Been having a couple skulls turn this black color in a couple places (on the front as seen in the picture and also on the back of the skull). I am beetle cleaning, and degreasing with dawn and water (with a touch of ammonia). They are a little dark right when they come out of the beetle tank, but it really comes through when they get wet. Any thoughts as to why this is happening and/or how to get rid of it. A real bugger to get white.