Amalgam
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An amalgam is any mixture or blending of mercury with another metal or with an alloy. Most metals are soluble in mercury, but some (such as iron) are not. Amalgams are commonly used in dental fillings.
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[edit] Dentistry
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For some centuries, dentists have been cleaning out decay and creating dental fillings using filling material such as stone chips, resin, cork, turpentine, gum, lead and gold leaf. The renowned physician Ambroise Paré (1510 – 1590) used lead or cork to fill teeth. Amalgams were the first true standard filling material.
Mercury (Hg) amalgams were used in dentistry because they were cheap, easy to use, durable, and regarded as safe. Modern low-copper amalgams have a powder component composed of 69.4% silver, 3.6% copper, 26.2% tin, and 0.8% zinc; and they have a liquid component of 42% to 45% mercury by weight[citation needed]. The amalgam remains soft for a short time so it can be packed to fill any irregular volume, and then forms a hard compound. There is an ongoing discussion about the use of mercury in dentistry due to the toxic content of amalgams. Amalgam fillings have been shown to increase mercury blood levels. Even though mercury is considered to be a very potent neurotoxin, mercury fillings are still considered safe by most dentists.[1]
Others, though, have quite a different take on mercury amalgam in dental fillings and contend that its safety is anything but certain. Says toxicologist Alan Stern, a contributor to a 2000 United States National Research Council report on mercury toxicity, "It’s really unclear what’s going on with dental amalgams...It’s a snake pit". He also notes that "the issue is complicated by the potential for panic and lawsuits" [2]. IAOMT, the International Academy of Oral Medicine and Toxicology, has put together a report which is "a review of the scientific evidence demonstrating significant exposure to mercury and resulting physiological harm from dental amalgam" [3]. See also Dental amalgam controversy.
In response to growing studies and pressure to investigate the issue further, the FDA held hearings September 6-7, 2006 on the safety of dental amalgam in children. Experts on both sides of the issue testified. The result was that the outside advisors the FDA had asked to review its prior reports declaring amalgam to be safe rejected those findings [4] stating that further study is needed.
The first people to use amalgam to fill cavities were the Chinese in the 7th century.[2] In 1816, Auguste Taveau developed a dental amalgam from silver coins and mercury. This early amalgam was low in mercury and had to be heated in order for the silver to dissolve at any appreciable rate. More modern dental amalgams are mixed cold. Current dental amalgams contain copper to eliminate the gamma-2 phase of the silver-mercury-tin alloy. The gamma-2 phase is weaker than the other phases, so a high-copper, low-gamma-2 dental amalgam has superior strength.
[edit] Dental amalgam controversy
The Dental amalgam controversy is a debate over the use of amalgams, which contain mercury, as a dental filling. A minority of dentists has always been opposed to the use of amalgam in dental fillings, since they first began to be used over 150 years ago.
During the years 1833 to 1850, two French brothers by the name Crawcours introduced the amalgam in the United States as dental filling material. According to the ADA, the brothers were charlatans with unscrupulous methods which sparked the “amalgam wars” over the use of amalgams as dental fillings. The American Society of Dental Surgeons (ASDS) was formed in 1840. Many of the members disapproved of the use of amalgams, and the ASDS was dissolved in 1856. Later, in 1859, 26 dentists formed the American Dental Association when they met in Niagara Falls, according to the ADA’s official website. The ADA claims that dental amalgam is safe since it has been used for over 160 years by now.
[edit] Chemical analysis
Mercury is the preferred electrode material for the analysis of metals by anodic stripping voltammetry. The formation of amalgams facilitates the reduction of most metal ions in aqueous solutions that is normally not possible because their reduction potentials are more negative than the potential for the reduction of the solution.
[edit] Use in organic chemistry
Formation of amalgams is used to increase the reactivity of metals. In the Grignard reaction, amalgamation of the magnesium makes the reaction more facile. In the Barbier reaction and other reactions using aluminum, it is absolutely necessary, as the surface oxide coating normally makes aluminum inert.
[edit] Mining
Mercury has been used in the gold and silver mining processes due to the ease with which mercury will amalgamate with them. In gold placer mining, in which small particles of gold are washed from sand or gravel deposits, mercury was often used to separate the gold from other heavy minerals.
After all of the usable metal had been extracted from the ore, mercury was poured down a long copper trough which formed a thin coating of mercury on the surface. The waste ore was then poured down the trough, and any gold in the waste amalgamated with the mercury. This coating was occasionally scraped off and distilled to remove the mercury, leaving behind fairly high-purity gold.
Mercury amalgamation was first applied to silver ores with the invention of the patio process in Mexico in 1557. Other amalgamation processes were invented for processing silver ores, including pan amalgamation and the Washoe process.
With the invention of mercury amalgamation to treat silver ore, mercury became essential to the silver mines of the New World. The Spanish Empire transported mercury from Almadén across the Atlantic to supply the silver mines of Zacatecas and Potosí. Another source for mercury in the Spanish Empire was the mine of Huancavelica in Peru, discovered in 1563. In 1648, the Viceroy of Peru declared that Potosí and Huancavelica were "the two pillars that support this kingdom and that of Spain."[3]
Today, mercury amalgamation has been replaced by other methods to recover gold and silver from ore. Dangers of mercury pollution have played a part in the near-disappearance of mercury amalgamation processes. Mercury amalgamation is still commonly used by small-scale gold placer miners, especially in less-developed countries, most notably Brazil.
[edit] Other uses
Thallium amalgam is used as liquid for thermometers, because it freezes at -58°C[citation needed], whereas pure mercury freezes at -38°C.