoir by Jose P. Ramirez. | Leprosy | Wikipedia | 501 | 44700 | https://en.wikipedia.org/wiki/Leprosy | Biology and health sciences | Infectious disease | null |
Infection of animals
Between 15 and 20% of nine-banded armadillos (Dasypus novemcinctus) in the south-central United States carry M. leprae.Liang, Jiayu (10/16/2023). "Leprosy in Florida: medical experts monitoring unusual, new cases of Hansen's disease". University of Florida Emerging Pathogens Institute. https://epi.ufl.edu/2023/10/16/leprosy-in-florida-medical-experts-monitoring-unusual-new-cases-of-hansens-disease/#:~:text=A%20major%20hint%20that%20someone,leprae%20bacteria. As a result of their low body temperature their tissues commonly contain massive numbers of organisms which helps in the dissemination of the infection. Armadillos were first demonstrated in 1971 to develop leprosy after inoculation with M. leprae. Because of armadillos' armor, skin lesions are difficult to ascertain. Abrasions around the eyes, nose, and feet are the most common signs. Infected armadillos make up a large reservoir of M. leprae and may be a source of infection for some humans in the United States or other locations in the armadillos' home range. In armadillo leprosy, lesions do not persist at the site of entry in animals; M. leprae multiply in macrophages at the site of inoculation and lymph nodes. | Leprosy | Wikipedia | 333 | 44700 | https://en.wikipedia.org/wiki/Leprosy | Biology and health sciences | Infectious disease | null |
Armadillos have been used in immunological research to fight leprosy. Some notable reagents include recombinant interleukin-2 and recombinant interferon-gamma reagents. Additionally, they have been key and have been useful models of leprosy for studies regarding neuropathy. In clinical procedures such as electrophysiological nerve conduction tests Armadillo's nerve function has been properly assessed. Despite the studies mentioned regarding Armadillo's relationship to neuropathy and other effects of leprosy, there is still a lack of proper study on armadillos, and in conducting more armadillo-specific regents our understanding of leprosy’s effects on armadillos and possible humans can be found. Armadillos are a key component of modern-day research on leprosy.
There is a stigma surrounding armadillos and the carrying of leprosy. Because many people do not understand armadillos very well it is common for people to think of them as being dangerous to society and as a result valuing their lives less than other animals. It has become more common in parts of America for people to eat raw or undercooked armadillo, making the chances high that if not properly handled with care, one may become infected.
An outbreak in chimpanzees in West Africa is showing that the bacteria can infect another species and also possibly have additional rodent hosts. Studies have demonstrated that the disease is endemic in the UK red Eurasian squirrel population, with M. leprae and M. lepromatosis appearing in different populations. The M. leprae strain discovered on Brownsea Island is equated to one thought to have died out in the human population in mediaeval times. Despite this, and speculation regarding past transmission through trade in squirrel furs, there does not seem to be a high risk of squirrel to human transmission from the wild population. Although leprosy continues to be diagnosed in immigrants to the UK, the last known human case of leprosy arising in the UK was recorded over 200 years ago. | Leprosy | Wikipedia | 441 | 44700 | https://en.wikipedia.org/wiki/Leprosy | Biology and health sciences | Infectious disease | null |
It has been shown that leprosy can reprogram cells in mouse and armadillo models, similar to how induced pluripotent stem cells are generated by the transcription factors Myc, Oct3/4, Sox2, and Klf4. A notable study conducted by Charles Shepard used mice to find how leprosy, an infection that has a preference for cooler areas of the body, would work in a warm-blooded animal. This mice study helped further the understanding of how leprosy works in humans. This was called "The Mouse Model." The main findings were that even in mice whose immune systems were severely impaired and at a perceived high risk of developing leprosy, the body was still in most cases able to fight off leprosy. The findings suggest that in mice, the body will use their body's energy to fight leprosy. Using The Mouse Model Shepard was able to conduct new research regarding leprosy. This model can now be used as a tool to further study M. leprae. The Mouse Model takes a more easily accessible animal model to better understand this complex disease. There are a few other up-and-coming models for M. leprae'' including the use of other animals including but not limited to mammals, birds, and cold-blooded animals. These animals do not tend to give as great results as armadillos and mice as different animals have different levels of disease resistance. | Leprosy | Wikipedia | 297 | 44700 | https://en.wikipedia.org/wiki/Leprosy | Biology and health sciences | Infectious disease | null |
Printing is a process for mass reproducing text and images using a master form or template. The earliest non-paper products involving printing include cylinder seals and objects such as the Cyrus Cylinder and the Cylinders of Nabonidus. The earliest known form of printing evolved from ink rubbings made on paper or cloth from texts on stone tablets, used during the sixth century. Printing by pressing an inked image onto paper (using woodblock printing) appeared later that century. Later developments in printing technology include the movable type invented by Bi Sheng around 1040 and the printing press invented by Johannes Gutenberg in the 15th century. The technology of printing played a key role in the development of the Renaissance and the Scientific Revolution and laid the material basis for the modern knowledge-based economy and the spread of learning to the masses.
History
Woodblock printing
Woodblock printing is a technique for printing text, images or patterns that was used widely throughout East Asia. It originated in China in antiquity as a method of printing on textiles and later on paper.
In East Asia
The earliest examples of ink-squeeze rubbings and potential stone printing blocks appear in the mid-sixth century in China. A type of printing called mechanical woodblock printing on paper started during the 7th century in the Tang dynasty, and subsequently spread throughout East Asia. Nara Japan printed the Hyakumantō Darani en masse around 770, and distributed them to temples throughout Japan. In Korea, an example of woodblock printing from the eighth century was discovered in 1966. A copy of the Buddhist Dharani Sutra called the Pure Light Dharani Sutra (), discovered in Gyeongju, in a Silla dynasty pagoda that was repaired in 751, was undated but must have been created sometime before the reconstruction of the Shakyamuni Pagoda of Bulguk Temple, Kyongju Province in 751. The document is estimated to have been created no later than 704.
By the ninth century, printing on paper had taken off, and the first completely surviving printed book is the Diamond Sutra (British Library) of 868, uncovered from Dunhuang. By the tenth century, 400,000 copies of some sutras and pictures were printed, and the Confucian classics were in print. A skilled printer could print up to 2,000 double-page sheets per day. | Printing | Wikipedia | 475 | 44723 | https://en.wikipedia.org/wiki/Printing | Technology | Media and communication | null |
Printing spread early to Korea and Japan, which also used Chinese logograms, but the technique was also used in Turpan and Vietnam using a number of other scripts. This technique then spread to Persia and Russia. This technique was transmitted to Europe by around 1400 and was used on paper for old master prints and playing cards.
In the Middle East
Block printing, called tarsh in Arabic, developed in Arabic Egypt during the ninth and tenth centuries, mostly for prayers and amulets. There is some evidence to suggest that these print blocks were made from non-wood materials, possibly tin, lead, or clay. The techniques employed are uncertain. Block printing later went out of use during the Timurid Renaissance. The printing technique in Egypt was embraced by reproducing texts on paper strips and supplying them in different copies to meet the demand.
In Europe
Block printing first came to Europe as a method for printing on cloth, where it was common by 1300. Images printed on cloth for religious purposes could be quite large and elaborate. When paper became relatively easily available, around 1400, the technique transferred very quickly to small woodcut religious images and playing cards printed on paper. These prints were produced in very large numbers from about 1425 onward.
Around the mid-fifteenth-century, block-books, woodcut books with both text and images, usually carved in the same block, emerged as a cheaper alternative to manuscripts and books printed with movable type. These were all short, heavily illustrated works, the bestsellers of the day, repeated in many different block-book versions: the and the Biblia pauperum were the most common. There is still some controversy among scholars as to whether their introduction preceded or, in the majority view, followed the introduction of movable type, with the estimated range of dates being between about 1440 and 1460.
Movable-type printing
Movable type is the system of printing and typography using movable pieces of metal type, made by casting from matrices struck by letterpunches. Movable type allowed for much more flexible processes than hand copying or block printing. | Printing | Wikipedia | 427 | 44723 | https://en.wikipedia.org/wiki/Printing | Technology | Media and communication | null |
Around 1040, the first known movable type system was created in China by Bi Sheng out of porcelain. Bi Sheng used clay type, which broke easily, but Wang Zhen by 1298 had carved a more durable type from wood. He also developed a complex system of revolving tables and number-association with written Chinese characters that made typesetting and printing more efficient. Still, the main method in use there remained woodblock printing (xylography), which "proved to be cheaper and more efficient for printing Chinese, with its thousands of characters".
Copper movable type printing originated in China at the beginning of the 12th century. It was used in large-scale printing of paper money
issued by the Northern Song dynasty. Movable type spread to Korea during the Goryeo dynasty.
Around 1230, Koreans invented a metal type movable printing using bronze. The Jikji, published in 1377, is the earliest known metal printed book. Type-casting was used, adapted from the method of casting coins. The character was cut in beech wood, which was then pressed into a soft clay to form a mould, and bronze poured into the mould, and finally the type was polished. Eastern metal movable type was spread to Europe between the late 14th and early 15th centuries. The Korean form of metal movable type was described by the French scholar Henri-Jean Martin as "extremely similar to Gutenberg's". Authoritative historians Frances Gies and Joseph Gies claimed that "The Asian priority of invention movable type is now firmly established, and that Chinese-Korean technique, or a report of it traveled westward is almost certain."
The printing press
Around 1450, Johannes Gutenberg introduced the first movable type printing system in Europe. He advanced innovations in casting type based on a matrix and hand mould, adaptations to the screw-press, the use of an oil-based ink, and the creation of a softer and more absorbent paper. Gutenberg was the first to create his type pieces from an alloy of lead, tin, antimony, copper and bismuth – the same components still used today. Johannes Gutenberg started work on his printing press around 1436, in partnership with Andreas Dritzehen – whom he had previously instructed in gem-cutting – and Andreas Heilmann, the owner of a paper mill. | Printing | Wikipedia | 478 | 44723 | https://en.wikipedia.org/wiki/Printing | Technology | Media and communication | null |
Compared to woodblock printing, movable type page setting and printing using a press was faster and more durable. Also, the metal type pieces were sturdier and the lettering more uniform, leading to typography and fonts. The high quality and relatively low price of the Gutenberg Bible (1455) established the superiority of movable type for Western languages. The printing press rapidly spread across Europe, leading up to the Renaissance, and later all around the world.
Time Life magazine called Gutenberg's innovations in movable type printing the most important invention of the second millennium.
Rotary printing press
The steam-powered rotary printing press, invented in 1843 in the United States by Richard M. Hoe, ultimately allowed millions of copies of a page in a single day. Mass production of printed works flourished after the transition to rolled paper, as continuous feed allowed the presses to run at a much faster pace. Hoe's original design operated at up to 2,000 revolutions per hour where each revolution deposited 4 page images, giving the press a throughput of 8,000 pages per hour. By 1891, The New York World and Philadelphia Item were operating presses producing either 90,000 4-page sheets per hour or 48,000 8-page sheets.
The rotary printing press uses impressions curved around a cylinder to print on long continuous rolls of paper or other substrates. Rotary drum printing was later significantly improved by William Bullock. There are multiple types of rotary printing press technologies that are still used today: sheetfed offset, rotogravure, and flexographic printing.
Printing capacity
The table lists the maximum number of pages which various press designs could print per hour.
Conventional printing technology
All printing process are concerned with two kinds of areas on the final output:
Image area (printing areas)
Non-image area (non-printing areas)
After the information has been prepared for production (the prepress step), each printing process has definitive means of separating the image from the non-image areas. | Printing | Wikipedia | 402 | 44723 | https://en.wikipedia.org/wiki/Printing | Technology | Media and communication | null |
Conventional printing has four types of process:
Planographics, in which the printing and non-printing areas are on the same plane surface and the difference between them is maintained chemically or by physical properties, the examples are: offset lithography, collotype, and screenless printing.
Relief, in which the printing areas are on a plane surface and the non printing areas are below the surface, examples: flexography and letterpress.
Intaglio, in which the non-printing areas are on a plane surface and the printing area are etched or engraved below the surface, examples: steel die engraving, gravure, etching, collagraph.
Porous or Stencil, in which the printing areas are on fine mesh screens through which ink can penetrate, and the non-printing areas are a stencil over the screen to block the flow of ink in those areas, examples: screen printing, stencil duplicator, risograph.
Crop marks
To print an image without a blank area around the image, the non-printing areas must be trimmed after printing. Crop marks can be used to show the printer where the printing area ends, and the non-printing area begins. The part of the image which is trimmed off is called bleed.
Letterpress
Letterpress printing is a technique of relief printing. A worker composes and locks movable type into the bed of a press, inks it, and presses paper against it to transfer the ink from the type which creates an impression on the paper.
There is different paper for different works the quality of paper shows different ink to use.
Letterpress printing was the normal form of printing text from its invention by Johannes Gutenberg in the mid-15th century and remained in wide use for books and other uses until the second half of the 20th century, when offset printing was developed. More recently, letterpress printing has seen a revival in an artisanal form.
Offset | Printing | Wikipedia | 394 | 44723 | https://en.wikipedia.org/wiki/Printing | Technology | Media and communication | null |
Offset printing is a widely used modern printing process. This technology is best described as when a positive (right-reading) image on a printing plate is inked and transferred (or "offset") from the plate to a rubber blanket. The blanket image becomes a mirror image of the plate image. An offset transfer moves the image to a printing substrate (typically paper), making the image right-reading again. Offset printing uses a lithographic process which is based on the repulsion of oil and water. The offset process employs a flat (planographic) image carrier (plate) which is mounted on a press cylinder. The image to be printed obtains ink from ink rollers, while the non-printing area attracts an (acidic) film of water, keeping the non-image areas ink-free. Most offset presses use three cylinders: Plate, blanket, impression. Currently, most books and newspapers are printed using offset lithography.
Gravure
Gravure printing is an intaglio printing technique, where the image being printed is made up of small depressions in the surface of the printing plate. The cells are filled with ink, and the excess is scraped off the surface with a doctor blade. Then a rubber-covered roller presses paper onto the surface of the plate and into contact with the ink in the cells. The printing cylinders are usually made from copper plated steel, which is subsequently chromed, and may be produced by diamond engraving; etching, or laser ablation.
Gravure printing is known for its ability to produce high-quality, high-resolution images with accurate color reproduction and using viscosity control equipment during production.
Ink evaporation control affects the change in the color of the printed image.
Gravure printing is used for long, high-quality print runs such as magazines, mail-order catalogues, packaging and printing onto fabric and wallpaper. It is also used for printing postage stamps and decorative plastic laminates, such as kitchen worktops.
Flexography
Flexography is a type of relief printing. The relief plates are typically made from photopolymers.
The process is used for flexible packaging, corrugated board, labels, newspapers and more. In this market it competes with gravure printing by holding 80% of the market in US, 50% in Europe but only 20% in Asia. | Printing | Wikipedia | 477 | 44723 | https://en.wikipedia.org/wiki/Printing | Technology | Media and communication | null |
Other printing techniques
The other significant printing techniques include:
Dye-sublimation printer
Inkjet, used typically to print a small number of books or packaging, and also to print a variety of materials: from high quality papers simulating offset printing, to floor tiles. Inkjet is also used to apply mailing addresses to direct mail pieces
Laser printing (toner printing) mainly used in offices and for transactional printing (bills, bank documents). Laser printing is commonly used by direct mail companies to create variable data letters or coupons.
Pad printing, popular for its ability to print on complex three-dimensional surfaces
Relief print, mainly used for catalogues
Screen printing for a variety of applications ranging from T-shirts to floor tiles, and on uneven surfaces
Intaglio, used mainly for high value documents such as currencies.
Thermal printing, popular in the 1990s for fax printing. Used today for printing labels such as airline baggage tags and individual price labels in supermarket deli counters.
Impact of German movable type printing press
Quantitative aspects
It is estimated that following the innovation of Gutenberg's printing press, the European book output rose from a few million to around one billion copies within a span of less than four centuries.
Religious impact
Samuel Hartlib, who was exiled in Britain and enthusiastic about social and cultural reforms, wrote in 1641 that "the art of printing will so spread knowledge that the common people, knowing their own rights and liberties, will not be governed by way of oppression". | Printing | Wikipedia | 301 | 44723 | https://en.wikipedia.org/wiki/Printing | Technology | Media and communication | null |
In the Muslim world, printing, especially in Arabic scripts, was strongly opposed throughout the early modern period, partially due to the high artistic renown of the art of traditional calligraphy. However, printing in Hebrew or Armenian script was often permitted. Thus, the first movable type printing in the Ottoman Empire was in Hebrew in 1493, after which both religious and non-religious texts were able to be printed in Hebrew. According to an imperial ambassador to Istanbul in the middle of the sixteenth century, it was a sin for the Turks, particularly Turkish Muslims, to print religious books. In 1515, Sultan Selim I issued a decree under which the practice of printing would be punishable by death. At the end of the sixteenth century, Sultan Murad III permitted the sale of non-religious printed books in Arabic characters, yet the majority were imported from Italy. Ibrahim Muteferrika established the first press for printing in Arabic in the Ottoman Empire, against opposition from the calligraphers and parts of the Ulama. It operated until 1742, producing altogether seventeen works, all of which were concerned with non-religious, utilitarian matters. Printing did not become common in the Islamic world until the 19th century.
Hebrew language printers were banned from printing guilds in some Germanic states; as a result, Hebrew printing flourished in Italy, beginning in 1470 in Rome, then spreading to other cities including Bari, Pisa, Livorno, and Mantua. Local rulers had the authority to grant or revoke licenses to publish Hebrew books, and many of those printed during this period carry the words 'con licenza de superiori' (indicating their printing having been officially licensed) on their title pages.
It was thought that the introduction of printing 'would strengthen religion and enhance the power of monarchs.' The majority of books were of a religious nature, with the church and crown regulating the content. The consequences of printing 'wrong' material were extreme. Meyrowitz used the example of William Carter who in 1584 printed a pro-Catholic pamphlet in Protestant-dominated England. The consequence of his action was hanging. | Printing | Wikipedia | 427 | 44723 | https://en.wikipedia.org/wiki/Printing | Technology | Media and communication | null |
Social impact
Print gave a broader range of readers access to knowledge and enabled later generations to build directly on the intellectual achievements of earlier ones without the changes arising within verbal traditions. Print, according to Acton in his 1895 lecture On the Study of History, gave "assurance that the work of the Renaissance would last, that what was written would be accessible to all, that such an occultation of knowledge and ideas as had depressed the Middle Ages would never recur, that not an idea would be lost".
Print was instrumental in changing the social nature of reading.
Elizabeth Eisenstein identifies two long-term effects of the invention of printing. She claims that print created a sustained and uniform reference for knowledge and allowed comparisons of incompatible views.
Asa Briggs and Peter Burke identify five kinds of reading that developed in relation to the introduction of print:
Critical reading: Because texts finally became accessible to the general population, critical reading emerged as people were able to form their own opinions on texts.
