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Chemical Properties

Calomel is a powder that is white when pure. When it is exposed to light or contains impurities it takes on a darker tint. Calomel is made up of mercury and chloride with the chemical formula Hg2Cl2. Depending on how calomel was administered, it affected the body in different ways. Taken orally, calomel damaged mainly the lining of the gastrointestinal tract. Mercury salts (such as calomel) are insoluble in water and therefore do not absorb well through the wall of the small intestine. Some of the calomel in the digestive system will likely be oxidized into a former of mercury that can be absorbed through the intestine, but most of it will not. Oral calomel was actually the safest form of the drug to take, especially in low doses. Most of the calomel ingested will be excreted through urine and stool.

Powdered forms of calomel were much more toxic, as their vapors damaged the brain. Once inhaled, the calomel enters the bloodstream and the mercury binds with the amino acids methionine, cysteine, homocysteine and taurine. This is because of the sulfur group these amino acids contain, which mercury has a high affinity for. It is able to pass through the blood brain barrier and builds up in the brain. Mercury also has the ability to pass through the placenta, causing damage to unborn babies if a pregnant mother is taking calomel.

Calomel was manufactured in two ways - sublimation and precipitation. When calomel first started being manufactured it was done through sublimation. Calomel made through sublimation tends to be a very fine white powder. There was some controversy over the sublimation of calomel. Many argued that the more times calomel was sublimed, the purer it got. Opponents believed that the repeated sublimation made calomel lose some of its therapeutic ability. In 1788 pharmacist Carl Wilhelm Scheele came up with the mechanism to make precipitated calomel. This became rapidly popular in the pharmaceutical industry because it was both a cheaper and safer form of production. Precipitation also tended to form very pure calomel salts.