Putrefaction is the decomposition of animal proteins, especially by anaerobic microorganisms, described as putrefying bacteria. Decomposition is a more general process. Putrefaction usually results in amines such as putrescine and cadaverine, which have a putrid odor. Material that is subject to putrefaction is called putrescible.
In alchemy, putrefaction is the same as fermentation, basically meaning to allow the substance to rot or decompose, sometimes with a small sample of the desired original pure material to act as a "seed".
Brief description of putrefaction of a human body with respect to time of death
2-3 days: Staining begins on the abdomen. The body begins to swell owing to gas formation.
3-4 days: The staining spreads and veins become discoloured.
5-6 days: The abdomen swells with gas (produced by the bacteria to decompose the body), and the skin blisters.
2 weeks : The abdomen becomes very tight and swollen.
3 weeks : Tissues begin to soften. Organs and cavities are bursting. The nails fall off.
4 weeks : Soft tissues begin to liquefy, and the face becomes unrecognisable.
The exact rate of putrefaction is, of course, dependent upon many factors, such as weather, exposure, and location.
The University of Tennessee's Forensic Anthropology Facility is a body farm made in 1981 to study human putrefaction. Several others have been built since.