A note from a varnish geek:
Linseed oil cooked long enough at the proper temperature gradient when cooled will form a gel dense enough to be cut into cubes. This was used to polish patent leather shoes.
on we go,
Well said. Varnish is a combination of oil, resin, and turpentine. Each may be modified (or not) prior to cooking. The results will vary according to cooking procedures and the skill level of the maker.