I haven't tried this yet for a relief, but another material (besides soap) that can be carved nicely is Delta Sand. I think this is sold in some stores as "Moon Sand." It can be baked, allowed to harden, and then carved (it's like a very soft stone). It can even be re-warmed and broken up to re-use. Great stuff! When we get to this project, I'm going to try it. I'll let you know how it goes.