Copal is a name given to tree resin, particularly the aromatic resins from the copal tree Protium copal (Burseraceae) used by the cultures of pre-Columbian Mesoamerica as ceremonially burned incense and for other purposes. More generally, the term copal describes resinous substances in an intermediate stage of polymerization and hardening between "gummier" resins and amber. Wikipedia, Copal

Ramsay
A very important thing in the making of a violin, after a good form, a right balancing of part against part, and all of wood in skillful condition, is the violin varnish. Composition:-

Linseed oil boiled,... ... ... 1 part.
Isinglass, ... ... ... ... 1/2 part.
Turpentine, ... ... ... ... Quantum suf.

Give two coats with this, then rub down with fine sandpaper. Then, best copal varnish, one coat. Finish then with boiled linseed oil, thickened with sifted 'rotten stone.' This gives a fine, smooth, and dull surface. Ramsay's violins are of surpassing tone; and he considered the varnish an important element in violin-making. [Scientific Basis and Build of Music, page 85]