Dangerous reading: Reading was seen as a dangerous pursuit because it was considered rebellious and unsociable, especially in the case of women, because reading could stir up dangerous emotions such as love, and if women could read, they could read love notes.
Creative reading: Printing allowed people to read texts and interpret them creatively, often in very different ways than the author intended.
Extensive reading: Once print made a wide range of texts available, earlier habits of intensive reading of texts from start to finish began to change, and people began reading selected excerpts, allowing much more extensive reading on a wider range of topics.
Private reading: Reading was linked to the rise of individualism because, before print, reading was often a group event in which one person would read to a group. With print, both literacy and the availability of texts increased, and solitary reading became the norm.
The invention of printing also changed the occupational structure of European cities. Printers emerged as a new group of artisans for whom literacy was essential, while the much more labour-intensive occupation of the scribe naturally declined. Proof-correcting arose as a new occupation, while a rise in the numbers of booksellers and librarians naturally followed the explosion in the numbers of books.
Educational impact
Gutenberg's printing press had profound impacts on universities as well. Universities were influenced in their "language of scholarship, libraries, curriculum, [and] pedagogy" | Printing | Wikipedia | 481 | 44723 | https://en.wikipedia.org/wiki/Printing | Technology | Media and communication | null |
The language of scholarship
Before the invention of the printing press, most written material was in Latin. However, after the invention of printing the number of books printed expanded as well as the vernacular. Latin was not replaced completely, but remained an international language until the eighteenth century.
University libraries
At this time, universities began establishing accompanying libraries. "Cambridge made the chaplain responsible for the library in the fifteenth century but this position was abolished in 1570 and in 1577 Cambridge established the new office of university librarian. Although, the University of Leuven did not see a need for a university library based on the idea that professor were the library. Libraries also began receiving so many books from gifts and purchases that they began to run out of room. However, the issue was solved in 1589 by a man named Merton who decided books should be stored on horizontal shelves rather than lecterns.
Curriculum
The printed press changed university libraries in many ways. Professors were finally able to compare the opinions of different authors rather than being forced to look at only one or two specific authors. Textbooks themselves were also being printed in different levels of difficulty, rather than just one introductory text being made available.
Comparison of printing methods
Digital printing
By 2005, digital printing accounted for approximately 9% of the 45 trillion pages printed annually around the world.
Printing at home, an office, or an engineering environment is subdivided into:
small format (up to ledger size paper sheets), as used in business offices and libraries
wide format (up to 3' or 914mm wide rolls of paper), as used in drafting and design establishments.
Some of the more common printing technologies are:
blueprint – and related chemical technologies
daisy wheel – where pre-formed characters are applied individually
dot-matrix – which produces arbitrary patterns of dots with an array of printing studs
line printing – where formed characters are applied to the paper by lines
heat transfer – such as early fax machines or modern receipt printers that apply heat to special paper, which turns black to form the printed image
inkjet – including bubble-jet, where ink is sprayed onto the paper to create the desired image
electrophotography – where toner is attracted to a charged image and then developed
laser – a type of xerography where the charged image is written pixel by pixel using a laser
solid ink printer – where solid sticks of ink are melted to make liquid ink or toner | Printing | Wikipedia | 477 | 44723 | https://en.wikipedia.org/wiki/Printing | Technology | Media and communication | null |
Vendors typically stress the total cost to operate the equipment, involving complex calculations that include all cost factors involved in the operation as well as the capital equipment costs, amortization, etc. For the most part, toner systems are more economical than inkjet in the long run, even though inkjets are less expensive in the initial purchase price.
Professional digital printing (using toner) primarily uses an electrical charge to transfer toner or liquid ink to the substrate onto which it is printed. Digital print quality has steadily improved from early color and black and white copiers to sophisticated colour digital presses such as the Xerox iGen3, the Kodak Nexpress, the HP Indigo Digital Press series, and the InfoPrint 5000. The iGen3 and Nexpress use toner particles and the Indigo uses liquid ink. The InfoPrint 5000 is a full-color, continuous forms inkjet drop-on-demand printing system. All handle variable data, and rival offset in quality. Digital offset presses are also called direct imaging presses, although these presses can receive computer files and automatically turn them into print-ready plates, they cannot insert variable data.
Small press and fanzines generally use digital printing. Prior to the introduction of cheap photocopying, the use of machines such as the spirit duplicator, hectograph, and mimeograph was common.
3D printing
3D printing is a form of manufacturing technology where physical objects are created from three-dimensional digital models using 3D printers. The objects are created by laying down or building up many thin layers of material in succession. The technique is also known as additive manufacturing, rapid prototyping, or fabricating.
In the 1980s, 3D printing techniques were considered suitable only for the production of functional or aesthetic prototypes, and a more appropriate term for it at the time was rapid prototyping. , the precision, repeatability, and material range of 3D printing have increased to the point that some 3D printing processes are considered viable as an industrial-production technology, whereby the term additive manufacturing can be used synonymously with 3D printing. One of the key advantages of 3D printing is the ability to produce very complex shapes or geometries that would be otherwise infeasible to construct by hand, including hollow parts or parts with internal truss structures to reduce weight. Fused deposition modeling (FDM), which uses a continuous filament of a thermoplastic material, is the most common 3D printing process in use . | Printing | Wikipedia | 504 | 44723 | https://en.wikipedia.org/wiki/Printing | Technology | Media and communication | null |
Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skeletons and pearls. Materials containing much calcium carbonate or resembling it are described as calcareous. Calcium carbonate is the active ingredient in agricultural lime and is produced when calcium ions in hard water react with carbonate ions to form limescale. It has medical use as a calcium supplement or as an antacid, but excessive consumption can be hazardous and cause hypercalcemia and digestive issues.
Chemistry
Calcium carbonate shares the typical properties of other carbonates. Notably it
reacts with acids, releasing carbonic acid which quickly disintegrates into carbon dioxide and water:
releases carbon dioxide upon heating, called a thermal decomposition reaction, or calcination (to above 840 °C in the case of ), to form calcium oxide, CaO, commonly called quicklime, with reaction enthalpy 178 kJ/mol:
reacts with gaseous hydrogen to form methane and water vapor plus solid calcium oxide or calcium hydroxide depending on temperature and product gas composition. Various metals including palladium and nickel are catalysts for the reaction.
Calcium carbonate reacts with water that is saturated with carbon dioxide to form the soluble calcium bicarbonate.
This reaction is important in the erosion of carbonate rock, forming caverns, and leads to hard water in many regions.
An unusual form of calcium carbonate is the hexahydrate ikaite, . Ikaite is stable only below 8 °C.
Preparation
The vast majority of calcium carbonate used in industry is extracted by mining or quarrying. Pure calcium carbonate (such as for food or pharmaceutical use), can be produced from a pure quarried source (usually marble).
Alternatively, calcium carbonate is prepared from calcium oxide. Water is added to give calcium hydroxide then carbon dioxide is passed through this solution to precipitate the desired calcium carbonate, referred to in the industry as precipitated calcium carbonate (PCC) This process is called carbonatation: | Calcium carbonate | Wikipedia | 430 | 44731 | https://en.wikipedia.org/wiki/Calcium%20carbonate | Physical sciences | Salts | null |
In a laboratory, calcium carbonate can easily be crystallized from calcium chloride (), by placing an aqueous solution of in a desiccator alongside ammonium carbonate . In the desiccator, ammonium carbonate is exposed to air and decomposes into ammonia, carbon dioxide, and water. The carbon dioxide then diffuses into the aqueous solution of calcium chloride, reacts with the calcium ions and the water, and forms calcium carbonate.
Structure
The thermodynamically stable form of under normal conditions is hexagonal β- (the mineral calcite). Other forms can be prepared, the denser (2.83 g/cm3) orthorhombic λ- (the mineral aragonite) and hexagonal μ-, occurring as the mineral vaterite. The aragonite form can be prepared by precipitation at temperatures above 85 °C; the vaterite form can be prepared by precipitation at 60 °C. Calcite contains calcium atoms coordinated by six oxygen atoms; in aragonite they are coordinated by nine oxygen atoms. The vaterite structure is not fully understood. Magnesium carbonate () has the calcite structure, whereas strontium carbonate () and barium carbonate () adopt the aragonite structure, reflecting their larger ionic radii.
Polymorphs
Calcium carbonate crystallizes in three anhydrous polymorphs, of which calcite is the thermodynamically most stable at room temperature, aragonite is only slightly less so, and vaterite is the least stable.
Crystal structure
The calcite crystal structure is trigonal, with space group Rc (No. 167 in the International Tables for Crystallography), and Pearson symbol hR10. Aragonite is orthorhombic, with space group Pmcn (No 62), and Pearson Symbol oP20. Vaterite is composed of at least two different coexisting crystallographic structures. The major structure exhibits hexagonal symmetry in space group P63/mmc, the minor structure is still unknown.
Crystallization
All three polymorphs crystallize simultaneously from aqueous solutions under ambient conditions. In additive-free aqueous solutions, calcite forms easily as the major product, while aragonite appears only as a minor product. | Calcium carbonate | Wikipedia | 471 | 44731 | https://en.wikipedia.org/wiki/Calcium%20carbonate | Physical sciences | Salts | null |
At high saturation, vaterite is typically the first phase precipitated, which is followed by a transformation of the vaterite to calcite. This behavior seems to follow Ostwald's rule, in which the least stable polymorph crystallizes first, followed by the crystallization of different polymorphs via a sequence of increasingly stable phases. However, aragonite, whose stability lies between those of vaterite and calcite, seems to be the exception to this rule, as aragonite does not form as a precursor to calcite under ambient conditions.
Aragonite occurs in majority when the reaction conditions inhibit the formation of calcite and/or promote the nucleation of aragonite. For example, the formation of aragonite is promoted by the presence of magnesium ions, or by using proteins and peptides derived from biological calcium carbonate. Some polyamines such as cadaverine and Poly(ethylene imine) have been shown to facilitate the formation of aragonite over calcite.
Selection by organisms
Organisms, such as molluscs and arthropods, have shown the ability to grow all three crystal polymorphs of calcium carbonate, mainly as protection (shells) and muscle attachments. Moreover, they exhibit a remarkable capability of phase selection over calcite and aragonite, and some organisms can switch between the two polymorphs. The ability of phase selection is usually attributed to the use of specific macromolecules or combinations of macromolecules by such organisms.
Occurrence
Geological sources
Calcite, aragonite and vaterite are pure calcium carbonate minerals. Industrially important source rocks which are predominantly calcium carbonate include limestone, chalk, marble and travertine.
Biological sources
Eggshells, snail shells and most seashells are predominantly calcium carbonate and can be used as industrial sources of that chemical. Oyster shells have enjoyed recent recognition as a source of dietary calcium, but are also a practical industrial source. Dark green vegetables such as broccoli and kale contain dietarily significant amounts of calcium carbonate, but they are not practical as an industrial source. | Calcium carbonate | Wikipedia | 433 | 44731 | https://en.wikipedia.org/wiki/Calcium%20carbonate | Physical sciences | Salts | null |
Annelids in the family Lumbricidae, earthworms, possess a regionalization of the digestive track called calciferous glands, Kalkdrüsen, or glandes de Morren, that processes calcium and into calcium carbonate, which is later excreted into the dirt. The function of these glands is unknown but is believed to serve as a regulation mechanism within the animals' tissues. This process is ecologically significant, stabilizing the pH of acid soils.
Extraterrestrial
Beyond Earth, strong evidence suggests the presence of calcium carbonate on Mars. Signs of calcium carbonate have been detected at more than one location (notably at Gusev and Huygens craters). This provides some evidence for the past presence of liquid water.
Geology
Carbonate is found frequently in geologic settings and constitutes an enormous carbon reservoir. Calcium carbonate occurs as aragonite, calcite and dolomite as significant constituents of the calcium cycle. The carbonate minerals form the rock types: limestone, chalk, marble, travertine, tufa, and others.
In warm, clear tropical waters corals are more abundant than towards the poles where the waters are cold. Calcium carbonate contributors, including plankton (such as coccoliths and planktic foraminifera), coralline algae, sponges, brachiopods, echinoderms, bryozoa and mollusks, are typically found in shallow water environments where sunlight and filterable food are more abundant. Cold-water carbonates do exist at higher latitudes but have a very slow growth rate. The calcification processes are changed by ocean acidification.
Where the oceanic crust is subducted under a continental plate sediments will be carried down to warmer zones in the asthenosphere and lithosphere. Under these conditions calcium carbonate decomposes to produce carbon dioxide which, along with other gases, give rise to explosive volcanic eruptions.
Carbonate compensation depth
The carbonate compensation depth (CCD) is the point in the ocean where the rate of precipitation of calcium carbonate is balanced by the rate of dissolution due to the conditions present. Deep in the ocean, the temperature drops and pressure increases. Increasing pressure also increases the solubility of calcium carbonate. Calcium carbonate is unusual in that its solubility increases with decreasing temperature. The carbonate compensation depth ranges from 4,000 to 6,000 meters below sea level in modern oceans, and the various polymorphs (calcite, aragonite) have different compensation depths based on their stability. | Calcium carbonate | Wikipedia | 509 | 44731 | https://en.wikipedia.org/wiki/Calcium%20carbonate | Physical sciences | Salts | null |
Role in taphonomy
Calcium carbonate can preserve fossils through permineralization. Most of the vertebrate fossils of the Two Medicine Formation—a geologic formation known for its duck-billed dinosaur eggs—are preserved by permineralization. This type of preservation conserves high levels of detail, even down to the microscopic level. However, it also leaves specimens vulnerable to weathering when exposed to the surface.
Trilobite populations were once thought to have composed the majority of aquatic life during the Cambrian, due to the fact that their calcium carbonate-rich shells were more easily preserved than those of other species, which had purely chitinous shells.
Uses
Construction
The main use of calcium carbonate is in the construction industry, either as a building material, or limestone aggregate for road building, as an ingredient of cement, or as the starting material for the preparation of builders' lime by burning in a kiln. However, because of weathering mainly caused by acid rain, calcium carbonate (in limestone form) is no longer used for building purposes on its own, but only as a raw primary substance for building materials.
Calcium carbonate is also used in the purification of iron from iron ore in a blast furnace. The carbonate is calcined in situ to give calcium oxide, which forms a slag with various impurities present, and separates from the purified iron.
In the oil industry, calcium carbonate is added to drilling fluids as a formation-bridging and filtercake-sealing agent; it is also a weighting material which increases the density of drilling fluids to control the downhole pressure. Calcium carbonate is added to swimming pools, as a pH corrector for maintaining alkalinity and offsetting the acidic properties of the disinfectant agent.
It is also used as a raw material in the refining of sugar from sugar beet; it is calcined in a kiln with anthracite to produce calcium oxide and carbon dioxide. This burnt lime is then slaked in fresh water to produce a calcium hydroxide suspension for the precipitation of impurities in raw juice during carbonatation. | Calcium carbonate | Wikipedia | 425 | 44731 | https://en.wikipedia.org/wiki/Calcium%20carbonate | Physical sciences | Salts | null |
Calcium carbonate in the form of chalk has traditionally been a major component of blackboard chalk. However, modern manufactured chalk is mostly gypsum, hydrated calcium sulfate . Calcium carbonate is a main source for growing biorock. Precipitated calcium carbonate (PCC), pre-dispersed in slurry form, is a common filler material for latex gloves with the aim of achieving maximum saving in material and production costs.
Fine ground calcium carbonate (GCC) is an essential ingredient in the microporous film used in diapers and some building films, as the pores are nucleated around the calcium carbonate particles during the manufacture of the film by biaxial stretching. GCC and PCC are used as a filler in paper because they are cheaper than wood fiber. Printing and writing paper can contain 10–20% calcium carbonate. In North America, calcium carbonate has begun to replace kaolin in the production of glossy paper. Europe has been practicing this as alkaline papermaking or acid-free papermaking for some decades. PCC used for paper filling and paper coatings is precipitated and prepared in a variety of shapes and sizes having characteristic narrow particle size distributions and equivalent spherical diameters of 0.4 to 3 micrometers. | Calcium carbonate | Wikipedia | 259 | 44731 | https://en.wikipedia.org/wiki/Calcium%20carbonate | Physical sciences | Salts | null |
Calcium carbonate is widely used as an extender in paints, in particular matte emulsion paint where typically 30% by weight of the paint is either chalk or marble. It is also a popular filler in plastics. Some typical examples include around 15–20% loading of chalk in unplasticized polyvinyl chloride (uPVC) drainpipes, 5–15% loading of stearate-coated chalk or marble in uPVC window profile. PVC cables can use calcium carbonate at loadings of up to 70 phr (parts per hundred parts of resin) to improve mechanical properties (tensile strength and elongation) and electrical properties (volume resistivity). Polypropylene compounds are often filled with calcium carbonate to increase rigidity, a requirement that becomes important at high usage temperatures. Here the percentage is often 20–40%. It also routinely used as a filler in thermosetting resins (sheet and bulk molding compounds) and has also been mixed with ABS, and other ingredients, to form some types of compression molded "clay" poker chips. Precipitated calcium carbonate, made by dropping calcium oxide into water, is used by itself or with additives as a white paint, known as whitewashing.
Calcium carbonate is added to a wide range of trade and do it yourself adhesives, sealants, and decorating fillers. Ceramic tile adhesives typically contain 70% to 80% limestone. Decorating crack fillers contain similar levels of marble or dolomite. It is also mixed with putty in setting stained glass windows, and as a resist to prevent glass from sticking to kiln shelves when firing glazes and paints at high temperature. | Calcium carbonate | Wikipedia | 350 | 44731 | https://en.wikipedia.org/wiki/Calcium%20carbonate | Physical sciences | Salts | null |
In ceramic glaze applications, calcium carbonate is known as whiting, and is a common ingredient for many glazes in its white powdered form. When a glaze containing this material is fired in a kiln, the whiting acts as a flux material in the glaze. Ground calcium carbonate is an abrasive (both as scouring powder and as an ingredient of household scouring creams), in particular in its calcite form, which has the relatively low hardness level of 3 on the Mohs scale, and will therefore not scratch glass and most other ceramics, enamel, bronze, iron, and steel, and have a moderate effect on softer metals like aluminium and copper. A paste made from calcium carbonate and deionized water can be used to clean tarnish on silver.
Health and diet
Calcium carbonate is widely used medicinally as an inexpensive dietary calcium supplement for gastric antacid (such as Tums and Eno). It may be used as a phosphate binder for the treatment of hyperphosphatemia (primarily in patients with chronic kidney failure). It is used in the pharmaceutical industry as an inert filler for tablets and other pharmaceuticals.
Calcium carbonate is used in the production of calcium oxide as well as toothpaste and has seen a resurgence as a food preservative and color retainer, when used in or with products such as organic apples.
