Here's an example of something I ran in to... A year or two ago I degreased a 4 month old lamb skull. When I was finished with the skull it looked perfectly white and clean. Now, a couple years later the entire skull is a uniform shade of yellow. It's yellow all over. I did not use any sealer yet the bone yellowed. What are the chances of soap left in the bone turning it another color? I know, probably a long shot. When a skull is done, do any of you soak the skull in plain water to remove any trapped soap or is it a non issue?