Fat and grease are fine in the right places. Not in skulls we want to make pretty though. To help folks understand why you don't need super high temperatures to get fat out of bone (and that cold temperatures are a poor choice) I put together some information on melting temperatures of fats. If you base assumptions of degreasing on lard (animal fat), soft lard melts at between 86 to 104 F. The harder, solid, "leaf lard" melts at 109 to 118 F and the fat in general melts at between 97 to 113 F. PIG fat melting points: backfat: 86–104 °F leaf fat: 109–118 °F (solid kidney fat deposits) mixed fat: 97–113 °F) Lard is the rendered belly fat from pigs [Sus domestica] and is soluble in benzene, chloroform, ether, slightly in alcohol and insoluble in water and melts at 97 to 107° (F). Tallow is the internal fat (suet) surrounding the kidneys and intestines of sheep, goats, deer and oxen that is rendered down at a very low heat, just enough to cause the grease to melt away from the connective tissues. Tallow is also harder than lard and has a higher melting point. 109 - 114°F for beef tallow as opposed to pork lard in general. Bear fat, commonly called bear grease is the material rendered from the body fat of any species of bears [Ursus spp.], and was commonly used as a lubricant, lamp oil and to make the finest croissant, according to the French. Bear fat melts at or a little above 80°. Bird fats generally have a melting point of 98°F. I don't know of anyone that has a tank of benzene to soak skulls in so we make do with other, safer solvents for the fats in bone. Notably, detergents. Detergents do not dissolve the fat but break it down into smaller particles and float it away in a water solution. To do this as fast as possible, the fat has to be at its melting point to allow the detergent to break it down and move it out of the bone. Fat that is not at it's melting point will remain solid and unresponsive to the action of detergents. Take two cups. Put detergent solution in both. Heat one and leave the other at even room temperature. Add a half teaspoon of lard to each one and let it sit for a few hours. Stir and you will find that the heated cup now will have the fat in solution. The unheated one will still be a chunk of lard staring at you. There is no need to be heating your degreasing solutions to over 120°F. Heating the bone above that point over a long term as in degreasing will begin to break down the supportive structure of the bone itself and it may eventually fail and degrade. Simply keeping the degreasing solution heated to at least 115°F and not higher than 120°F should be enough to cover the range of fat melting points in any species that you work with without damaging the bone structure itself. Cervids and Caprinae will still require a temperature of 120 degrees to break down the fat type they have. Other techniques such as circulating the degreasing solution can also help to speed the process.