One of the points I'd like to make about traditional finishes is the historical availability of ingredients. Finishes containing copal in particular. African copal was widely available in Europe during the late nineteenth and early twentieth centuries. This was due to the European colonies in Africa. I suspect that this was one of the major reasons it was widely used as an ingredient. The copal "Trade" nowadays is considerably different, and it is not as readily available, nor inexpensive. There are many other resins available as well, from other areas of the world. From what I've been able to determine, all were considerably softer, and therefore less desirable than African copal, especially copal coming from East Africa. Copal, being very hard and fairly pure, was (as it appears to me) the resin of choice. The only other one of real note was the very-expensive use of Baltic Amber. Copal was considered to be partially fossilized, with amber being completely fossilized. Copal was also the base for the early spar varnish used on ships. Nowadays, with the development of modern (and less expensive) poly-type coatings, true copal finishes are really a thing of the past.
Shellac finishes were more popular in America because, beginning in the 1800s, seedlac was being directly imported from the Far East, thereby avoiding the hefty duties previously imposed by the British Crown.
Here's an interesting reference work from 1921 that covers quite a bit of info on older finishes:
https://openlibrary.org/works/OL10663281W/Rubber_resins_paints_and_varnishesRegards
Ken
