What you are seeing here on this rabbit skull is called adipocere AKA corpse wax, grave wax or mortuary wax. It is a crumbly, waxy, water-insoluble material consisting mostly of saturated fatty acids.
is the product of a chemical reaction in which fats react with water
and hydrogen in the presence of bacterial enzymes, breaking down into
fatty acids and soaps. Adipocere is resistant to bacteria and can
protect a corpse, slowing further decomposition. The transformation of
fats into adipocere occurs best in an environment that has an absence of
oxygen and high levels of moisture. Adipocere formation begins within a
month of death, and, in the absence of air, it can persist for
is not uncommon for adipocere to be found on bones. Either found in a
natural setting or while being processed. I've found that the best time
to remove it is either A) when the bones are freshly out of maceration
or peroxide and you use a toothbrush to literally brush the adipocere
off while submerged in water or B) After the bones have been degreased,
had a peroxide bath and then dried. The adipocere become less waxy and
more flaky and you can more easily remove it with your fingernail or
again, with a toothbrush. This time dry.
careful how much force you use on a bone with a brush or fingernail.
This rabbit skull for example is going to be a real challenge for me to
remove the adipocere as it is a very thin/fragile skull.