Calcium carbonate is used therapeutically as phosphate binder in patients on maintenance haemodialysis. It is the most common form of phosphate binder prescribed, particularly in non-dialysis chronic kidney disease. Calcium carbonate is the most commonly used phosphate binder, but clinicians are increasingly prescribing the more expensive, non-calcium-based phosphate binders, particularly sevelamer. | Calcium carbonate | Wikipedia | 367 | 44731 | https://en.wikipedia.org/wiki/Calcium%20carbonate | Physical sciences | Salts | null |
Excess calcium from supplements, fortified food, and high-calcium diets can cause milk-alkali syndrome, which has serious toxicity and can be fatal. In 1915, Bertram Sippy introduced the "Sippy regimen" of hourly ingestion of milk and cream, and the gradual addition of eggs and cooked cereal, for 10 days, combined with alkaline powders, which provided symptomatic relief for peptic ulcer disease. Over the next several decades, the Sippy regimen resulted in kidney failure, alkalosis, and hypercalcaemia, mostly in men with peptic ulcer disease. These adverse effects were reversed when the regimen stopped, but it was fatal in some patients with protracted vomiting. Milk-alkali syndrome declined in men after effective treatments for peptic ulcer disease arose. Since the 1990s it has been most frequently reported in women taking calcium supplements above the recommended range of 1.2 to 1.5 grams daily, for prevention and treatment of osteoporosis, and is exacerbated by dehydration. Calcium has been added to over-the-counter products, which contributes to inadvertent excessive intake. Excessive calcium intake can lead to hypercalcemia, complications of which include vomiting, abdominal pain and altered mental status.
As a food additive it is designated E170, and it has an INS number of 170. Used as an acidity regulator, anticaking agent, stabilizer or color it is approved for usage in the EU, US and Australia and New Zealand. It is "added by law to all UK milled bread flour except wholemeal". It is used in some soy milk and almond milk products as a source of dietary calcium; at least one study suggests that calcium carbonate might be as bioavailable as the calcium in cow's milk. Calcium carbonate is also used as a firming agent in many canned and bottled vegetable products.
Several calcium supplement formulations have been documented to contain the chemical element lead, posing a public health concern. Lead is commonly found in natural sources of calcium.
Agriculture and aquaculture
Agricultural lime, powdered chalk or limestone, is used as a cheap method of neutralising acidic soil, making it suitable for planting, also used in aquaculture industry for pH regulation of pond soil before initiating culture. There is interest in understanding whether or not it can affect pesticide adsorption and desorption in calcareous soil. | Calcium carbonate | Wikipedia | 502 | 44731 | https://en.wikipedia.org/wiki/Calcium%20carbonate | Physical sciences | Salts | null |
Household cleaning
Calcium carbonate is a key ingredient in many household cleaning powders like Comet and is used as a scrubbing agent.
Pollution mitigation
In 1989, a researcher, Ken Simmons, introduced into the Whetstone Brook in Massachusetts. His hope was that the calcium carbonate would counter the acid in the stream from acid rain and save the trout that had ceased to spawn. Although his experiment was a success, it did increase the amount of aluminium ions in the area of the brook that was not treated with the limestone. This shows that can be added to neutralize the effects of acid rain in river ecosystems. Currently calcium carbonate is used to neutralize acidic conditions in both soil and water. Since the 1970s, such liming has been practiced on a large scale in Sweden to mitigate acidification and several thousand lakes and streams are limed repeatedly.
Calcium carbonate is also used in flue-gas desulfurization applications eliminating harmful and emissions from coal and other fossil fuels burnt in large fossil fuel power stations.
Plastics
Calcium carbonate is commonly used in the plastic industry as a filler. When it is incorporated in a plastic material, it can improve the hardness, stiffness, dimensional stability and processability of the material.
Calcination equilibrium
Calcination of limestone using charcoal fires to produce quicklime has been practiced since antiquity by cultures all over the world. The temperature at which limestone yields calcium oxide is usually given as 825 °C, but stating an absolute threshold is misleading. Calcium carbonate exists in equilibrium with calcium oxide and carbon dioxide at any temperature. At each temperature there is a partial pressure of carbon dioxide that is in equilibrium with calcium carbonate. At room temperature the equilibrium overwhelmingly favors calcium carbonate, because the equilibrium pressure is only a tiny fraction of the partial pressure in air, which is about 0.035 kPa.
At temperatures above 550 °C the equilibrium pressure begins to exceed the pressure in air. So above 550 °C, calcium carbonate begins to outgas into air. However, in a charcoal fired kiln, the concentration of will be much higher than it is in air. Indeed, if all the oxygen in the kiln is consumed in the fire, then the partial pressure of in the kiln can be as high as 20 kPa. | Calcium carbonate | Wikipedia | 454 | 44731 | https://en.wikipedia.org/wiki/Calcium%20carbonate | Physical sciences | Salts | null |
The table shows that this partial pressure is not achieved until the temperature is nearly 800 °C. For the outgassing of from calcium carbonate to happen at an economically useful rate, the equilibrium pressure must significantly exceed the ambient pressure of . And for it to happen rapidly, the equilibrium pressure must exceed total atmospheric pressure of 101 kPa, which happens at 898 °C.
{| class="wikitable"
|+ Equilibrium pressure of over (P) versus temperature (T).
|-
!P (kPa)
|0.055||0.13||0.31||1.80||5.9||9.3||14||24||34||51||72 ||80||91||101||179||901||3961
|-
!T (°C)
|550||587||605||680||727||748||777||800||830||852||871||881||891||898||937||1082||1241
|}
Solubility
With varying pressure
Calcium carbonate is poorly soluble in pure water (47 mg/L at normal atmospheric partial pressure as shown below).
The equilibrium of its solution is given by the equation (with dissolved calcium carbonate on the right):
{| width="500"
| style="width:50%; height:30px;"|
| Ksp = to at 25 °C
|}
where the solubility product for is given as anywhere from Ksp = to Ksp = at 25 °C, depending upon the data source. What the equation means is that the product of molar concentration of calcium ions (moles of dissolved per liter of solution) with the molar concentration of dissolved cannot exceed the value of Ksp. This seemingly simple solubility equation, however, must be taken along with the more complicated equilibrium of carbon dioxide with water (see carbonic acid). Some of the combines with in the solution according to
{| width="500"
| style="width:50%; height:25px;"|
| Ka2 = at 25 °C
|}
is known as the bicarbonate ion. Calcium bicarbonate is many times more soluble in water than calcium carbonate—indeed it exists only in solution.
Some of the combines with in solution according to | Calcium carbonate | Wikipedia | 510 | 44731 | https://en.wikipedia.org/wiki/Calcium%20carbonate | Physical sciences | Salts | null |
{|
| width=250|
| Ka1 = at 25 °C
|}
Some of the breaks up into water and dissolved carbon dioxide according to
{|
|width=250|
| Kh = at 25 °C
|}
And dissolved carbon dioxide is in equilibrium with atmospheric carbon dioxide according to
{|
| width=250| =
| where = 29.76 atm/(mol/L) at 25 °C (Henry volatility), and P is the partial pressure.
|}
For ambient air, P is around atm (or equivalently 35 Pa). The last equation above fixes the concentration of dissolved as a function of P, independent of the concentration of dissolved . At atmospheric partial pressure of , dissolved concentration is moles per liter. The equation before that fixes the concentration of as a function of concentration. For [] = , it results in = moles per liter. When is known, the remaining three equations together with
{| width="450"
| style="width:50%; height:25px;"|
| K = 10−14 at 25 °C
|}
(which is true for all aqueous solutions), and the constraint that the solution must be electrically neutral, i.e., the overall charge of dissolved positive ions must be cancelled out by the overall charge of dissolved negative ions , make it possible to solve simultaneously for the remaining five unknown concentrations (the previously mentioned form of the neutrality is valid only if calcium carbonate has been put in contact with pure water or with a neutral pH solution; in the case where the initial water solvent pH is not neutral, the balance is not neutral). | Calcium carbonate | Wikipedia | 335 | 44731 | https://en.wikipedia.org/wiki/Calcium%20carbonate | Physical sciences | Salts | null |
The adjacent table shows the result for and (in the form of pH) as a function of ambient partial pressure of (Ksp = has been taken for the calculation).
At atmospheric levels of ambient the table indicates that the solution will be slightly alkaline with a maximum solubility of 47 mg/L.
As ambient partial pressure is reduced below atmospheric levels, the solution becomes more and more alkaline. At extremely low P, dissolved , bicarbonate ion, and carbonate ion largely evaporate from the solution, leaving a highly alkaline solution of calcium hydroxide, which is more soluble than . For P = 10−12 atm, the product is still below the solubility product of (). For still lower pressure, precipitation will occur before precipitation.
As ambient partial pressure increases to levels above atmospheric, pH drops, and much of the carbonate ion is converted to bicarbonate ion, which results in higher solubility of .
The effect of the latter is especially evident in day-to-day life of people who have hard water. Water in aquifers underground can be exposed to levels of much higher than atmospheric. As such water percolates through calcium carbonate rock, the dissolves according to one of the trends above. When that same water then emerges from the tap, in time it comes into equilibrium with levels in the air by outgassing its excess . The calcium carbonate becomes less soluble as a result, and the excess precipitates as lime scale. This same process is responsible for the formation of stalactites and stalagmites in limestone caves.
Two hydrated phases of calcium carbonate, monohydrocalcite and ikaite , may precipitate from water at ambient conditions and persist as metastable phases.
With varying pH, temperature and salinity: scaling in swimming pools
In contrast to the open equilibrium scenario above, many swimming pools are managed by addition of sodium bicarbonate () to the concentration of about 2 mmol/L as a buffer, then control of pH through use of HCl, , , NaOH or chlorine formulations that are acidic or basic. In this situation, dissolved inorganic carbon (total inorganic carbon) is far from equilibrium with atmospheric . Progress towards equilibrium through outgassing of is slowed by
In this situation, the dissociation constants for the much faster reactions | Calcium carbonate | Wikipedia | 484 | 44731 | https://en.wikipedia.org/wiki/Calcium%20carbonate | Physical sciences | Salts | null |
allow the prediction of concentrations of each dissolved inorganic carbon species in solution, from the added concentration of (which constitutes more than 90% of Bjerrum plot species from pH 7 to pH 8 at 25 °C in fresh water). Addition of will increase concentration at any pH. Rearranging the equations given above, we can see that = , and [] = . Therefore, when concentration is known, the maximum concentration of ions before scaling through precipitation can be predicted from the formula:
[]max = ×
The solubility product for (Ksp) and the dissociation constants for the dissolved inorganic carbon species (including Ka2) are all substantially affected by temperature and salinity, with the overall effect that []max increases from freshwater to saltwater, and decreases with rising temperature, pH, or added bicarbonate level, as illustrated in the accompanying graphs.
The trends are illustrative for pool management, but whether scaling occurs also depends on other factors including interactions with , and other ions in the pool, as well as supersaturation effects. Scaling is commonly observed in electrolytic chlorine generators, where there is a high pH near the cathode surface and scale deposition further increases temperature. This is one reason that some pool operators prefer borate over bicarbonate as the primary pH buffer, and avoid the use of pool chemicals containing calcium. | Calcium carbonate | Wikipedia | 279 | 44731 | https://en.wikipedia.org/wiki/Calcium%20carbonate | Physical sciences | Salts | null |
Solubility in a strong or weak acid solution
Solutions of strong (HCl), moderately strong (sulfamic) or weak (acetic, citric, sorbic, lactic, phosphoric) acids are commercially available. They are commonly used as descaling agents to remove limescale deposits. The maximum amount of that can be "dissolved" by one liter of an acid solution can be calculated using the above equilibrium equations.
In the case of a strong monoacid with decreasing acid concentration [A] = [], we obtain (with molar mass = 100 g/mol):
{| class="wikitable"
! [A] (mol/L)
| 1
| 10−1
| 10−2
| 10−3
| 10−4
| 10−5
| 10−6
| 10−7
| 10−10
|-
! Initial pH
| 0.00||1.00||2.00||3.00||4.00||5.00||6.00||6.79||7.00
|-
!Final pH
| 6.75||7.25||7.75||8.14||8.25||8.26||8.26||8.26||8.27
|-
! Dissolved (g/L of acid)
| 50.0||5.00||0.514||0.0849||0.0504||0.0474||0.0471||0.0470||0.0470
|}
where the initial state is the acid solution with no (not taking into account possible dissolution) and the final state is the solution with saturated . For strong acid concentrations, all species have a negligible concentration in the final state with respect to and so that the neutrality equation reduces approximately to 2[] = [] yielding [] ≈ 0.5 []. When the concentration decreases, [] becomes non-negligible so that the preceding expression is no longer valid. For vanishing acid concentrations, one can recover the final pH and the solubility of in pure water.
In the case of a weak monoacid (here we take acetic acid with pKa = 4.76) with decreasing total acid concentration [A] = [] + [AH], we obtain: | Calcium carbonate | Wikipedia | 504 | 44731 | https://en.wikipedia.org/wiki/Calcium%20carbonate | Physical sciences | Salts | null |
{| class="wikitable"
![A] (mol/L)
| [] ≈ 0.5 []
| 10−1
| 10−2
| 10−3
| 10−4
| 10−5
| 10−6
| 10−7
| 10−10
|-
!Initial pH
| 2.38||2.88||3.39||3.91||4.47||5.15||6.02||6.79||7.00
|-
!Final pH
| 6.75||7.25||7.75||8.14||8.25||8.26||8.26||8.26||8.27
|-
!|Dissolved (g/L of acid)
| 49.5||4.99||0.513||0.0848||0.0504||0.0474||0.0471||0.0470||0.0470
|}
For the same total acid concentration, the initial pH of the weak acid is less acid than the one of the strong acid; however, the maximum amount of which can be dissolved is approximately the same. This is because in the final state, the pH is larger than the pKa, so that the weak acid is almost completely dissociated, yielding in the end as many ions as the strong acid to "dissolve" the calcium carbonate.
The calculation in the case of phosphoric acid (which is the most widely used for domestic applications) is more complicated since the concentrations of the four dissociation states corresponding to this acid must be calculated together with [], [], [], [] and []. The system may be reduced to a seventh degree equation for [] the numerical solution of which gives | Calcium carbonate | Wikipedia | 391 | 44731 | https://en.wikipedia.org/wiki/Calcium%20carbonate | Physical sciences | Salts | null |
{| class="wikitable"
! [A] (mol/L)
| 1
| 10−1
| 10−2
| 10−3
| 10−4
| 10−5
| 10−6
| 10−7
| 10−10
|-
! Initial pH
| 1.08||1.62||2.25||3.05||4.01||5.00||5.97||6.74||7.00
|-
! Final pH
| 6.71||7.17||7.63||8.06||8.24||8.26||8.26||8.26||8.27
|-
! Dissolved (g/L of acid)
| 62.0||7.39||0.874||0.123||0.0536||0.0477||0.0471||0.0471||0.0470
|}
where [A] = is the total acid concentration. Thus phosphoric acid is more efficient than a monoacid since at the final almost neutral pH, the second dissociated state concentration [] is not negligible (see phosphoric acid). | Calcium carbonate | Wikipedia | 273 | 44731 | https://en.wikipedia.org/wiki/Calcium%20carbonate | Physical sciences | Salts | null |
Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Chalk is common throughout Western Europe, where deposits underlie parts of France, and steep cliffs are often seen where they meet the sea in places such as the Dover cliffs on the Kent coast of the English Channel.
Chalk is mined for use in industry, such as for quicklime, bricks and builder's putty, and in agriculture, for raising pH in soils with high acidity. It is also used for "blackboard chalk" for writing and drawing on various types of surfaces, although these can also be manufactured from other carbonate-based minerals, or gypsum.
Description
Chalk is a fine-textured, earthy type of limestone distinguished by its light colour, softness, and high porosity. It is composed mostly of tiny fragments of the calcite shells or skeletons of plankton, such as foraminifera or coccolithophores. These fragments mostly take the form of calcite plates ranging from 0.5 to 4 microns in size, though about 10% to 25% of a typical chalk is composed of fragments that are 10 to 100 microns in size. The larger fragments include intact plankton skeletons and skeletal fragments of larger organisms, such as molluscs, echinoderms, or bryozoans.
Chalk is typically almost pure calcite, , with just 2% to 4% of other minerals. These are usually quartz and clay minerals, though collophane (cryptocrystalline apatite, a phosphate mineral) is also sometimes present, as nodules or as small pellets interpreted as fecal pellets. In some chalk beds, the calcite has been converted to dolomite, , and in a few cases the dolomitized chalk has been dedolomitized back to calcite.
Chalk is highly porous, with typical values of porosity ranging from 35 to 47 per cent. While it is similar in appearance to both gypsum and diatomite, chalk is identifiable by its hardness, fossil content, and its reaction to acid (it produces effervescence on contact). | Chalk | Wikipedia | 474 | 44734 | https://en.wikipedia.org/wiki/Chalk | Physical sciences | Sedimentary rocks | Earth science |
Formation
In Western Europe, chalk was formed in the Late Cretaceous Epoch and the early Palaeocene Epoch (between 100 and 61 million years ago). It was deposited on extensive continental shelves at depths between , during a time of nonseasonal (likely arid) climate that reduced the amount of erosion from nearby exposed rock. The lack of nearby erosion explains the high purity of chalk. The coccolithophores, foraminifera, and other microscopic organisms from which the chalk came mostly form low-magnesium calcite skeletons, so the sediments were already in the form of highly stable low-magnesium calcite when deposited. This is in contrast with most other limestones, which formed from high-magnesium calcite or aragonite that rapidly converted to the more stable low-magnesium calcite after deposition, resulting in the early cementation of such limestones. In chalk, the absence of calcium carbonate conversion process prevented early cementation, and it accounts for chalk's high porosity. Additionally, chalk is the only form of limestone that commonly shows signs of compaction.
Flint (a type of chert) is very common as bands parallel to the bedding or as nodules in seams, or linings to fractures, embedded in chalk. It is probably derived from sponge spicules or other siliceous organisms as water is expelled upwards during compaction. Flint is often deposited around larger fossils such as Echinoidea which may be silicified (i.e. replaced molecule by molecule by flint).
Geology and geographic distribution
Chalk is so common in Cretaceous marine beds that the Cretaceous Period was named for these deposits. The name Cretaceous was derived from Latin creta, meaning chalk. Some deposits of chalk were formed after the Cretaceous.
The Chalk Group is a European stratigraphic unit deposited during the late Cretaceous Period. It forms the famous White Cliffs of Dover in Kent, England, as well as their counterparts of the Cap Blanc Nez on the other side of the Dover Strait. The Champagne region of France is mostly underlain by chalk deposits, which contain artificial caves used for wine storage. Some of the highest chalk cliffs in the world occur at Jasmund National Park in Germany and at Møns Klint in Denmark.
Chalk deposits are also found in Cretaceous beds on other continents, such as the Austin Chalk, Selma Group, and Niobrara Formations of the North American interior. Chalk is also found in western Egypt (Khoman Formation) and western Australia (Miria Formation). | Chalk | Wikipedia | 511 | 44734 | https://en.wikipedia.org/wiki/Chalk | Physical sciences | Sedimentary rocks | Earth science |
Chalk of Oligocene to Neogene age has been found in drill cores of rock under the Pacific Ocean at Stewart Arch in the Solomon Islands.
