Shellac

Shellac is a product used by painters and restorers as a varnish or as a fixative for charcoal and chalk drawings. It is obtained from the shells of a type of insect. You can dissolve shellac in alcohol. In ceramics, shellac is used to decorate pieces and create a relief. After the shellac has dried, you can wipe away the uncoated clay with a wet sponge. Where shellac has been applied, the clay does not disappear, and you achieve a relief on your work. You can see examples in this article.