as Dave mentioned they were of a polymerizing linseed base. Most finishes based on my observations appear to be linseed based.
18th century finishes contained a variety of different recipients, which would have likely included linseed oil, bees wax, Venice turpentine, pine gum and resins, spirits, and other organic such as egg whites.
I’ve viewed 3 original brown bess muskets, a few original 1816’s and an original charlevilles last year at the Baltimore Gun Show and compared some test pieces on small knife blanks of Walnut.
The knife blanks I had used true oil, polymerizing linseed, polymerizing tung oil, straight linseed oil, Windsor and Newton Artwork Varnish. The artwork varnish is very thin very much like mineral oils.
The risk here is that i have know way of knowing if the originals were refinished and I’m using american walnut.
The Brown Bess muskets (2 third models and 1 second model) ironically seemed to varnished with a very thin yellowish colored finish / varnish. The one it matched the closest to was the Windsor and newton varnish, which is polymerizing linseed oil used on art work.
The 1816’s, two had a very matte like finish, which seemed to be closest to the straight linseed oil, one looked like it was varnished with a tung oil, which was very shiny and dark.
the Charleville was varnished with what looked to me like it matched the tru-oil pretty close, with the major inconsistency being that the steel parts were also varnished. I think that is likely a protective varnish applied much later than its original production.
Per Jess Melot of the Rifle Shoppe, he feels that the finish that closest resembles original varnishes is by Dem Bart, and its their checkering oil. I’ve not had much luck finding any, contacted the manufacture and their shipping charges are almost equal to the cost of one very small jar.
Lastly, i think the best way to go to recreate the most ideal historical accurate finish is with a polymerizing linseed or tung oil. I prefer tung oil, it tends to cure stronger and more durable and can be burnished easier to the stocks.
If your goal is to maximize the protection of the gun, i think that is the way to go. If your goal is to be as historically accurate as possible, i think a very thinned oil like the artwork varnish is the way to go, however it will provide very little to no long term protection.