This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
This is a spirit varnish, preferably made by a cold process, and requires less technical knowledge than the preparation of fatty varnishes. The chief dependence is upon the choice of the raw materials. These raw materials, copal, shellac, etc., are first broken up small and placed in a barrel adapted for turning upon an axis, with a hand crank, or with a belt and pulley from a power shaft. The barrel is of course simply mounted in a frame of wood or iron, whichever is the most convenient. After the barrel has received its raw material, it may be started and kept revolving for 24 hours. Long interruptions in the turning must be carefully avoided, particularly in summer, for the material in the barrel, when at rest, will, at this season, soon form a large lump, to dissolve which will consume much time and labor. To prevent the formation of a semi-solid mass, as well as to facilitate the dissolving of the gum, it would be well to put some hard, smooth stones into the barrel with the varnish ingredients.
Take 50 parts, by weight, of Syrian asphalt; 50 parts, by weight, of copal oil; 50 parts, by weight, of thick varnish oil, and 105 parts, by weight, turpentine oil, to which add 7 parts, by weight, of drier. When the asphalt is melted through and through, add the copal oil and heat it until the water is driven off, as copal oil is seldom free from water. Now take it off the fire and allow it to cool; add first the siccative, then the turpentine and linseed oil, which have been previously thoroughly mixed together. This bicycle varnish does not get completely black until it is baked.