You can get a layer of hot melt glue and yarn to cover the clay, but it doesn't stick well to the clay. When finished, it is easy to remove because of that. The inside of the mask just had a little clay dust stuck to it, which washed off easily with water.
This kind of mask material is tough, leathery, and more damage resistant than paper mache. It is very light-weight and comfortable. To get the mask off the clay mold and over my own head, I made a couple of cuts behind the ears, which allow the mask to flex open. It then just springs closes over my head and stays in place pretty well. This material should have very good longevity, which is good, because laying on the yarn is pretty time consuming.
Step 1: Sculpting the clay
After sculpting the clay, you can see some of the first yarn going on.