There are layers of chalk, containing Globorotalia, in the Nicosia Formation of Cyprus, which formed during the Pliocene.
Mining
Chalk is mined from chalk deposits both above ground and underground. Chalk mining boomed during the Industrial Revolution, due to the need for chalk products such as quicklime and bricks.
Uses
Most people first encounter chalk in school where it refers to blackboard chalk, which was originally made of mineral chalk, since it readily crumbles and leaves particles that stick loosely to rough surfaces, allowing it to make writing that can be readily erased. Blackboard chalk manufacturers now may use mineral chalk, other mineral sources of calcium carbonate, or the mineral gypsum (calcium sulfate). While gypsum-based blackboard chalk is the lowest cost to produce, and thus widely used in the developing world, use of carbonate-based chalk produces larger particles and thus less dust, and it is marketed as "dustless chalk".
Coloured chalks, pastel chalks, and sidewalk chalk (shaped into larger sticks and often coloured), used to draw on sidewalks, streets, and driveways, are primarily made of gypsum rather than calcium carbonate chalk.
Magnesium carbonate chalk is commonly used as a drying agent to obtain better grip by gymnasts and rock climbers.
Glazing putty mainly contains chalk as a filler in linseed oil.
Chalk and other forms of limestone may be used for their properties as a base. Chalk is a source of quicklime by thermal decomposition, or slaked lime following quenching of quicklime with water. In agriculture, chalk is used for raising pH in soils with high acidity. Small doses of chalk can also be used as an antacid. Additionally, the small particles of chalk make it a substance ideal for cleaning and polishing. For example, toothpaste commonly contains small amounts of chalk, which serves as a mild abrasive. Polishing chalk is chalk prepared with a carefully controlled grain size, for very fine polishing of metals.
French chalk (also known as tailor's chalk) is traditionally a hard chalk used to make temporary markings on cloth, mainly by tailors. It is now usually made of talc (magnesium silicate).
Chalk beds form important petroleum reservoirs in the North Sea and along the Gulf Coast of North America. | Chalk | Wikipedia | 509 | 44734 | https://en.wikipedia.org/wiki/Chalk | Physical sciences | Sedimentary rocks | Earth science |
Previous uses
In southeast England, deneholes are a notable example of ancient chalk pits. Such bell pits may also mark the sites of ancient flint mines, where the prime object was to remove flint nodules for stone tool manufacture. The surface remains at Cissbury are one such example, but perhaps the most famous is the extensive complex at Grimes Graves in Norfolk.
Chalk was traditionally used in recreation. In field sports, such as tennis played on grass, powdered chalk was used to mark the boundary lines of the playing field or court. If a ball hits the line, a cloud of chalk or pigment dust will be visible. In recent years, powdered chalk has been replaced with titanium dioxide. In gymnastics, rock-climbing, weightlifting and tug of war, chalk — now usually magnesium carbonate — is applied to the hands and feet to remove perspiration and reduce slipping.
Chalk may also be used as a house construction material instead of brick or wattle and daub: quarried chalk was cut into blocks and used as ashlar, or loose chalk was rammed into blocks and laid in mortar. There are still houses standing which have been constructed using chalk as the main building material. Most are pre-Victorian though a few are more recent.
A mixture of chalk and mercury can be used as fingerprint powder. However, because of the toxicity of the mercury, the use of such mixtures for fingerprinting was abandoned in 1967. | Chalk | Wikipedia | 290 | 44734 | https://en.wikipedia.org/wiki/Chalk | Physical sciences | Sedimentary rocks | Earth science |
Goldbach's conjecture is one of the oldest and best-known unsolved problems in number theory and all of mathematics. It states that every even natural number greater than 2 is the sum of two prime numbers.
The conjecture has been shown to hold for all integers less than but remains unproven despite considerable effort.
History
Origins
On 7 June 1742, the Prussian mathematician Christian Goldbach wrote a letter to Leonhard Euler (letter XLIII), in which he proposed the following conjecture:
Goldbach was following the now-abandoned convention of considering 1 to be a prime number, so that a sum of units would be a sum of primes.
He then proposed a second conjecture in the margin of his letter, which implies the first:
Euler replied in a letter dated 30 June 1742 and reminded Goldbach of an earlier conversation they had had (""), in which Goldbach had remarked that the first of those two conjectures would follow from the statement
This is in fact equivalent to his second, marginal conjecture.
In the letter dated 30 June 1742, Euler stated:
Similar conjecture by Descartes
René Descartes wrote that "Every even number can be expressed as the sum of at most three primes." The proposition is equivalent to Goldbach's conjecture, and Paul Erdős said that "Descartes actually discovered this before Goldbach... but it is better that the conjecture was named for Goldbach because, mathematically speaking, Descartes was infinitely rich and Goldbach was very poor." | Goldbach's conjecture | Wikipedia | 311 | 44758 | https://en.wikipedia.org/wiki/Goldbach%27s%20conjecture | Mathematics | Sums and products | null |
Partial results
The strong Goldbach conjecture is much more difficult than the weak Goldbach conjecture, which says that every odd integer greater than 5 is the sum of three primes. Using Vinogradov's method, Nikolai Chudakov, Johannes van der Corput, and Theodor Estermann showed (1937–1938) that almost all even numbers can be written as the sum of two primes (in the sense that the fraction of even numbers up to some which can be so written tends towards 1 as increases). In 1930, Lev Schnirelmann proved that any natural number greater than 1 can be written as the sum of not more than prime numbers, where is an effectively computable constant; see Schnirelmann density. Schnirelmann's constant is the lowest number with this property. Schnirelmann himself obtained . This result was subsequently enhanced by many authors, such as Olivier Ramaré, who in 1995 showed that every even number is in fact the sum of at most 6 primes. The best known result currently stems from the proof of the weak Goldbach conjecture by Harald Helfgott, which directly implies that every even number is the sum of at most 4 primes.
In 1924, Hardy and Littlewood showed under the assumption of the generalized Riemann hypothesis that the number of even numbers up to violating the Goldbach conjecture is much less than for small .
In 1948, using sieve theory methods, Alfréd Rényi showed that every sufficiently large even number can be written as the sum of a prime and an almost prime with at most factors. Chen Jingrun showed in 1973 using sieve theory that every sufficiently large even number can be written as the sum of either two primes, or a prime and a semiprime (the product of two primes). See Chen's theorem for further information.
In 1975, Hugh Lowell Montgomery and Bob Vaughan showed that "most" even numbers are expressible as the sum of two primes. More precisely, they showed that there exist positive constants and such that for all sufficiently large numbers , every even number less than is the sum of two primes, with at most exceptions. In particular, the set of even integers that are not the sum of two primes has density zero. | Goldbach's conjecture | Wikipedia | 465 | 44758 | https://en.wikipedia.org/wiki/Goldbach%27s%20conjecture | Mathematics | Sums and products | null |
In 1951, Yuri Linnik proved the existence of a constant such that every sufficiently large even number is the sum of two primes and at most powers of 2. János Pintz and Imre Ruzsa found in 2020 that works. Assuming the generalized Riemann hypothesis, also works, as shown by Roger Heath-Brown and Jan-Christoph Schlage-Puchta in 2002.
A proof for the weak conjecture was submitted in 2013 by Harald Helfgott to Annals of Mathematics Studies series. Although the article was accepted, Helfgott decided to undertake the major modifications suggested by the referee. Despite several revisions, Helfgott's proof has not yet appeared in a peer-reviewed publication. The weak conjecture is implied by the strong conjecture, as if is a sum of two primes, then is a sum of three primes. However, the converse implication and thus the strong Goldbach conjecture would remain unproven if Helfgott's proof is correct.
Computational results
For small values of , the strong Goldbach conjecture (and hence the weak Goldbach conjecture) can be verified directly. For instance, in 1938, Nils Pipping laboriously verified the conjecture up to . With the advent of computers, many more values of have been checked; T. Oliveira e Silva ran a distributed computer search that has verified the conjecture for (and double-checked up to ) as of 2013. One record from this search is that is the smallest number that cannot be written as a sum of two primes where one is smaller than 9781.
In popular culture
Goldbach's Conjecture () is the title of the biography of Chinese mathematician and number theorist Chen Jingrun, written by Xu Chi.
The conjecture is a central point in the plot of the 1992 novel Uncle Petros and Goldbach's Conjecture by Greek author Apostolos Doxiadis, in the short story "Sixty Million Trillion Combinations" by Isaac Asimov and also in the 2008 mystery novel No One You Know by Michelle Richmond.
Goldbach's conjecture is part of the plot of the 2007 Spanish film Fermat's Room.
Goldbach's conjecture is featured as the main topic of research of actress Ella Rumpf's character Marguerite in the 2023 French-Swiss film Marguerite's Theorem.
Formal statement
Each of the three conjectures has a natural analog in terms of the modern definition of a prime, under which 1 is excluded. A modern version of the first conjecture is: | Goldbach's conjecture | Wikipedia | 505 | 44758 | https://en.wikipedia.org/wiki/Goldbach%27s%20conjecture | Mathematics | Sums and products | null |
A modern version of the marginal conjecture is:
And a modern version of Goldbach's older conjecture of which Euler reminded him is:
These modern versions might not be entirely equivalent to the corresponding original statements. For example, if there were an even integer larger than 4, for a prime, that could not be expressed as the sum of two primes in the modern sense, then it would be a counterexample to the modern version of the third conjecture (without being a counterexample to the original version). The modern version is thus probably stronger (but in order to confirm that, one would have to prove that the first version, freely applied to any positive even integer , could not possibly rule out the existence of such a specific counterexample ). In any case, the modern statements have the same relationships with each other as the older statements did. That is, the second and third modern statements are equivalent, and either implies the first modern statement.
The third modern statement (equivalent to the second) is the form in which the conjecture is usually expressed today. It is also known as the "strong", "even", or "binary" Goldbach conjecture. A weaker form of the second modern statement, known as "Goldbach's weak conjecture", the "odd Goldbach conjecture", or the "ternary Goldbach conjecture", asserts that
Heuristic justification
Statistical considerations that focus on the probabilistic distribution of prime numbers present informal evidence in favour of the conjecture (in both the weak and strong forms) for sufficiently large integers: the greater the integer, the more ways there are available for that number to be represented as the sum of two or three other numbers, and the more "likely" it becomes that at least one of these representations consists entirely of primes.
A very crude version of the heuristic probabilistic argument (for the strong form of the Goldbach conjecture) is as follows. The prime number theorem asserts that an integer selected at random has roughly a chance of being prime. Thus if is a large even integer and is a number between 3 and , then one might expect the probability of and simultaneously being prime to be . If one pursues this heuristic, one might expect the total number of ways to write a large even integer as the sum of two odd primes to be roughly | Goldbach's conjecture | Wikipedia | 480 | 44758 | https://en.wikipedia.org/wiki/Goldbach%27s%20conjecture | Mathematics | Sums and products | null |
Since , this quantity goes to infinity as increases, and one would expect that every large even integer has not just one representation as the sum of two primes, but in fact very many such representations.
This heuristic argument is actually somewhat inaccurate because it assumes that the events of and being prime are statistically independent of each other. For instance, if is odd, then is also odd, and if is even, then is even, a non-trivial relation because, besides the number 2, only odd numbers can be prime. Similarly, if is divisible by 3, and was already a prime other than 3, then would also be coprime to 3 and thus be slightly more likely to be prime than a general number. Pursuing this type of analysis more carefully, G. H. Hardy and John Edensor Littlewood in 1923 conjectured (as part of their Hardy–Littlewood prime tuple conjecture) that for any fixed , the number of representations of a large integer as the sum of primes with should be asymptotically equal to
where the product is over all primes , and is the number of solutions to the equation in modular arithmetic, subject to the constraints . This formula has been rigorously proven to be asymptotically valid for from the work of Ivan Matveevich Vinogradov, but is still only a conjecture when . In the latter case, the above formula simplifies to 0 when is odd, and to
when is even, where is Hardy–Littlewood's twin prime constant
This is sometimes known as the extended Goldbach conjecture. The strong Goldbach conjecture is in fact very similar to the twin prime conjecture, and the two conjectures are believed to be of roughly comparable difficulty.
The Goldbach partition function is the function that associates to each even integer the number of ways it can be decomposed into a sum of two primes. Its graph looks like a comet and is therefore called Goldbach's comet.
Goldbach's comet suggests tight upper and lower bounds on the number of representations of an even number as the sum of two primes, and also that the number of these representations depend strongly on the value modulo 3 of the number. | Goldbach's conjecture | Wikipedia | 450 | 44758 | https://en.wikipedia.org/wiki/Goldbach%27s%20conjecture | Mathematics | Sums and products | null |
Related problems
Although Goldbach's conjecture implies that every positive integer greater than one can be written as a sum of at most three primes, it is not always possible to find such a sum using a greedy algorithm that uses the largest possible prime at each step. The Pillai sequence tracks the numbers requiring the largest number of primes in their greedy representations.
Similar problems to Goldbach's conjecture exist in which primes are replaced by other particular sets of numbers, such as the squares:
It was proven by Lagrange that every positive integer is the sum of four squares. See Waring's problem and the related Waring–Goldbach problem on sums of powers of primes.
Hardy and Littlewood listed as their Conjecture I: "Every large odd number () is the sum of a prime and the double of a prime". This conjecture is known as Lemoine's conjecture and is also called Levy's conjecture.
The Goldbach conjecture for practical numbers, a prime-like sequence of integers, was stated by Margenstern in 1984, and proved by Melfi in 1996: every even number is a sum of two practical numbers.
Harvey Dubner proposed a strengthening of the Goldbach conjecture that states that every even integer greater than 4208 is the sum of two twin primes (not necessarily belonging to the same pair). Only 34 even integers less than 4208 are not the sum of two twin primes; Dubner has verified computationally that this list is complete up to A proof of this stronger conjecture would not only imply Goldbach's conjecture, but also the twin prime conjecture.
According to Bertrand's postulate, for every integer , there is always at least one prime such that If the postulate were false, there would exist some integer for which no prime numbers lie between and , making it impossible to express as a sum of two primes. | Goldbach's conjecture | Wikipedia | 383 | 44758 | https://en.wikipedia.org/wiki/Goldbach%27s%20conjecture | Mathematics | Sums and products | null |
Goldbach's conjecture is used when studying computation complexity. The connection is made through the Busy Beaver function, where BB(n) is the maximum number of steps taken by any n state Turing machine that halts. There is a 27-state Turing machine that halts if and only if Goldbach's conjecture is false. Hence if BB(27) was known, and the Turing machine did not stop in that number of steps, it would be known to run forever and hence no counterexamples exist (which proves the conjecture true). This is a completely impractical way to settle the conjecture; instead it is used to suggest that BB(27) will be very hard to compute, at least as difficult as settling the Goldbach conjecture. | Goldbach's conjecture | Wikipedia | 154 | 44758 | https://en.wikipedia.org/wiki/Goldbach%27s%20conjecture | Mathematics | Sums and products | null |
Luminosity is an absolute measure of radiated electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a star, galaxy, or other astronomical objects.
In SI units, luminosity is measured in joules per second, or watts. In astronomy, values for luminosity are often given in the terms of the luminosity of the Sun, L⊙. Luminosity can also be given in terms of the astronomical magnitude system: the absolute bolometric magnitude (Mbol) of an object is a logarithmic measure of its total energy emission rate, while absolute magnitude is a logarithmic measure of the luminosity within some specific wavelength range or filter band.
In contrast, the term brightness in astronomy is generally used to refer to an object's apparent brightness: that is, how bright an object appears to an observer. Apparent brightness depends on both the luminosity of the object and the distance between the object and observer, and also on any absorption of light along the path from object to observer. Apparent magnitude is a logarithmic measure of apparent brightness. The distance determined by luminosity measures can be somewhat ambiguous, and is thus sometimes called the luminosity distance.
Measurement
When not qualified, the term "luminosity" means bolometric luminosity, which is measured either in the SI units, watts, or in terms of solar luminosities (). A bolometer is the instrument used to measure radiant energy over a wide band by absorption and measurement of heating. A star also radiates neutrinos, which carry off some energy (about 2% in the case of the Sun), contributing to the star's total luminosity. The IAU has defined a nominal solar luminosity of to promote publication of consistent and comparable values in units of the solar luminosity. | Luminosity | Wikipedia | 411 | 44790 | https://en.wikipedia.org/wiki/Luminosity | Physical sciences | Observational astronomy | null |
While bolometers do exist, they cannot be used to measure even the apparent brightness of a star because they are insufficiently sensitive across the electromagnetic spectrum and because most wavelengths do not reach the surface of the Earth. In practice bolometric magnitudes are measured by taking measurements at certain wavelengths and constructing a model of the total spectrum that is most likely to match those measurements. In some cases, the process of estimation is extreme, with luminosities being calculated when less than 1% of the energy output is observed, for example with a hot Wolf-Rayet star observed only in the infrared. Bolometric luminosities can also be calculated using a bolometric correction to a luminosity in a particular passband.
The term luminosity is also used in relation to particular passbands such as a visual luminosity of K-band luminosity. These are not generally luminosities in the strict sense of an absolute measure of radiated power, but absolute magnitudes defined for a given filter in a photometric system. Several different photometric systems exist. Some such as the UBV or Johnson system are defined against photometric standard stars, while others such as the AB system are defined in terms of a spectral flux density.
Stellar luminosity
A star's luminosity can be determined from two stellar characteristics: size and effective temperature. The former is typically represented in terms of solar radii, R⊙, while the latter is represented in kelvins, but in most cases neither can be measured directly. To determine a star's radius, two other metrics are needed: the star's angular diameter and its distance from Earth. Both can be measured with great accuracy in certain cases, with cool supergiants often having large angular diameters, and some cool evolved stars having masers in their atmospheres that can be used to measure the parallax using VLBI. However, for most stars the angular diameter or parallax, or both, are far below our ability to measure with any certainty. Since the effective temperature is merely a number that represents the temperature of a black body that would reproduce the luminosity, it obviously cannot be measured directly, but it can be estimated from the spectrum. | Luminosity | Wikipedia | 462 | 44790 | https://en.wikipedia.org/wiki/Luminosity | Physical sciences | Observational astronomy | null |
An alternative way to measure stellar luminosity is to measure the star's apparent brightness and distance. A third component needed to derive the luminosity is the degree of interstellar extinction that is present, a condition that usually arises because of gas and dust present in the interstellar medium (ISM), the Earth's atmosphere, and circumstellar matter. Consequently, one of astronomy's central challenges in determining a star's luminosity is to derive accurate measurements for each of these components, without which an accurate luminosity figure remains elusive. Extinction can only be measured directly if the actual and observed luminosities are both known, but it can be estimated from the observed colour of a star, using models of the expected level of reddening from the interstellar medium.
