Nice job! I've tried doing similar stuff with clay (packed earth floors out of DAS Pronto to be precise) but found it too hard to work with. It took a bit too much force to press as flat as I wanted it (which risked making the underlying material buckle) and once it had dried, I found it had shrunk enough to need retouching around the edges. That crackling effect is very cool though. I might just reconsider this modelling material again.