I have had good luck boiling horn scales.

I think that the success of reheating and straightening depends on if the material is thermoplastic or thermosetting. Thermoplastic materials such as horn, celluloid (cellulose nitrate), bone (to a lesser degree) have a good chance of reheating and straitening. Thermosetting materials such as bakelite or vulcanized hard rubber are a lot harder to work with.

I would/have used wet heat to bend horn and bone. I would/have used a dry heat to soften man made resins.

Early plastics are a finicky bunch, it is hard to tell what you have and determine what their properties are.

Charlie