In the current system of stellar classification, stars are grouped according to temperature, with the massive, very young and energetic Class O stars boasting temperatures in excess of 30,000 K while the less massive, typically older Class M stars exhibit temperatures less than 3,500 K. Because luminosity is proportional to temperature to the fourth power, the large variation in stellar temperatures produces an even vaster variation in stellar luminosity. Because the luminosity depends on a high power of the stellar mass, high mass luminous stars have much shorter lifetimes. The most luminous stars are always young stars, no more than a few million years for the most extreme. In the Hertzsprung–Russell diagram, the x-axis represents temperature or spectral type while the y-axis represents luminosity or magnitude. The vast majority of stars are found along the main sequence with blue Class O stars found at the top left of the chart while red Class M stars fall to the bottom right. Certain stars like Deneb and Betelgeuse are found above and to the right of the main sequence, more luminous or cooler than their equivalents on the main sequence. Increased luminosity at the same temperature, or alternatively cooler temperature at the same luminosity, indicates that these stars are larger than those on the main sequence and they are called giants or supergiants. | Luminosity | Wikipedia | 442 | 44790 | https://en.wikipedia.org/wiki/Luminosity | Physical sciences | Observational astronomy | null |
Blue and white supergiants are high luminosity stars somewhat cooler than the most luminous main sequence stars. A star like Deneb, for example, has a luminosity around 200,000 L⊙, a spectral type of A2, and an effective temperature around 8,500 K, meaning it has a radius around . For comparison, the red supergiant Betelgeuse has a luminosity around 100,000 L⊙, a spectral type of M2, and a temperature around 3,500 K, meaning its radius is about . Red supergiants are the largest type of star, but the most luminous are much smaller and hotter, with temperatures up to 50,000 K and more and luminosities of several million L⊙, meaning their radii are just a few tens of R⊙. For example, R136a1 has a temperature over 46,000 K and a luminosity of more than 6,100,000 L⊙ (mostly in the UV), it is only .
Radio luminosity
The luminosity of a radio source is measured in , to avoid having to specify a bandwidth over which it is measured. The observed strength, or flux density, of a radio source is measured in Jansky where .
For example, consider a 10W transmitter at a distance of 1 million metres, radiating over a bandwidth of 1 MHz. By the time that power has reached the observer, the power is spread over the surface of a sphere with area or about , so its flux density is .
More generally, for sources at cosmological distances, a k-correction must be made for the spectral index α of the source, and a relativistic correction must be made for the fact that the frequency scale in the emitted rest frame is different from that in the observer's rest frame. So the full expression for radio luminosity, assuming isotropic emission, is
where Lν is the luminosity in , Sobs is the observed flux density in , DL is the luminosity distance in metres, z is the redshift, α is the spectral index (in the sense , and in radio astronomy, assuming thermal emission the spectral index is typically equal to 2.) | Luminosity | Wikipedia | 447 | 44790 | https://en.wikipedia.org/wiki/Luminosity | Physical sciences | Observational astronomy | null |
For example, consider a 1 Jy signal from a radio source at a redshift of 1, at a frequency of 1.4 GHz.
Ned Wright's cosmology calculator calculates a luminosity distance for a redshift of 1 to be 6701 Mpc = 2×1026 m giving a radio luminosity of .
To calculate the total radio power, this luminosity must be integrated over the bandwidth of the emission. A common assumption is to set the bandwidth to the observing frequency, which effectively assumes the power radiated has uniform intensity from zero frequency up to the observing frequency. In the case above, the total power is . This is sometimes expressed in terms of the total (i.e. integrated over all wavelengths) luminosity of the Sun which is , giving a radio power of .
Luminosity formulae
The Stefan–Boltzmann equation applied to a black body gives the value for luminosity for a black body, an idealized object which is perfectly opaque and non-reflecting:
where A is the surface area, T is the temperature (in kelvins) and is the Stefan–Boltzmann constant, with a value of
Imagine a point source of light of luminosity that radiates equally in all directions. A hollow sphere centered on the point would have its entire interior surface illuminated. As the radius increases, the surface area will also increase, and the constant luminosity has more surface area to illuminate, leading to a decrease in observed brightness.
where
is the area of the illuminated surface.
is the flux density of the illuminated surface.
The surface area of a sphere with radius r is , so for stars and other point sources of light:
where is the distance from the observer to the light source.
For stars on the main sequence, luminosity is also related to mass approximately as below:
Relationship to magnitude | Luminosity | Wikipedia | 384 | 44790 | https://en.wikipedia.org/wiki/Luminosity | Physical sciences | Observational astronomy | null |
Luminosity is an intrinsic measurable property of a star independent of distance. The concept of magnitude, on the other hand, incorporates distance. The apparent magnitude is a measure of the diminishing flux of light as a result of distance according to the inverse-square law. The Pogson logarithmic scale is used to measure both apparent and absolute magnitudes, the latter corresponding to the brightness of a star or other celestial body as seen if it would be located at an interstellar distance of . In addition to this brightness decrease from increased distance, there is an extra decrease of brightness due to extinction from intervening interstellar dust.
By measuring the width of certain absorption lines in the stellar spectrum, it is often possible to assign a certain luminosity class to a star without knowing its distance. Thus a fair measure of its absolute magnitude can be determined without knowing its distance nor the interstellar extinction.
In measuring star brightnesses, absolute magnitude, apparent magnitude, and distance are interrelated parameters—if two are known, the third can be determined. Since the Sun's luminosity is the standard, comparing these parameters with the Sun's apparent magnitude and distance is the easiest way to remember how to convert between them, although officially, zero point values are defined by the IAU.
The magnitude of a star, a unitless measure, is a logarithmic scale of observed visible brightness. The apparent magnitude is the observed visible brightness from Earth which depends on the distance of the object. The absolute magnitude is the apparent magnitude at a distance of , therefore the bolometric absolute magnitude is a logarithmic measure of the bolometric luminosity.
The difference in bolometric magnitude between two objects is related to their luminosity ratio according to:
where:
is the bolometric magnitude of the first object
is the bolometric magnitude of the second object.
is the first object's bolometric luminosity
is the second object's bolometric luminosity
The zero point of the absolute magnitude scale is actually defined as a fixed luminosity of . Therefore, the absolute magnitude can be calculated from a luminosity in watts:
where is the zero point luminosity
and the luminosity in watts can be calculated from an absolute magnitude (although absolute magnitudes are often not measured relative to an absolute flux): | Luminosity | Wikipedia | 488 | 44790 | https://en.wikipedia.org/wiki/Luminosity | Physical sciences | Observational astronomy | null |
Welding is a fabrication process that joins materials, usually metals or thermoplastics, primarily by using high temperature to melt the parts together and allow them to cool, causing fusion. Common alternative methods include solvent welding (of thermoplastics) using chemicals to melt materials being bonded without heat, and solid-state welding processes which bond without melting, such as pressure, cold welding, and diffusion bonding.
Metal welding is distinct from lower temperature bonding techniques such as brazing and soldering, which do not melt the base metal (parent metal) and instead require flowing a filler metal to solidify their bonds.
In addition to melting the base metal in welding, a filler material is typically added to the joint to form a pool of molten material (the weld pool) that cools to form a joint that can be stronger than the base material. Welding also requires a form of shield to protect the filler metals or melted metals from being contaminated or oxidized.
Many different energy sources can be used for welding, including a gas flame (chemical), an electric arc (electrical), a laser, an electron beam, friction, and ultrasound. While often an industrial process, welding may be performed in many different environments, including in open air, under water, and in outer space. Welding is a hazardous undertaking and precautions are required to avoid burns, electric shock, vision damage, inhalation of poisonous gases and fumes, and exposure to intense ultraviolet radiation.
Until the end of the 19th century, the only welding process was forge welding, which blacksmiths had used for millennia to join iron and steel by heating and hammering. Arc welding and oxy-fuel welding were among the first processes to develop late in the century, and electric resistance welding followed soon after. Welding technology advanced quickly during the early 20th century, as world wars drove the demand for reliable and inexpensive joining methods. Following the wars, several modern welding techniques were developed, including manual methods like shielded metal arc welding, now one of the most popular welding methods, as well as semi-automatic and automatic processes such as gas metal arc welding, submerged arc welding, flux-cored arc welding and electroslag welding. Developments continued with the invention of laser beam welding, electron beam welding, magnetic pulse welding, and friction stir welding in the latter half of the century. Today, as the science continues to advance, robot welding is commonplace in industrial settings, and researchers continue to develop new welding methods and gain greater understanding of weld quality. | Welding | Wikipedia | 508 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
Etymology
The term weld is derived from the Middle English verb well (; plural/present tense: ) or welling (), meaning 'to heat' (to the maximum temperature possible); 'to bring to a boil'. The modern word was probably derived from the past-tense participle welled (), with the addition of d for this purpose being common in the Germanic languages of the Angles and Saxons. It was first recorded in English in 1590. A fourteenth century translation of the Christian Bible into English by John Wycliffe translates Isaiah 2:4 as "" (they shall beat together their swords into plowshares). In the 1590 version this was changed to "" (they shall weld together their swords into plowshares), suggesting this particular use of the word probably became popular in English sometime between these periods.
The Old English word for welding iron was ('to bring together') or ('to bring together hot').
The word is related to the Old Swedish word , meaning 'to boil', which could refer to joining metals, as in (literally 'to boil iron'). Sweden was a large exporter of iron during the Middle Ages, so the word may have entered English from the Swedish iron trade, or may have been imported with the thousands of Viking settlements that arrived in England before and during the Viking Age, as more than half of the most common English words in everyday use are Scandinavian in origin.
History
The history of joining metals goes back several millennia. The earliest examples of this come from the Bronze and Iron Ages in Europe and the Middle East. The ancient Greek historian Herodotus states in The Histories of the 5th century BC that Glaucus of Chios "was the man who single-handedly invented iron welding". Forge welding was used in the construction of the Iron pillar of Delhi, erected in Delhi, India about 310 AD and weighing 5.4 metric tons.
The Middle Ages brought advances in forge welding, in which blacksmiths pounded heated metal repeatedly until bonding occurred. In 1540, Vannoccio Biringuccio published De la pirotechnia, which includes descriptions of the forging operation. Renaissance craftsmen were skilled in the process, and the industry continued to grow during the following centuries. | Welding | Wikipedia | 467 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
In 1800, Sir Humphry Davy discovered the short-pulse electrical arc and presented his results in 1801. In 1802, Russian scientist Vasily Petrov created the continuous electric arc, and subsequently published "News of Galvanic-Voltaic Experiments" in 1803, in which he described experiments carried out in 1802. Of great importance in this work was the description of a stable arc discharge and the indication of its possible use for many applications, one being melting metals. In 1808, Davy, who was unaware of Petrov's work, rediscovered the continuous electric arc. In 1881–82 inventors Nikolai Benardos (Russian) and Stanisław Olszewski (Polish) created the first electric arc welding method known as carbon arc welding using carbon electrodes. The advances in arc welding continued with the invention of metal electrodes in the late 1800s by a Russian, Nikolai Slavyanov (1888), and an American, C. L. Coffin (1890). Around 1900, A. P. Strohmenger released a coated metal electrode in Britain, which gave a more stable arc. In 1905, Russian scientist Vladimir Mitkevich proposed using a three-phase electric arc for welding. Alternating current welding was invented by C. J. Holslag in 1919, but did not become popular for another decade.
Resistance welding was also developed during the final decades of the 19th century, with the first patents going to Elihu Thomson in 1885, who produced further advances over the next 15 years. Thermite welding was invented in 1893, and around that time another process, oxyfuel welding, became well established. Acetylene was discovered in 1836 by Edmund Davy, but its use was not practical in welding until about 1900, when a suitable torch was developed. At first, oxyfuel welding was one of the more popular welding methods due to its portability and relatively low cost. As the 20th century progressed, however, it fell out of favor for industrial applications. It was largely replaced with arc welding, as advances in metal coverings (known as flux) were made. Flux covering the electrode primarily shields the base material from impurities, but also stabilizes the arc and can add alloying components to the weld metal. | Welding | Wikipedia | 457 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
World War I caused a major surge in the use of welding, with the various military powers attempting to determine which of the several new welding processes would be best. The British primarily used arc welding, even constructing a ship, the "Fullagar" with an entirely welded hull. Arc welding was first applied to aircraft during the war as well, as some German airplane fuselages were constructed using the process. Also noteworthy is the first welded road bridge in the world, the Maurzyce Bridge in Poland (1928).
During the 1920s, significant advances were made in welding technology, including the introduction of automatic welding in 1920, in which electrode wire was fed continuously. Shielding gas became a subject receiving much attention, as scientists attempted to protect welds from the effects of oxygen and nitrogen in the atmosphere. Porosity and brittleness were the primary problems, and the solutions that developed included the use of hydrogen, argon, and helium as welding atmospheres. During the following decade, further advances allowed for the welding of reactive metals like aluminum and magnesium. This in conjunction with developments in automatic welding, alternating current, and fluxes fed a major expansion of arc welding during the 1930s and then during World War II. In 1930, the first all-welded merchant vessel, M/S Carolinian, was launched.
During the middle of the century, many new welding methods were invented. In 1930, Kyle Taylor was responsible for the release of stud welding, which soon became popular in shipbuilding and construction. Submerged arc welding was invented the same year and continues to be popular today. In 1932 a Russian, Konstantin Khrenov eventually implemented the first underwater electric arc welding. Gas tungsten arc welding, after decades of development, was finally perfected in 1941, and gas metal arc welding followed in 1948, allowing for fast welding of non-ferrous materials but requiring expensive shielding gases. Shielded metal arc welding was developed during the 1950s, using a flux-coated consumable electrode, and it quickly became the most popular metal arc welding process. In 1957, the flux-cored arc welding process debuted, in which the self-shielded wire electrode could be used with automatic equipment, resulting in greatly increased welding speeds, and that same year, plasma arc welding was invented by Robert Gage. Electroslag welding was introduced in 1958, and it was followed by its cousin, electrogas welding, in 1961. In 1953, the Soviet scientist N. F. Kazakov proposed the diffusion bonding method. | Welding | Wikipedia | 502 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
Other recent developments in welding include the 1958 breakthrough of electron beam welding, making deep and narrow welding possible through the concentrated heat source. Following the invention of the laser in 1960, laser beam welding debuted several decades later, and has proved to be especially useful in high-speed, automated welding. Magnetic pulse welding (MPW) has been industrially used since 1967. Friction stir welding was invented in 1991 by Wayne Thomas at The Welding Institute (TWI, UK) and found high-quality applications all over the world. All of these four new processes continue to be quite expensive due to the high cost of the necessary equipment, and this has limited their applications.
Processes
Welding joins two pieces of metal using heat, pressure, or both. The most common modern welding methods use heat sufficient to melt the base metals to be joined and the filler metal. This includes gas welding and all forms of arc welding. The area where the base and filler metals melt is called the weld pool or puddle. Most welding methods involve pushing the puddle along a joint to create a weld bead. Overlapping pieces of metal can be joined by forming the weld pool within a hole made in the topmost piece of base metal. This is called a plug weld. Overlapping base metals are commonly joined using electric resistance welding, a process that combines heat and pressure and does not require a filler metal. Solid-state welding processes join two pieces of metal using pressure.
Gas welding
The most common gas welding process is oxyfuel welding, also known as oxyacetylene welding. It is one of the oldest and most versatile welding processes, but in recent years it has become less popular in industrial applications. It is still widely used for welding pipes and tubes, as well as repair work.
The equipment is relatively inexpensive and simple, generally employing the combustion of acetylene in oxygen to produce a welding flame temperature of about 3100 °C (5600 °F). The flame, since it is less concentrated than an electric arc, causes slower weld cooling, which can lead to greater residual stresses and weld distortion, though it eases the welding of high alloy steels. A similar process, generally called oxyfuel cutting, is used to cut metals.
Arc welding | Welding | Wikipedia | 457 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
These processes use a welding power supply to create and maintain an electric arc between an electrode and the base material to melt metals at the welding point. They can use either direct current (DC) or alternating current (AC), and consumable or non-consumable electrodes. The welding region is sometimes protected by some type of inert or semi-inert gas, known as a shielding gas, and filler material is sometimes used as well.
Arc welding processes
One of the most common types of arc welding is shielded metal arc welding (SMAW); it is also known as manual metal arc welding (MMAW) or stick welding. Electric current is used to strike an arc between the base material and consumable electrode rod, which is made of filler material (typical steel) and is covered with a flux that protects the weld area from oxidation and contamination by producing carbon dioxide (CO2) gas during the welding process. The electrode core itself acts as filler material, making a separate filler unnecessary.
The process is versatile and can be performed with relatively inexpensive equipment, making it well suited to shop jobs and field work. An operator can become reasonably proficient with a modest amount of training and can achieve mastery with experience. Weld times are rather slow, since the consumable electrodes must be frequently replaced and because slag, the residue from the flux, must be chipped away after welding. Furthermore, the process is generally limited to welding ferrous materials, though special electrodes have made possible the welding of cast iron, stainless steel, aluminum, and other metals.
Gas metal arc welding (GMAW), also known as metal inert gas or MIG welding, is a semi-automatic or automatic process that uses a continuous wire feed as an electrode and an inert or semi-inert gas mixture to protect the weld from contamination. Since the electrode is continuous, welding speeds are greater for GMAW than for SMAW.
A related process, flux-cored arc welding (FCAW), uses similar equipment but uses wire consisting of a steel electrode surrounding a powder fill material. This cored wire is more expensive than the standard solid wire and can generate fumes and/or slag, but it permits even higher welding speed and greater metal penetration. | Welding | Wikipedia | 466 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
Gas tungsten arc welding (GTAW), or tungsten inert gas (TIG) welding, is a manual welding process that uses a non-consumable tungsten electrode, an inert or semi-inert gas mixture, and a separate filler material. Especially useful for welding thin materials, this method is characterized by a stable arc and high-quality welds, but it requires significant operator skill and can only be accomplished at relatively low speeds.
GTAW can be used on nearly all weldable metals, though it is most often applied to stainless steel and light metals. It is often used when quality welds are extremely important, such as in bicycle, aircraft and naval applications. A related process, plasma arc welding, also uses a tungsten electrode but uses plasma gas to make the arc. The arc is more concentrated than the GTAW arc, making transverse control more critical and thus generally restricting the technique to a mechanized process. Because of its stable current, the method can be used on a wider range of material thicknesses than can the GTAW process and it is much faster. It can be applied to all of the same materials as GTAW except magnesium, and automated welding of stainless steel is one important application of the process. A variation of the process is plasma cutting, an efficient steel cutting process.
