As far as I know there is no way to completely repair a cracked piece of ivory. Plus, there is a special glue a piano tech. would use that would cause you a bunch. There is a form of synthetic ivory that can take the place of the old piece for a flawless look. But it's rather tricky to get the old one off in one piece. I would suggest a piano tech. for this job. Good luck with it!
anom., June 2008
