Adipocere or grave wax or mortuary wax is the water-insoluble material consisting mostly of saturated fatty acids. It is formed by the slow hydrolysis of fats in decomposing material such as a human cadaver by action of anaerobic bacteria. The transformation of fats into adipocere occurs best in the absence of oxygen in cold and humid environment, such as in wet ground or mud at the bottom of a lake or a sealed cask, and it can occur with both embalmed and untreated bodies. Corpes of infants and overweight persons are particularly prone to adipocere transformation. Adipocere formation begins within a month of death, and in the absence of air it can persist for centuries. An exposed, infested body or a body in a warm environment is unlikely to form deposits of adipocere. The process of adipocere formation is also known as saponification.

It is generally believed to have first been discovered by the Frenchman Fourcroy in the 18th century; however, Sir Thomas Browne describes this substance in his discourse, Hydriotaphia, Urn Burial of 1658:

"In a Hydropicall body ten years buried in a Church-yard, we met with a fat concretion, where the nitre of the Earth, and the salt and lixivious liquor of the body, had coagulated large lumps of fat, into the consistence of the hardest castle-soap: wherof part remaineth with us."

In essence, in this process the usual dissolution of putrefaction is replaced by a permanent firm cast of fatty tissues and even internal organs and face. This allows some estimation of body shape and facial features, and injuries are often well-preserved.

The Mütter Museum possesses the Soap Lady, the body of an extremely obese woman, which was almost entirely saponified.

Sources

• J.S.Finch. A Doctor's life of Science and Faith. Princeton 1950
• C.A.Patrides, ed. Sir Thomas Browne The Major Works. Penguin 1977