Melt 8 parts of powdered copal gum in an iron pot by slow heat, and 2 parts balsam capivi previously warmed. Then remove from the fire, and add 10 parts spirits of turpentine, also warmed, in order to reduce to the necessaiy degree of thickness for working.
Gum Copal is made more soluble in spirits of turpentine "by melting the powdered crude gum, and allowing it to stand for some time loosely covered. Powdered copal 24 parts, spirits of turpentine 40, camphor 1, 4 ounces copal, 1/2 ounce camphor, 3 ounces white drying oil, 2 ounces essential oil of turpentine. Reduce the copal to powder, mix the camphor and drying oil, then heat it on a slow fire, add the turpentine and strain. As other soft resins are sometimes substituted for mastic, so inferior hard resins are sometimes employed in the place of copal, in the composition of varnishes celebrated as copal varnishes. Copal is difficult of solution in turpentine and linseed oils, both of which enter into the composition of the ordinary Copal Varnishes, which are employed by the coach painter and afford the best varnishes used by the house painter and grainer. Combined, however, with linseed oil and oil of turpentine, copal varnish affords a vehicle superior in texture, strength, and durability to mastic and its megilp, though in its application it is a less attractive instrument, and of more difficult management. As copal swells while dissolving, so its solutions and varnishes contract, and consequently crack, in drying, and thence linseed oil is essential to prevent its cracking.
The mixture of copal varnish and linseed oil is best effected by the medium of oil of turpentine, and for this purpose heat is sometimes requisite.