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.
Step 2: Yarn Application
The yarn colors change, darkening when saturated with glue. Usually, there is enough glue used in sticking down the yarn to also saturate the yarn in the flattening procedure. You can always add more glue if the yarn is too dry.
When finished, the surface is shinier than the original yarn surface, flatter, and easier to keep clean. For those who object to the shiny surface, one could probably just spray the finished mask with matte spray varnish, available at art stores.
Step 3: Shots of the Finished Mask
In the process of removing it from the clay inside, I made a couple of cuts up the neck behind the ears. The complicated forms are not much of a problem to remove, because the clay can be flexed and broken off of the head mold while still inside the mask shell. The head mold itself has no complicated details that the yarn layer would lock onto. Therefore, even if the yarn layer sticks to protruding clay forms, such as the horn-like shapes over the eyes, the clay breaks off and can be removed from the inside of the shell after everything comes off the simpler head mold shape.
You can always use more cuts to remove things, if need be, and patch with more glue later.