Have you tried Hydrogen Peroxide?
We used it on skulls. We took a gallon jug like antifreeze, and rinsed it out and cut the side out. we put the skull in the jug layed on its ide and then wetted down paper towls with peroxide and filled the jug part way up. the paper towl wicked the peroxide up to keep it wet. We would rotate different skulls into out of the pot so that they woudl whiten dry.