I have used Tea Tree Oil in the past. I am not sure if it was here or on another site I ran into a post where someone that worked in a museum said this is what they used for preserving things like powder horns, eating utensils and such to keep them from drying out and cracking, it also rejuvenated old horn.
I have used it with excellent results on trigger guards, buttplates and horn insets on forends. I got mine at one of the local Vitamin/health stores. Not cheap but it doesn't take much. Coat the item and let it sink in. Smells good too.
Hope this helps..
WBLDon
