Adipocere

Adipocere (/ˈædᵻpəˌsɪər, -poʊ-/), also known as corpse, grave or mortuary wax, is a wax-like organic substance formed by the anaerobic bacterial hydrolysis of fat in tissue, such as body fat in corpses. In its formation, putrefaction is replaced by a permanent firm cast of fatty tissues, internal organs, and the face.

Adipocere was first described by Sir Thomas Browne in his discourse Hydriotaphia, Urn Burial (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 castile-soap: wherof part remaineth with us.

The chemical process of adipocere formation, saponification, came to be understood in the 17th century when microscopes became widely available.

In 1825, physician and lecturer Augustus Granville is believed to have (somewhat unwittingly) made candles from the adipocere of a mummy and used them to light the public lecture he gave to report on the mummy's dissection. Granville apparently thought that the waxy material from which he made the candles had been used to preserve the mummy, rather than its being a product of the saponification of the mummified body.

...
Wikipedia