I've not worked with mammoth yet, but I can tell you that it's more current cousin, elephant ivory, is also prone to cracking. The primary cause is dehydration. For the ivory I have used, I make sure I moisten it about 4 times a year with plain water on a damp sponge. I've had no cracking of the pieces since I started this. BTW, if you are going to try the moisture route, it is best to not put a finish on elephant ivory. I can't say for sure if this is true with mammoth.