Submerged arc welding (SAW) is a high-productivity welding method in which the arc is struck beneath a covering layer of flux. This increases arc quality since contaminants in the atmosphere are blocked by the flux. The slag that forms on the weld generally comes off by itself, and combined with the use of a continuous wire feed, the weld deposition rate is high. Working conditions are much improved over other arc welding processes, since the flux hides the arc and almost no smoke is produced. The process is commonly used in industry, especially for large products and in the manufacture of welded pressure vessels. Other arc welding processes include atomic hydrogen welding, electroslag welding (ESW), electrogas welding, and stud arc welding. ESW is a highly productive, single-pass welding process for thicker materials between 1 inch (25 mm) and 12 inches (300 mm) in a vertical or close to vertical position. | Welding | Wikipedia | 459 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
Arc welding power supplies
To supply the electrical power necessary for arc welding processes, a variety of different power supplies can be used. The most common welding power supplies are constant current power supplies and constant voltage power supplies. In arc welding, the length of the arc is directly related to the voltage, and the amount of heat input is related to the current. Constant current power supplies are most often used for manual welding processes such as gas tungsten arc welding and shielded metal arc welding, because they maintain a relatively constant current even as the voltage varies. This is important because in manual welding, it can be difficult to hold the electrode perfectly steady, and as a result, the arc length and thus voltage tend to fluctuate. Constant voltage power supplies hold the voltage constant and vary the current, and as a result, are most often used for automated welding processes such as gas metal arc welding, flux-cored arc welding, and submerged arc welding. In these processes, arc length is kept constant, since any fluctuation in the distance between the wire and the base material is quickly rectified by a large change in current. For example, if the wire and the base material get too close, the current will rapidly increase, which in turn causes the heat to increase and the tip of the wire to melt, returning it to its original separation distance. | Welding | Wikipedia | 271 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
The type of current used plays an important role in arc welding. Consumable electrode processes such as shielded metal arc welding and gas metal arc welding generally use direct current, but the electrode can be charged either positively or negatively. In welding, the positively charged anode will have a greater heat concentration, and as a result, changing the polarity of the electrode affects weld properties. If the electrode is positively charged, the base metal will be hotter, increasing weld penetration and welding speed. Alternatively, a negatively charged electrode results in more shallow welds. Non-consumable electrode processes, such as gas tungsten arc welding, can use either type of direct current, as well as alternating current. However, with direct current, because the electrode only creates the arc and does not provide filler material, a positively charged electrode causes shallow welds, while a negatively charged electrode makes deeper welds. Alternating current rapidly moves between these two, resulting in medium-penetration welds. One disadvantage of AC, the fact that the arc must be re-ignited after every zero crossings, has been addressed with the invention of special power units that produce a square wave pattern instead of the normal sine wave, making rapid zero crossings possible and minimizing the effects of the problem.
Resistance welding
Resistance welding involves the generation of heat by passing current through the resistance caused by the contact between two or more metal surfaces. Small pools of molten metal are formed at the weld area as high current (1,000–100,000 A) is passed through the metal. In general, resistance welding methods are efficient and cause little pollution, but their applications are somewhat limited and the equipment cost can be high.
Resistance spot welding is a popular method used to join overlapping metal sheets of up to 3 mm thick. Two electrodes are simultaneously used to clamp the metal sheets together and to pass current through the sheets. The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. Weld strength is significantly lower than with other welding methods, making the process suitable for only certain applications. It is used extensively in the automotive industry—ordinary cars can have several thousand spot welds made by industrial robots. A specialized process called shot welding, can be used to spot weld stainless steel. | Welding | Wikipedia | 477 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
Seam welding also relies on two electrodes to apply pressure and current to join metal sheets. However, instead of pointed electrodes, wheel-shaped electrodes roll along and often feed the workpiece, making it possible to make long continuous welds. In the past, this process was used in the manufacture of beverage cans, but now its uses are more limited. Other resistance welding methods include butt welding, flash welding, projection welding, and upset welding.
Energy beam welding
Energy beam welding methods, namely laser beam welding and electron beam welding, are relatively new processes that have become quite popular in high production applications. The two processes are quite similar, differing most notably in their source of power. Laser beam welding employs a highly focused laser beam, while electron beam welding is done in a vacuum and uses an electron beam. Both have a very high energy density, making deep weld penetration possible and minimizing the size of the weld area. Both processes are extremely fast, and are easily automated, making them highly productive. The primary disadvantages are their very high equipment costs (though these are decreasing) and a susceptibility to thermal cracking. Developments in this area include laser-hybrid welding, which uses principles from both laser beam welding and arc welding for even better weld properties, laser cladding, and x-ray welding.
Solid-state welding
Like forge welding (the earliest welding process discovered), some modern welding methods do not involve the melting of the materials being joined. One of the most popular, ultrasonic welding, is used to connect thin sheets or wires made of metal or thermoplastic by vibrating them at high frequency and under high pressure. The equipment and methods involved are similar to that of resistance welding, but instead of electric current, vibration provides energy input. When welding metals, the vibrations are introduced horizontally, and the materials are not melted; with plastics, which should have similar melting temperatures, vertically. Ultrasonic welding is commonly used for making electrical connections out of aluminum or copper, and it is also a very common polymer welding process.
Another common process, explosion welding, involves the joining of materials by pushing them together under extremely high pressure. The energy from the impact plasticizes the materials, forming a weld, even though only a limited amount of heat is generated. The process is commonly used for welding dissimilar materials, including bonding aluminum to carbon steel in ship hulls and stainless steel or titanium to carbon steel in petrochemical pressure vessels. | Welding | Wikipedia | 502 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
Other solid-state welding processes include friction welding (including friction stir welding and friction stir spot welding), magnetic pulse welding, co-extrusion welding, cold welding, diffusion bonding, exothermic welding, high frequency welding, hot pressure welding, induction welding, and roll bonding.
Geometry
Welds can be geometrically prepared in many different ways. The five basic types of weld joints are the butt joint, lap joint, corner joint, edge joint, and T-joint (a variant of this last is the cruciform joint). Other variations exist as well—for example, double-V preparation joints are characterized by the two pieces of material each tapering to a single center point at one-half their height. Single-U and double-U preparation joints are also fairly common—instead of having straight edges like the single-V and double-V preparation joints, they are curved, forming the shape of a U. Lap joints are also commonly more than two pieces thick—depending on the process used and the thickness of the material, many pieces can be welded together in a lap joint geometry.
Many welding processes require the use of a particular joint design; for example, resistance spot welding, laser beam welding, and electron beam welding are most frequently performed on lap joints. Other welding methods, like shielded metal arc welding, are extremely versatile and can weld virtually any type of joint. Some processes can also be used to make multipass welds, in which one weld is allowed to cool, and then another weld is performed on top of it. This allows for the welding of thick sections arranged in a single-V preparation joint, for example.
After welding, a number of distinct regions can be identified in the weld area. The weld itself is called the fusion zone—more specifically, it is where the filler metal was laid during the welding process. The properties of the fusion zone depend primarily on the filler metal used, and its compatibility with the base materials. It is surrounded by the heat-affected zone, the area that had its microstructure and properties altered by the weld. These properties depend on the base material's behavior when subjected to heat. The metal in this area is often weaker than both the base material and the fusion zone, and is also where residual stresses are found.
Quality | Welding | Wikipedia | 478 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
Many distinct factors influence the strength of welds and the material around them, including the welding method, the amount and concentration of energy input, the weldability of the base material, filler material, and flux material, the design of the joint, and the interactions between all these factors.
For example, the factor of welding position influences weld quality, that welding codes & specifications may require testing—both welding procedures and welders—using specified welding positions: 1G (flat), 2G (horizontal), 3G (vertical), 4G (overhead), 5G (horizontal fixed pipe), or 6G (inclined fixed pipe).
To test the quality of a weld, either destructive or nondestructive testing methods are commonly used to verify that welds are free of defects, have acceptable levels of residual stresses and distortion, and have acceptable heat-affected zone (HAZ) properties. Types of welding defects include cracks, distortion, gas inclusions (porosity), non-metallic inclusions, lack of fusion, incomplete penetration, lamellar tearing, and undercutting.
The metalworking industry has instituted codes and specifications to guide welders, weld inspectors, engineers, managers, and property owners in proper welding technique, design of welds, how to judge the quality of welding procedure specification, how to judge the skill of the person performing the weld, and how to ensure the quality of a welding job. Methods such as visual inspection, radiography, ultrasonic testing, phased-array ultrasonics, dye penetrant inspection, magnetic particle inspection, or industrial computed tomography can help with detection and analysis of certain defects. | Welding | Wikipedia | 345 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
Heat-affected zone
The heat-affected zone (HAZ) is a ring surrounding the weld in which the temperature of the welding process, combined with the stresses of uneven heating and cooling, alters the heat-treatment properties of the alloy. The effects of welding on the material surrounding the weld can be detrimental—depending on the materials used and the heat input of the welding process used, the HAZ can be of varying size and strength. The thermal diffusivity of the base material plays a large role—if the diffusivity is high, the material cooling rate is high and the HAZ is relatively small. Conversely, a low diffusivity leads to slower cooling and a larger HAZ. The amount of heat injected by the welding process plays an important role as well, as processes like oxyacetylene welding have an unconcentrated heat input and increase the size of the HAZ. Processes like laser beam welding give a highly concentrated, limited amount of heat, resulting in a small HAZ. Arc welding falls between these two extremes, with the individual processes varying somewhat in heat input. To calculate the heat input for arc welding procedures, the following formula can be used:
where Q = heat input (kJ/mm), V = voltage (V), I = current (A), and S = welding speed (mm/min). The efficiency is dependent on the welding process used, with shielded metal arc welding having a value of 0.75, gas metal arc welding and submerged arc welding, 0.9, and gas tungsten arc welding, 0.8. Methods of alleviating the stresses and brittleness created in the HAZ include stress relieving and tempering.
One major defect concerning the HAZ would be cracking at the toes , due to the rapid expansion (heating) and contraction (cooling) the material may not have the ability to withstand the stress and could cause cracking, one method the control these stress would be to control the heating and cooling rate, such as pre-heating and post- heating
Lifetime extension with after treatment methods
The durability and life of dynamically loaded, welded steel structures is determined in many cases by the welds, in particular the weld transitions. Through selective treatment of the transitions by grinding (abrasive cutting), shot peening, High-frequency impact treatment, Ultrasonic impact treatment, etc. the durability of many designs increases significantly.
Metallurgy | Welding | Wikipedia | 505 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
Most solids used are engineering materials consisting of crystalline solids in which the atoms or ions are arranged in a repetitive geometric pattern which is known as a lattice structure. The only exception is material that is made from glass which is a combination of a supercooled liquid and polymers which are aggregates of large organic molecules.
Crystalline solids cohesion is obtained by a metallic or chemical bond that is formed between the constituent atoms. Chemical bonds can be grouped into two types consisting of ionic and covalent. To form an ionic bond, either a valence or bonding electron separates from one atom and becomes attached to another atom to form oppositely charged ions. The bonding in the static position is when the ions occupy an equilibrium position where the resulting force between them is zero. When the ions are exerted in tension force, the inter-ionic spacing increases creating an electrostatic attractive force, while a repulsing force under compressive force between the atomic nuclei is dominant.
Covalent bonding takes place when one of the constituent atoms loses one or more electrons, with the other atom gaining the electrons, resulting in an electron cloud that is shared by the molecule as a whole. In both ionic and covalent bonding the location of the ions and electrons are constrained relative to each other, thereby resulting in the bond being characteristically brittle.
Metallic bonding can be classified as a type of covalent bonding for which the constituent atoms are of the same type and do not combine with one another to form a chemical bond. Atoms will lose an electron(s) forming an array of positive ions. These electrons are shared by the lattice which makes the electron cluster mobile, as the electrons are free to move as well as the ions. For this, it gives metals their relatively high thermal and electrical conductivity as well as being characteristically ductile.
Three of the most commonly used crystal lattice structures in metals are the body-centred cubic, face-centred cubic and close-packed hexagonal. Ferritic steel has a body-centred cubic structure and austenitic steel, non-ferrous metals like aluminium, copper and nickel have the face-centred cubic structure.
Ductility is an important factor in ensuring the integrity of structures by enabling them to sustain local stress concentrations without fracture. In addition, structures are required to be of an acceptable strength, which is related to a material's yield strength. In general, as the yield strength of a material increases, there is a corresponding reduction in fracture toughness. | Welding | Wikipedia | 498 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
A reduction in fracture toughness may also be attributed to the embrittlement effect of impurities, or for body-centred cubic metals, from a reduction in temperature. Metals and in particular steels have a transitional temperature range where above this range the metal has acceptable notch-ductility while below this range the material becomes brittle. Within the range, the materials behavior is unpredictable. The reduction in fracture toughness is accompanied by a change in the fracture appearance. When above the transition, the fracture is primarily due to micro-void coalescence, which results in the fracture appearing fibrous. When the temperatures falls the fracture will show signs of cleavage facets. These two appearances are visible by the naked eye. Brittle fracture in steel plates may appear as chevron markings under the microscope. These arrow-like ridges on the crack surface point towards the origin of the fracture.
Fracture toughness is measured using a notched and pre-cracked rectangular specimen, of which the dimensions are specified in standards, for example ASTM E23. There are other means of estimating or measuring fracture toughness by the following: The Charpy impact test per ASTM A370; The crack-tip opening displacement (CTOD) test per BS 7448–1; The J integral test per ASTM E1820; The Pellini drop-weight test per ASTM E208.
Unusual conditions | Welding | Wikipedia | 282 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
While many welding applications are done in controlled environments such as factories and repair shops, some welding processes are commonly used in a wide variety of conditions, such as open air, underwater, and vacuums (such as space). In open-air applications, such as construction and outdoors repair, shielded metal arc welding is the most common process. Processes that employ inert gases to protect the weld cannot be readily used in such situations, because unpredictable atmospheric movements can result in a faulty weld. Shielded metal arc welding is also often used in underwater welding in the construction and repair of ships, offshore platforms, and pipelines, but others, such as flux cored arc welding and gas tungsten arc welding, are also common. Welding in space is also possible—it was first attempted in 1969 by Russian cosmonauts during the Soyuz 6 mission, when they performed experiments to test shielded metal arc welding, plasma arc welding, and electron beam welding in a depressurized environment. Further testing of these methods was done in the following decades, and today researchers continue to develop methods for using other welding processes in space, such as laser beam welding, resistance welding, and friction welding. Advances in these areas may be useful for future endeavours similar to the construction of the International Space Station, which could rely on welding for joining in space the parts that were manufactured on Earth.
Safety issues
Welding can be dangerous and unhealthy if the proper precautions are not taken. However, using new technology and proper protection greatly reduces risks of injury and death associated with welding. | Welding | Wikipedia | 313 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
Since many common welding procedures involve an open electric arc or flame, the risk of burns and fire is significant; this is why it is classified as a hot work process. To prevent injury, welders wear personal protective equipment in the form of heavy leather gloves and protective long-sleeve jackets to avoid exposure to extreme heat and flames. Synthetic clothing such as polyester should not be worn since it may burn, causing injury. Additionally, the brightness of the weld area leads to a condition called arc eye or flash burns in which ultraviolet light causes inflammation of the cornea and can burn the retinas of the eyes. Goggles and welding helmets with dark UV-filtering face plates are worn to prevent this exposure. Since the 2000s, some helmets have included a face plate which instantly darkens upon exposure to the intense UV light. To protect bystanders, the welding area is often surrounded with translucent welding curtains. These curtains, made of a polyvinyl chloride plastic film, shield people outside the welding area from the UV light of the electric arc, but cannot replace the filter glass used in helmets. Depending on the type of material, welding varieties, and other factors, welding can produce over 100 dB(A) of noise. Long term or continuous exposure to higher decibels can lead to noise-induced hearing loss.
Welders are often exposed to dangerous gases and particulate matter. Processes like flux-cored arc welding and shielded metal arc welding produce smoke containing particles of various types of oxides. The size of the particles in question tends to influence the toxicity of the fumes, with smaller particles presenting a greater danger. This is because smaller particles have the ability to cross the blood–brain barrier. Fumes and gases, such as carbon dioxide, ozone, and fumes containing heavy metals, can be dangerous to welders lacking proper ventilation and training. Exposure to manganese welding fumes, for example, even at low levels (<0.2 mg/m3), may lead to neurological problems or to damage to the lungs, liver, kidneys, or central nervous system. Nano particles can become trapped in the alveolar macrophages of the lungs and induce pulmonary fibrosis. The use of compressed gases and flames in many, welding processes poses an explosion and fire risk. Some common precautions include limiting the amount of oxygen in the air, and keeping combustible materials away from the workplace. | Welding | Wikipedia | 491 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
Costs and trends
As an industrial process, the cost of welding plays a crucial role in manufacturing decisions. Many different variables affect the total cost, including equipment cost, labor cost, material cost, and energy cost. Depending on the process, equipment cost can vary, from inexpensive for methods like shielded metal arc welding and oxyfuel welding, to extremely expensive for methods like laser beam welding and electron beam welding. Because of their high cost, they are only used in high production operations. Similarly, because automation and robots increase equipment costs, they are only implemented when high production is necessary. Labor cost depends on the deposition rate (the rate of welding), the hourly wage, and the total operation time, including time spent fitting, welding, and handling the part. The cost of materials includes the cost of the base and filler material, and the cost of shielding gases. Finally, energy cost depends on arc time and welding power demand.
For manual welding methods, labor costs generally make up the vast majority of the total cost. As a result, many cost-saving measures are focused on minimizing operation time. To do this, welding procedures with high deposition rates can be selected, and weld parameters can be fine-tuned to increase welding speed. Mechanization and automation are often implemented to reduce labor costs, but this frequently increases the cost of equipment and creates additional setup time. Material costs tend to increase when special properties are necessary, and energy costs normally do not amount to more than several percent of the total welding cost.
In recent years, in order to minimize labor costs in high production manufacturing, industrial welding has become increasingly more automated, most notably with the use of robots in resistance spot welding (especially in the automotive industry) and in arc welding. In robot welding, mechanized devices both hold the material and perform the weld and at first, spot welding was its most common application, but robotic arc welding increases in popularity as technology advances. Other key areas of research and development include the welding of dissimilar materials (such as steel and aluminum, for example) and new welding processes, such as friction stir, magnetic pulse, conductive heat seam, and laser-hybrid welding. Furthermore, progress is desired in making more specialized methods like laser beam welding practical for more applications, such as in the aerospace and automotive industries. Researchers also hope to better understand the often unpredictable properties of welds, especially microstructure, residual stresses, and a weld's tendency to crack or deform. | Welding | Wikipedia | 508 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
The trend of accelerating the speed at which welds are performed in the steel erection industry comes at a risk to the integrity of the connection. Without proper fusion to the base materials provided by sufficient arc time on the weld, a project inspector cannot ensure the effective diameter of the puddle weld therefore he or she cannot guarantee the published load capacities unless they witness the actual installation. This method of puddle welding is common in the United States and Canada for attaching steel sheets to bar joist and structural steel members. Regional agencies are responsible for ensuring the proper installation of puddle welding on steel construction sites. Currently there is no standard or weld procedure which can ensure the published holding capacity of any unwitnessed connection, but this is under review by the American Welding Society.
