A stoat skull someone sent me. Currently in the "degreasing" process.
For skulls to be museum quality its important to remove any grease or fat that may have soaked into the bone. Excess fats can decay and turn acidic which will cause the bone to degrade and decompose over time. The process involves soaking the bone in solvents such as acetone however smaller specimens (such as a tiny stoat) require a gentle, but long soak in concentrated dish soap.
Trophy animal skulls often degrade over the years, where as museum specimens cleaned professionally can be kept indefinitely as long as the cleanliness is maintained.