The University of Tennessee maintains an experimental area where observations are made on decomposing corpses. They find that flies lay their eggs in wounds on dead bodies, and maggots appear before 30 hours at about 23ºC. This approximates the time required for liquid decomposition products to begin to appear on the surface of a body. We could not find any evidence for the migration of liquid decomposition products through the cloth; therefore, the cloth could not have been in contact with the body for very long.
Decomposing bodies start producing ammonia (NH3) in the lungs quite soon after death, and the ammonia diffuses outward through the nose and mouth. Ammonia is lighter than air, and it diffuses rapidly. The rate of production of ammonia decreases with time after death.
Within a few hours, depending on weather conditions, a body starts to produce heavier amines in its tissues, e.g., putrescine (1,4-diaminobutane), and cadaverine (1,5-diaminopentane). These amines are much heavier than air, and they diffuse relatively slowly. Experiments prove that slow diffusion relates to increased resolution in image formation. The early appearance and rapid diffusion of low-molecular-weight ammonia from the nose and mouth might help explain the greater amount of image color between the nose and mouth, in the beard, and into the nearby hair. It will also diffuse through the cloth more quickly and reach the back side of the cloth in greater concentration. Ammonia will diffuse about 20 cm through air while cadaverine is diffusing only 6 cm.