Tom:
I have heard similar things about wood cracks. My father was an amateur wood worker and did a little cabinet making. He was a big fan of restoring old cracked furniture using copious amounts of a penetrating oil, such as teak oil, which made a difference in many cases depending on the wood and degree of damage.
The problem with horn, I understand, is that it is not nearly as "porous" as wood and so does not absorb moisture very well. It is basically made of layers of a protein (
keratin) that is water repellant. Keratin is what our finger and toe nails are made from as well as our hair, and it is the protein that covers our skin and keeps water from being absorbed when we bathe. Without the outer layer of keratin in our skin we would absorb water like a sponge whenever we took a shower or went swimming.
I have had no success trying to rehydrate horn and close cracks with oil or water. If others have a formula for doing that I would really like to hear about it -- have several hilts that need such treatment.
Ian.