Castilene?

casteline is a stage 2 sculpting medium for me. first off it's either hard plastercine chavant clay or super sculpy or sculpy firm. I like to make a mold then switch to either a hard wax or casteline, then go for the finer details to make a production quality sculpt. starting off in casteline will drive you probably insane unless you have working with similar materials with similar temp tolerances. it's got a very slim sweet spot so expect a lot of trail and error finding it out. but if you are starting out sculpting use super sculpy.

It's really good material to sculpt in, but the learning curve on it is pretty steep. For every person I've talked to that loves Castilene, there are about 5 that hate it.

I'd never heard of it, but from what I just read its a sculpting wax which means it will never harden. Heat would make it soft again.