Step 4: Whitening and De-Greasing your skull
Now that the worst part is behind you, the rest should be easy. For this step you will need a small plastic tub or container as well as several bottles of 3% hydrogen peroxide ($0.99 per bottle at your local drug store).
1. Place your skull into the tub and pour in the bottles of hydrogen peroxide. You will want to make sure you use enough peroxide to completely cover the skull. Do NOT dilute the peroxide. 3% is very low strength but it is the cheapest and easiest to to get your hands on.
2. The peroxide will fizz like crazy. Allow the skull to soak over night, and the check to see the progress. You skull should begin to whiten nicely. Leave it in the solution until you reach you desired whiteness, but I would not leave it in for more than about 48 hours, as the peroxide will eventually begin to weaken the bone just enough so that it gets a few hairline cracks.
3. Once your skull has whitened nicely, remove it and wash it thoroughly under running water. If there are still small bits of flesh or cartilage on it, they should have turned a light yellow color by now and should come off easily during the final rinse. Allow your skull to dry for 24-48 hours.
4. If your skull develops any greasy dark stains, contact a taxidermist to purchase a professional de-greaser. I have herd that ammonia can be used, but I have never used it myself. The peroxide works as a de-greaser as well as a whitener, but some species have greasier bones than others.
*** I NEVER recommend using a bleach mix for whitening. This can seriously damage a skull. If you wish to use something stronger than drug-store peroxide, you can purchase 40% peroxide from a taxidermist supplier. Follow the instructions on the bottle.