I could not find the original article I read on skulls turning chalky/crumbling after bleach but I found this; hope it helps.
Sometimes you get skulls that got chalky during preparation, usually due to using natron, chlorine or too highly concentrated peroxide. If you leave them like this, they'll keep crumbling away. Someone asked me how to restore such a skull, so I asked around, and a friend gave
me this recipe I want to share with you all: First you take out the teeth if possible, and if not, cover them with rubber glue to protect them. Then you mix 2 parts of water with one part white glue (the kind you use to glue wood), hand-warm, and put the skull in for about half an hour. The warm diluted glue will get into all pores of the bone, and when it's dry the bone will be hard and smooth again, no longer crumbly. Then you glue the teeth back in or scrape away the rubber glue. Never tried this myself (don't have chalky skulls), but according to my friend it works well (the rubber glue was my own idea). One remark: you can't bleach a skull anymore after this procedure, so if
the skull needs bleaching, do it before.