I have two just out of the package Prismacolor erasers, one is so hard it cracked, but won't knead, and the other is pretty stiff. I was hoping there was something one could add to make it soft. Videos I looked at suggested working in graphite powder, which is impossible in one case. Other videos I watched didn't actually answer the question in their title. It would pretty much have to be a solvent in the case of the hardened one.