Glass and plastic welding
Glasses and certain types of plastics are commonly welded materials. Unlike metals, which have a specific melting point, glasses and plastics have a melting range, called the glass transition. When heating the solid material past the glass-transition temperature (Tg) into this range, it will generally become softer and more pliable. When it crosses through the range, above the glass-melting temperature (Tm), it will become a very thick, sluggish, viscous liquid, slowly decreasing in viscosity as temperature increases. Typically, this viscous liquid will have very little surface tension compared to metals, becoming a sticky, taffy to honey-like consistency, so welding can usually take place by simply pressing two melted surfaces together. The two liquids will generally mix and join at first contact. Upon cooling through the glass transition, the welded piece will solidify as one solid piece of amorphous material.
Glass welding | Welding | Wikipedia | 344 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
Glass welding is a common practice during glassblowing. It is used very often in the construction of lighting, neon signs, flashtubes, scientific equipment, and the manufacture of dishes and other glassware. It is also used during glass casting for joining the halves of glass molds, making items such as bottles and jars. Welding glass is accomplished by heating the glass through the glass transition, turning it into a thick, formable, liquid mass. Heating is usually done with a gas or oxy-gas torch, or a furnace, because the temperatures for melting glass are often quite high. This temperature may vary, depending on the type of glass. For example, lead glass becomes a weldable liquid at around , and can be welded with a simple propane torch. On the other hand, quartz glass (fused silica) must be heated to over , but quickly loses its viscosity and formability if overheated, so an oxyhydrogen torch must be used. Sometimes a tube may be attached to the glass, allowing it to be blown into various shapes, such as bulbs, bottles, or tubes. When two pieces of liquid glass are pressed together, they will usually weld very readily. Welding a handle onto a pitcher can usually be done with relative ease. However, when welding a tube to another tube, a combination of blowing and suction, and pressing and pulling is used to ensure a good seal, to shape the glass, and to keep the surface tension from closing the tube in on itself. Sometimes a filler rod may be used, but usually not.
Because glass is very brittle in its solid state, it is often prone to cracking upon heating and cooling, especially if the heating and cooling are uneven. This is because the brittleness of glass does not allow for uneven thermal expansion. Glass that has been welded will usually need to be cooled very slowly and evenly through the glass transition, in a process called annealing, to relieve any internal stresses created by a temperature gradient. | Welding | Wikipedia | 408 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
There are many types of glass, and it is most common to weld using the same types. Different glasses often have different rates of thermal expansion, which can cause them to crack upon cooling when they contract differently. For instance, quartz has very low thermal expansion, while soda-lime glass has very high thermal expansion. When welding different glasses to each other, it is usually important to closely match their coefficients of thermal expansion, to ensure that cracking does not occur. Also, some glasses will simply not mix with others, so welding between certain types may not be possible.
Glass can also be welded to metals and ceramics, although with metals the process is usually more adhesion to the surface of the metal rather than a commingling of the two materials. However, certain glasses will typically bond only to certain metals. For example, lead glass bonds readily to copper or molybdenum, but not to aluminum. Tungsten electrodes are often used in lighting but will not bond to quartz glass, so the tungsten is often wetted with molten borosilicate glass, which bonds to both tungsten and quartz. However, care must be taken to ensure that all materials have similar coefficients of thermal expansion to prevent cracking both when the object cools and when it is heated again. Special alloys are often used for this purpose, ensuring that the coefficients of expansion match, and sometimes thin, metallic coatings may be applied to a metal to create a good bond with the glass.
Plastic welding
Plastics are generally divided into two categories, which are "thermosets" and "thermoplastics." A thermoset is a plastic in which a chemical reaction sets the molecular bonds after first forming the plastic, and then the bonds cannot be broken again without degrading the plastic. Thermosets cannot be melted, therefore, once a thermoset has set it is impossible to weld it. Examples of thermosets include epoxies, silicone, vulcanized rubber, polyester, and polyurethane. | Welding | Wikipedia | 416 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
Thermoplastics, by contrast, form long molecular chains, which are often coiled or intertwined, forming an amorphous structure without any long-range, crystalline order. Some thermoplastics may be fully amorphous, while others have a partially crystalline/partially amorphous structure. Both amorphous and semicrystalline thermoplastics have a glass transition, above which welding can occur, but semicrystallines also have a specific melting point which is above the glass transition. Above this melting point, the viscous liquid will become a free-flowing liquid (see rheological weldability for thermoplastics). Examples of thermoplastics include polyethylene, polypropylene, polystyrene, polyvinylchloride (PVC), and fluoroplastics like Teflon and Spectralon.
Welding thermoplastic with heat is very similar to welding glass. The plastic first must be cleaned and then heated through the glass transition, turning the weld-interface into a thick, viscous liquid. Two heated interfaces can then be pressed together, allowing the molecules to mix through intermolecular diffusion, joining them as one. Then the plastic is cooled through the glass transition, allowing the weld to solidify. A filler rod may often be used for certain types of joints. The main differences between welding glass and plastic are the types of heating methods, the much lower melting temperatures, and the fact that plastics will burn if overheated. Many different methods have been devised for heating plastic to a weldable temperature without burning it. Ovens or electric heating tools can be used to melt the plastic. Ultrasonic, laser, or friction heating are other methods. Resistive metals may be implanted in the plastic, which respond to induction heating. Some plastics will begin to burn at temperatures lower than their glass transition, so welding can be performed by blowing a heated, inert gas onto the plastic, melting it while, at the same time, shielding it from oxygen.
Solvent welding | Welding | Wikipedia | 430 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
Many thermoplastics can also be welded using chemical solvents. When placed in contact with the plastic, the solvent will begin to soften it, bringing the surface into a thick, liquid solution. When two melted surfaces are pressed together, the molecules in the solution mix, joining them as one. Because the solvent can permeate the plastic, the solvent evaporates out through the surface of the plastic, causing the weld to drop out of solution and solidify. A common use for solvent welding is for joining PVC (polyvinyl chloride) or ABS (acrylonitrile butadiene styrene) pipes during plumbing, or for welding styrene and polystyrene plastics in the construction of models. Solvent welding is especially effective on plastics like PVC which burn at or below their glass transition, but may be ineffective on plastics like Teflon or polyethylene that are resistant to chemical decomposition. | Welding | Wikipedia | 193 | 44883 | https://en.wikipedia.org/wiki/Welding | Technology | Metallurgy | null |
Asthma is a common long-term inflammatory disease of the airways of the lungs. Asthma occurs when allergens, pollen, dust, or other particles, are inhaled into the lungs, causing the bronchioles to constrict and produce mucus, which then restricts oxygen flow to the alveoli. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wheezing, coughing, chest tightness, and shortness of breath. These may occur a few times a day or a few times per week. Depending on the person, asthma symptoms may become worse at night or with exercise.
Asthma is thought to be caused by a combination of genetic and environmental factors. Environmental factors include exposure to air pollution and allergens. Other potential triggers include medications such as aspirin and beta blockers. Diagnosis is usually based on the pattern of symptoms, response to therapy over time, and spirometry lung function testing. Asthma is classified according to the frequency of symptoms of forced expiratory volume in one second (FEV1), and peak expiratory flow rate. It may also be classified as atopic or non-atopic, where atopy refers to a predisposition toward developing a type 1 hypersensitivity reaction.
There is no known cure for asthma, but it can be controlled. Symptoms can be prevented by avoiding triggers, such as allergens and respiratory irritants, and suppressed with the use of inhaled corticosteroids. Long-acting beta agonists (LABA) or antileukotriene agents may be used in addition to inhaled corticosteroids if asthma symptoms remain uncontrolled. Treatment of rapidly worsening symptoms is usually with an inhaled short-acting beta2 agonist such as salbutamol and corticosteroids taken by mouth. In very severe cases, intravenous corticosteroids, magnesium sulfate, and hospitalization may be required.
In 2019 asthma affected approximately 262 million people and caused approximately 461,000 deaths. Most of the deaths occurred in the developing world. Asthma often begins in childhood, and the rates have increased significantly since the 1960s. Asthma was recognized as early as Ancient Egypt. The word asthma is from the Greek , , which means 'panting'.
Signs and symptoms | Asthma | Wikipedia | 489 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Asthma is characterized by recurrent episodes of wheezing, shortness of breath, chest tightness, and coughing. Sputum may be produced from the lung by coughing but is often hard to bring up. During recovery from an asthma attack (exacerbation), the sputum may appear pus-like due to high levels of white blood cells called eosinophils. Symptoms are usually worse at night and in the early morning or in response to exercise or cold air. Some people with asthma rarely experience symptoms, usually in response to triggers, whereas others may react frequently and readily and experience persistent symptoms.
Associated conditions
A number of other health conditions occur more frequently in people with asthma, including gastroesophageal reflux disease (GERD), rhinosinusitis, and obstructive sleep apnea. Psychological disorders are also more common, with anxiety disorders occurring in between 16 and 52% and mood disorders in 14–41%. It is not known whether asthma causes psychological problems or psychological problems lead to asthma. Current asthma, but not former asthma, is associated with increased all-cause mortality, heart disease mortality, and chronic lower respiratory tract disease mortality. Asthma, particularly severe asthma, is strongly associated with development of chronic obstructive pulmonary disease (COPD). Those with asthma, especially if it is poorly controlled, are at increased risk for radiocontrast reactions.
Cavities occur more often in people with asthma. This may be related to the effect of beta2-adrenergic agonists decreasing saliva. These medications may also increase the risk of dental erosions.
Causes
Asthma is caused by a combination of complex and incompletely understood environmental and genetic interactions. These influence both its severity and its responsiveness to treatment. It is believed that the recent increased rates of asthma are due to changing epigenetics (heritable factors other than those related to the DNA sequence) and a changing living environment. Asthma that starts before the age of 12 years old is more likely due to genetic influence, while onset after age 12 is more likely due to environmental influence.
Environmental | Asthma | Wikipedia | 434 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Many environmental factors have been associated with asthma's development and exacerbation, including allergens, air pollution, and other environmental chemicals. There are some substances that are known to cause asthma in exposed people and they are called asthmagens. Some common asthmagens include ammonia, latex, pesticides, solder and welding fumes, metal or wood dusts, spraying of isocyanate paint in vehicle repair, formaldehyde, glutaraldehyde, anhydrides, glues, dyes, metal working fluids, oil mists, moulds. Smoking during pregnancy and after delivery is associated with a greater risk of asthma-like symptoms. Low air quality from environmental factors such as traffic pollution or high ozone levels has been associated with both asthma development and increased asthma severity. Over half of cases in children in the United States occur in areas when air quality is below the EPA standards. Low air quality is more common in low-income and minority communities.
Exposure to indoor volatile organic compounds may be a trigger for asthma; formaldehyde exposure, for example, has a positive association. Phthalates in certain types of PVC are associated with asthma in both children and adults. While exposure to pesticides is linked to the development of asthma, a cause and effect relationship has yet to be established. A meta-analysis concluded gas stoves are a major risk factor for asthma, finding around one in eight cases in the U.S. could be attributed to these.
The majority of the evidence does not support a causal role between paracetamol (acetaminophen) or antibiotic use and asthma. A 2014 systematic review found that the association between paracetamol use and asthma disappeared when respiratory infections were taken into account. Maternal psychological stress during pregnancy is a risk factor for the child to develop asthma.
Asthma is associated with exposure to indoor allergens. Common indoor allergens include dust mites, cockroaches, animal dander (fragments of fur or feathers), and mould. Efforts to decrease dust mites have been found to be ineffective on symptoms in sensitized subjects. Weak evidence suggests that efforts to decrease mould by repairing buildings may help improve asthma symptoms in adults. Certain viral respiratory infections, such as respiratory syncytial virus and rhinovirus, may increase the risk of developing asthma when acquired as young children. Certain other infections, however, may decrease the risk. | Asthma | Wikipedia | 496 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Hygiene hypothesis
The hygiene hypothesis attempts to explain the increased rates of asthma worldwide as a direct and unintended result of reduced exposure, during childhood, to non-pathogenic bacteria and viruses. It has been proposed that the reduced exposure to bacteria and viruses is due, in part, to increased cleanliness and decreased family size in modern societies. Exposure to bacterial endotoxin in early childhood may prevent the development of asthma, but exposure at an older age may provoke bronchoconstriction. Evidence supporting the hygiene hypothesis includes lower rates of asthma on farms and in households with pets.
Use of antibiotics in early life has been linked to the development of asthma. Also, delivery via caesarean section is associated with an increased risk (estimated at 20–80%) of asthma – this increased risk is attributed to the lack of healthy bacterial colonization that the newborn would have acquired from passage through the birth canal. There is a link between asthma and the degree of affluence which may be related to the hygiene hypothesis as less affluent individuals often have more exposure to bacteria and viruses.
Genetic
Family history is a risk factor for asthma, with many different genes being implicated. If one identical twin is affected, the probability of the other having the disease is approximately 25%. By the end of 2005, 25 genes had been associated with asthma in six or more separate populations, including GSTM1, IL10, CTLA-4, SPINK5, LTC4S, IL4R and ADAM33, among others. Many of these genes are related to the immune system or modulating inflammation. Even among this list of genes supported by highly replicated studies, results have not been consistent among all populations tested. In 2006 over 100 genes were associated with asthma in one genetic association study alone; more continue to be found.
Some genetic variants may only cause asthma when they are combined with specific environmental exposures. An example is a specific single nucleotide polymorphism in the CD14 region and exposure to endotoxin (a bacterial product). Endotoxin exposure can come from several environmental sources including tobacco smoke, dogs, and farms. Risk for asthma, then, is determined by both a person's genetics and the level of endotoxin exposure. | Asthma | Wikipedia | 453 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Medical conditions
A triad of atopic eczema, allergic rhinitis and asthma is called atopy. The strongest risk factor for developing asthma is a history of atopic disease; with asthma occurring at a much greater rate in those who have either eczema or hay fever. Asthma has been associated with eosinophilic granulomatosis with polyangiitis (formerly known as Churg–Strauss syndrome), an autoimmune disease and vasculitis. Individuals with certain types of urticaria may also experience symptoms of asthma.
There is a correlation between obesity and the risk of asthma with both having increased in recent years. Several factors may be at play including decreased respiratory function due to a buildup of fat and the fact that adipose tissue leads to a pro-inflammatory state.
Beta blocker medications such as propranolol can trigger asthma in those who are susceptible. Cardioselective beta-blockers, however, appear safe in those with mild or moderate disease. Other medications that can cause problems in asthmatics are angiotensin-converting enzyme inhibitors, aspirin, and NSAIDs. Use of acid-suppressing medication (proton pump inhibitors and H2 blockers) during pregnancy is associated with an increased risk of asthma in the child.
Exacerbation
Some individuals will have stable asthma for weeks or months and then suddenly develop an episode of acute asthma. Different individuals react to various factors in different ways. Most individuals can develop severe exacerbation from a number of triggering agents.
Home factors that can lead to exacerbation of asthma include dust, animal dander (especially cat and dog hair), cockroach allergens and mold. Perfumes are a common cause of acute attacks in women and children. Both viral and bacterial infections of the upper respiratory tract can worsen the disease. Psychological stress may worsen symptoms – it is thought that stress alters the immune system and thus increases the airway inflammatory response to allergens and irritants.
Asthma exacerbations in school-aged children peak in autumn, shortly after children return to school. This might reflect a combination of factors, including poor treatment adherence, increased allergen and viral exposure, and altered immune tolerance. There is limited evidence to guide possible approaches to reducing autumn exacerbations, but while costly, seasonal omalizumab treatment from four to six weeks before school return may reduce autumn asthma exacerbations.
Pathophysiology | Asthma | Wikipedia | 508 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Asthma is the result of chronic inflammation of the conducting zone of the airways (most especially the bronchi and bronchioles), which subsequently results in increased contractability of the surrounding smooth muscles. This among other factors leads to bouts of narrowing of the airway and the classic symptoms of wheezing. The narrowing is typically reversible with or without treatment. Occasionally the airways themselves change. Typical changes in the airways include an increase in eosinophils and thickening of the lamina reticularis. Chronically the airways' smooth muscle may increase in size along with an increase in the numbers of mucous glands. Other cell types involved include T lymphocytes, macrophages, and neutrophils. There may also be involvement of other components of the immune system, including cytokines, chemokines, histamine, and leukotrienes among others.
Diagnosis
While asthma is a well-recognized condition, there is not one universal agreed-upon definition. It is defined by the Global Initiative for Asthma as "a chronic inflammatory disorder of the airways in which many cells and cellular elements play a role. The chronic inflammation is associated with airway hyper-responsiveness that leads to recurrent episodes of wheezing, breathlessness, chest tightness and coughing particularly at night or in the early morning. These episodes are usually associated with widespread but variable airflow obstruction within the lung that is often reversible either spontaneously or with treatment".
There is currently no precise test for the diagnosis, which is typically based on the pattern of symptoms and response to therapy over time. Asthma may be suspected if there is a history of recurrent wheezing, coughing or difficulty breathing and these symptoms occur or worsen due to exercise, viral infections, allergens or air pollution. Spirometry is then used to confirm the diagnosis. In children under the age of six the diagnosis is more difficult as they are too young for spirometry. | Asthma | Wikipedia | 405 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Spirometry
Spirometry is recommended to aid in diagnosis and management. It is the single best test for asthma. If the FEV1 measured by this technique improves more than 12% and increases by at least 200 millilitres following administration of a bronchodilator such as salbutamol, this is supportive of the diagnosis. It however may be normal in those with a history of mild asthma, not currently acting up. As caffeine is a bronchodilator in people with asthma, the use of caffeine before a lung function test may interfere with the results. Single-breath diffusing capacity can help differentiate asthma from COPD. It is reasonable to perform spirometry every one or two years to follow how well a person's asthma is controlled.
Others
The methacholine challenge involves the inhalation of increasing concentrations of a substance that causes airway narrowing in those predisposed. If negative it means that a person does not have asthma; if positive, however, it is not specific for the disease.
Other supportive evidence includes: a ≥20% difference in peak expiratory flow rate on at least three days in a week for at least two weeks, a ≥20% improvement of peak flow following treatment with either salbutamol, inhaled corticosteroids or prednisone, or a ≥20% decrease in peak flow following exposure to a trigger. Testing peak expiratory flow is more variable than spirometry, however, and thus not recommended for routine diagnosis. It may be useful for daily self-monitoring in those with moderate to severe disease and for checking the effectiveness of new medications. It may also be helpful in guiding treatment in those with acute exacerbations.
Classification | Asthma | Wikipedia | 360 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Asthma is clinically classified according to the frequency of symptoms, forced expiratory volume in one second (FEV1), and peak expiratory flow rate. Asthma may also be classified as atopic (extrinsic) or non-atopic (intrinsic), based on whether symptoms are precipitated by allergens (atopic) or not (non-atopic). While asthma is classified based on severity, at the moment there is no clear method for classifying different subgroups of asthma beyond this system. Finding ways to identify subgroups that respond well to different types of treatments is a current critical goal of asthma research. Recently, asthma has been classified based on whether it is associated with type 2 or non–type 2 inflammation. This approach to immunologic classification is driven by a developing understanding of the underlying immune processes and by the development of therapeutic approaches that target type 2 inflammation.
