I know of three products in this category: Fimo Gel, Translucent Liquid
Sculpey (TLS) og Kato Clear Liquid Medium. TLS is the least clear of the
three.
They are all loved for their many uses, especially for image transfers and,
mixed with pigments like powders, inks and paints, for making faux enamel or
paintings on clay.
They are widely used as a surface treatment instead of varnish, and as an
adhesive when joining cured and raw clay, they are practically
indispensable.