Although asthma is a chronic obstructive condition, it is not considered as a part of chronic obstructive pulmonary disease, as this term refers specifically to combinations of disease that are irreversible such as bronchiectasis and emphysema. Unlike these diseases, the airway obstruction in asthma is usually reversible; however, if left untreated, the chronic inflammation from asthma can lead the lungs to become irreversibly obstructed due to airway remodelling. In contrast to emphysema, asthma affects the bronchi, not the alveoli. The combination of asthma with a component of irreversible airway obstruction has been termed the asthma-chronic obstructive disease (COPD) overlap syndrome (ACOS). Compared to other people with "pure" asthma or COPD, people with ACOS exhibit increased morbidity, mortality and possibly more comorbidities.
Asthma exacerbation
An acute asthma exacerbation is commonly referred to as an asthma attack. The classic symptoms are shortness of breath, wheezing, and chest tightness. The wheezing is most often when breathing out. While these are the primary symptoms of asthma, some people present primarily with coughing, and in severe cases, air motion may be significantly impaired such that no wheezing is heard. In children, chest pain is often present. | Asthma | Wikipedia | 483 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Signs occurring during an asthma attack include the use of accessory muscles of respiration (sternocleidomastoid and scalene muscles of the neck), there may be a paradoxical pulse (a pulse that is weaker during inhalation and stronger during exhalation), and over-inflation of the chest. A blue colour of the skin and nails may occur from lack of oxygen.
In a mild exacerbation the peak expiratory flow rate (PEFR) is ≥200 L/min, or ≥50% of the predicted best. Moderate is defined as between 80 and 200 L/min, or 25% and 50% of the predicted best, while severe is defined as ≤ 80 L/min, or ≤25% of the predicted best.
Acute severe asthma, previously known as status asthmaticus, is an acute exacerbation of asthma that does not respond to standard treatments of bronchodilators and corticosteroids. Half of cases are due to infections with others caused by allergen, air pollution, or insufficient or inappropriate medication use.
Brittle asthma is a kind of asthma distinguishable by recurrent, severe attacks. Type 1 brittle asthma is a disease with wide peak flow variability, despite intense medication. Type 2 brittle asthma is background well-controlled asthma with sudden severe exacerbations.
Exercise-induced
Exercise can trigger bronchoconstriction both in people with or without asthma. It occurs in most people with asthma and up to 20% of people without asthma. Exercise-induced bronchoconstriction is common in professional athletes. The highest rates are among cyclists (up to 45%), swimmers, and cross-country skiers. While it may occur with any weather conditions, it is more common when it is dry and cold. Inhaled beta2 agonists do not appear to improve athletic performance among those without asthma; however, oral doses may improve endurance and strength.
Occupational | Asthma | Wikipedia | 396 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Asthma as a result of (or worsened by) workplace exposures is a commonly reported occupational disease. Many cases, however, are not reported or recognized as such. It is estimated that 5–25% of asthma cases in adults are work-related. A few hundred different agents have been implicated, with the most common being isocyanates, grain and wood dust, colophony, soldering flux, latex, animals, and aldehydes. The employment associated with the highest risk of problems include those who spray paint, bakers and those who process food, nurses, chemical workers, those who work with animals, welders, hairdressers and timber workers.
Aspirin-exacerbated respiratory disease
Aspirin-exacerbated respiratory disease (AERD), also known as aspirin-induced asthma, affects up to 9% of asthmatics. AERD consists of asthma, nasal polyps, sinus disease, and respiratory reactions to aspirin and other NSAID medications (such as ibuprofen and naproxen). People often also develop loss of smell and most experience respiratory reactions to alcohol.
Alcohol-induced asthma
Alcohol may worsen asthmatic symptoms in up to a third of people. This may be even more common in some ethnic groups such as the Japanese and those with aspirin-exacerbated respiratory disease. Other studies have found improvement in asthmatic symptoms from alcohol. | Asthma | Wikipedia | 298 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Non-atopic asthma
Non-atopic asthma, also known as intrinsic or non-allergic, makes up between 10 and 33% of cases. There is negative skin test to common inhalant allergens. Often it starts later in life, and women are more commonly affected than men. Usual treatments may not work as well. The concept that "non-atopic" is synonymous with "non-allergic" is called into question by epidemiological data that the prevalence of asthma is closely related to the serum IgE level standardized for age and sex (P<0.0001), indicating that asthma is almost always associated with some sort of IgE-related reaction and therefore has an allergic basis, although not all the allergic stimuli that cause asthma appear to have been included in the battery of aeroallergens studied (the "missing antigen(s)" hypothesis). For example, an updated systematic review and meta-analysis of population-attributable risk (PAR) of Chlamydia pneumoniae biomarkers in chronic asthma found that the PAR for C. pneumoniae-specific IgE was 47%.
Infectious asthma
Infectious asthma is an easily identified clinical presentation. When queried, asthma patients may report that their first asthma symptoms began after an acute lower respiratory tract illness. This type of history has been labelled the "infectious asthma" (IA) syndrome, or as "asthma associated with infection" (AAWI) to distinguish infection-associated asthma initiation from the well known association of respiratory infections with asthma exacerbations. Reported clinical prevalences of IA for adults range from around 40% in a primary care practice to 70% in a speciality practice treating mainly severe asthma patients. Additional information on the clinical prevalence of IA in adult-onset asthma is unavailable because clinicians are not trained to elicit this type of history routinely, and recollection in child-onset asthma is challenging. A population-based incident case-control study in a geographically defined area of Finland reported that 35.8% of new-onset asthma cases had experienced acute bronchitis or pneumonia in the year preceding asthma onset, representing a significantly higher risk compared to randomly selected controls (odds ratio 7.2, 95% confidence interval 5.2–10).
Phenotyping and endotyping | Asthma | Wikipedia | 471 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Asthma phenotyping and endotyping has emerged as a novel approach to asthma classification inspired by precision medicine which separates the clinical presentations of asthma, or asthma phenotypes, from their underlying causes, or asthma endotypes. The best-supported endotypic distinction is the type 2-high/type 2-low distinction. Classification based on type 2 inflammation is useful in predicting which patients will benefit from targeted biologic therapy.
Differential diagnosis
Many other conditions can cause symptoms similar to those of asthma. In children, symptoms may be due to other upper airway diseases such as allergic rhinitis and sinusitis, as well as other causes of airway obstruction including foreign body aspiration, tracheal stenosis, laryngotracheomalacia, vascular rings, enlarged lymph nodes or neck masses. Bronchiolitis and other viral infections may also produce wheezing. According to European Respiratory Society, it may not be suitable to label wheezing preschool children with the term asthma because there is lack of clinical data on inflammation in airways. In adults, COPD, congestive heart failure, airway masses, as well as drug-induced coughing due to ACE inhibitors may cause similar symptoms. In both populations vocal cord dysfunction may present similarly.
Chronic obstructive pulmonary disease can coexist with asthma and can occur as a complication of chronic asthma. After the age of 65, most people with obstructive airway disease will have asthma and COPD. In this setting, COPD can be differentiated by increased airway neutrophils, abnormally increased wall thickness, and increased smooth muscle in the bronchi. However, this level of investigation is not performed due to COPD and asthma sharing similar principles of management: corticosteroids, long-acting beta-agonists, and smoking cessation. It closely resembles asthma in symptoms, is correlated with more exposure to cigarette smoke, an older age, less symptom reversibility after bronchodilator administration, and decreased likelihood of family history of atopy. | Asthma | Wikipedia | 431 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Prevention
The evidence for the effectiveness of measures to prevent the development of asthma is weak. The World Health Organization recommends decreasing risk factors such as tobacco smoke, air pollution, chemical irritants including perfume, and the number of lower respiratory infections. Other efforts that show promise include: limiting smoke exposure in utero, breastfeeding, and increased exposure to daycare or large families, but none are well supported enough to be recommended for this indication.
Early pet exposure may be useful. Results from exposure to pets at other times are inconclusive and it is only recommended that pets be removed from the home if a person has allergic symptoms to said pet.
Dietary restrictions during pregnancy or when breastfeeding have not been found to be effective at preventing asthma in children and are not recommended. Omega-3 consumption, Mediterranean diet and antioxidants have been suggested by some studies to potentially help prevent crises but the evidence is still inconclusive.
Reducing or eliminating compounds known to sensitive people from the workplace may be effective. It is not clear if annual influenza vaccinations affect the risk of exacerbations. Immunization, however, is recommended by the World Health Organization. Smoking bans are effective in decreasing exacerbations of asthma. | Asthma | Wikipedia | 251 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Management
While there is no cure for asthma, symptoms can typically be improved. The most effective treatment for asthma is identifying triggers, such as cigarette smoke, pets or other allergens, and eliminating exposure to them. If trigger avoidance is insufficient, the use of medication is recommended. Pharmaceutical drugs are selected based on, among other things, the severity of illness and the frequency of symptoms. Specific medications for asthma are broadly classified into fast-acting and long-acting categories. The medications listed below have demonstrated efficacy in improving asthma symptoms; however, real world use-effectiveness is limited as around half of people with asthma worldwide remain sub-optimally controlled, even when treated. People with asthma may remain sub-optimally controlled either because optimum doses of asthma medications do not work (called "refractory" asthma) or because individuals are either unable (e.g. inability to afford treatment, poor inhaler technique) or unwilling (e.g., wish to avoid side effects of corticosteroids) to take optimum doses of prescribed asthma medications (called "difficult to treat" asthma). In practice, it is not possible to distinguish "refractory" from "difficult to treat" categories for patients who have never taken optimum doses of asthma medications. A related issue is that the asthma efficacy trials upon which the pharmacological treatment guidelines are based have systematically excluded the majority of people with asthma. For example, asthma efficacy treatment trials always exclude otherwise eligible people who smoke, and smoking diminishes the efficacy of inhaled corticosteroids, the mainstay of asthma control management.
Bronchodilators are recommended for short-term relief of symptoms. In those with occasional attacks, no other medication is needed. If mild persistent disease is present (more than two attacks a week), low-dose inhaled corticosteroids or alternatively, a leukotriene antagonist or a mast cell stabilizer by mouth is recommended. For those who have daily attacks, a higher dose of inhaled corticosteroids is used. In a moderate or severe exacerbation, corticosteroids by mouth are added to these treatments.
People with asthma have higher rates of anxiety, psychological stress, and depression. This is associated with poorer asthma control. Cognitive behavioural therapy may improve quality of life, asthma control, and anxiety levels in people with asthma. | Asthma | Wikipedia | 488 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Improving people's knowledge about asthma and using a written action plan has been identified as an important component of managing asthma. Providing educational sessions that include information specific to a person's culture is likely effective. More research is necessary to determine if increasing preparedness and knowledge of asthma among school staff and families using home-based and school interventions results in long term improvements in safety for children with asthma. School-based asthma self-management interventions, which attempt to improve knowledge of asthma, its triggers and the importance of regular practitioner review, may reduce hospital admissions and emergency department visits. These interventions may also reduce the number of days children experience asthma symptoms and may lead to small improvements in asthma-related quality of life. More research is necessary to determine if shared decision-making is helpful for managing adults with asthma or if a personalized asthma action plan is effective and necessary. Some people with asthma use pulse oximeters to monitor their own blood oxygen levels during an asthma attack. However, there is no evidence regarding the use in these instances.
Lifestyle modification
Avoidance of triggers is a key component of improving control and preventing attacks. The most common triggers include allergens, smoke (from tobacco or other sources), air pollution, nonselective beta-blockers, and sulfite-containing foods. Cigarette smoking and second-hand smoke (passive smoke) may reduce the effectiveness of medications such as corticosteroids. Laws that limit smoking decrease the number of people hospitalized for asthma. Dust mite control measures, including air filtration, chemicals to kill mites, vacuuming, mattress covers and other methods had no effect on asthma symptoms. There is insufficient evidence to suggest that dehumidifiers are helpful for controlling asthma.
Overall, exercise is beneficial in people with stable asthma. Yoga could provide small improvements in quality of life and symptoms in people with asthma. More research is necessary to determine how effective weight loss is in improving quality of life, the usage of health care services, and adverse effects for people of all ages with asthma.
Findings suggest that the Wim Hof Method may reduce inflammation in healthy and non-healthy participants as it increases epinephrine levels, causing an increase in interleukin-10 and a decrease in pro-inflammatory cytokines. | Asthma | Wikipedia | 460 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Medications
Medications used to treat asthma are divided into two general classes: quick-relief medications used to treat acute symptoms; and long-term control medications used to prevent further exacerbation. Antibiotics are generally not needed for sudden worsening of symptoms or for treating asthma at any time.
Medications for asthma exacerbations | Asthma | Wikipedia | 64 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Short-acting beta2-adrenoceptor agonists (SABAs), such as salbutamol (albuterol USAN) are the first-line treatment for asthma symptoms. They are recommended before exercise in those with exercise-induced symptoms.
Anticholinergic medications, such as ipratropium, provide additional benefit when used in combination with SABA in those with moderate or severe symptoms and may prevent hospitalizations. Anticholinergic bronchodilators can also be used if a person cannot tolerate a SABA. If a child requires admission to hospital additional ipratropium does not appear to help over a SABA. For children over 2 years old with acute asthma symptoms, inhaled anticholinergic medications taken alone is safe but is not as effective as inhaled SABA or SABA combined with inhaled anticholinergic medication. Adults who receive combined inhaled medications, which include short-acting anticholinergics and SABA, may be at risk for increased adverse effects such as experiencing a tremor, agitation, and heart beat palpitations compared to people who are treated with SABAs alone.
Older, less selective adrenergic agonists, such as inhaled epinephrine, have similar efficacy to SABAs. They are, however, not recommended due to concerns regarding excessive cardiac stimulation.
Corticosteroids can also help with the acute phase of an exacerbation because of their antiinflammatory properties. The benefit of systemic and oral corticosteroids is well established. Inhaled or nebulized corticosteroids can also be used. For adults and children who are in the hospital due to acute asthma, systemic (IV) corticosteroids improve symptoms. A short course of corticosteroids after an acute asthma exacerbation may help prevent relapses and reduce hospitalizations.
Other remedies, less established, are intravenous or nebulized magnesium sulfate and helium mixed with oxygen. Aminophylline could be used with caution as well.
Mechanical ventilation is the last resort in case of severe hypoxemia.
Intravenous administration of the drug aminophylline does not provide an improvement in bronchodilation when compared to standard inhaled beta2 agonist treatment. Aminophylline treatment is associated with more adverse effects compared to inhaled beta2 agonist treatment.
Long–term control | Asthma | Wikipedia | 494 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Corticosteroids are generally considered the most effective treatment available for long-term control. Inhaled forms are usually used except in the case of severe persistent disease, in which oral corticosteroids may be needed. Dosage depends on the severity of symptoms. High dosage and long-term use might lead to the appearance of common adverse effects which are growth delay, adrenal suppression, and osteoporosis. Continuous (daily) use of an inhaled corticosteroid, rather than its intermitted use, seems to provide better results in controlling asthma exacerbations. Commonly used corticosteroids are budesonide, fluticasone, mometasone and ciclesonide.
Long-acting beta-adrenoceptor agonists (LABA) such as salmeterol and formoterol can improve asthma control, at least in adults, when given in combination with inhaled corticosteroids. In children this benefit is uncertain. When used without steroids they increase the risk of severe side-effects, and with corticosteroids they may slightly increase the risk. Evidence suggests that for children who have persistent asthma, a treatment regime that includes LABA added to inhaled corticosteroids may improve lung function but does not reduce the amount of serious exacerbations. Children who require LABA as part of their asthma treatment may need to go to the hospital more frequently. | Asthma | Wikipedia | 297 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Leukotriene receptor antagonists (anti-leukotriene agents such as montelukast and zafirlukast) may be used in addition to inhaled corticosteroids, typically also in conjunction with a LABA. For adults or adolescents who have persistent asthma that is not controlled very well, the addition of anti-leukotriene agents along with daily inhaled corticosteriods improves lung function and reduces the risk of moderate and severe asthma exacerbations. Anti-leukotriene agents may be effective alone for adolescents and adults; however, there is no clear research suggesting which people with asthma would benefit from anti-leukotriene receptor alone. In those under five years of age, anti-leukotriene agents were the preferred add-on therapy after inhaled corticosteroids. A 2013 Cochrane systematic review concluded that anti-leukotriene agents appear to be of little benefit when added to inhaled steroids for treating children. A similar class of drugs, 5-LOX inhibitors, may be used as an alternative in the chronic treatment of mild to moderate asthma among older children and adults. there is one medication in this family known as zileuton.
Mast cell stabilizers (such as cromolyn sodium) are safe alternatives to corticosteroids but not preferred because they have to be administered frequently.
Oral theophyllines are sometimes used for controlling chronic asthma, but their used is minimized due to side effects.
Omalizumab, a monoclonal antibody against IgE, is a novel way to lessen exacerbations by decreasing the levels of circulating IgE that play a significant role at allergic asthma.
Anticholinergic medications such as ipratropium bromide have not been shown to be beneficial for treating chronic asthma in children over 2 years old, and are not suggested for routine treatment of chronic asthma in adults.
There is no strong evidence to recommend chloroquine medication as a replacement for taking corticosteroids by mouth (for those who are not able to tolerate inhaled steroids). Methotrexate is not suggested as a replacement for taking corticosteriods by mouth ("steroid-sparing") due to the adverse effects associated with taking methotrexate and the minimal relief provided for asthma symptoms. | Asthma | Wikipedia | 490 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
Macrolide antibiotics, particularly the azalide macrolide azithromycin, are a recently added Global Initiative for Asthma (GINA)-recommended treatment option for both eosinophilic and non-eosinophilic severe, refractory asthma based on azithromycin's efficacy in reducing moderate and severe exacerbations combined. Azithromycin's mechanism of action is not established, and could involve pathogen- and/or host-directed anti-inflammatory activities. Limited clinical observations suggest that some patients with new-onset asthma and with "difficult-to-treat" asthma (including those with the asthma-COPD overlap syndrome – ACOS) may respond dramatically to azithromycin. However, these groups of asthma patients have not been studied in randomized treatment trials and patient selection needs to be carefully individualized.
A 2024 study indicates that commonly used diabetes medications may lower asthma attacks by up to 70%. The research examined metformin and GLP-1 drugs such as Ozempic (semaglutide), Mounjaro (tirzepatide), and Saxenda (liraglutide). Among nearly 13,000 participants with both diabetes and asthma, metformin reduced the risk of asthma attacks by 30%, with an additional 40% reduction when combined with a GLP-1 drug. | Asthma | Wikipedia | 286 | 44905 | https://en.wikipedia.org/wiki/Asthma | Biology and health sciences | Illness and injury | null |
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