A hard, shining, transparent concrete juice, obtained from an American tree; but which, although it is commonly considered a gum, has neither the solubility in water common to gums, nor the solubility in alcohol common to resins, except in a very slight degree. It may be dissolved by digestion in linseed oil, rendered drying by quick lime, with a heat very little less than is sufficient to boil or decompose the oil. This solution, diluted with oil of turpentine, forms a beautiful transparent varnish, which, when properly applied, and slowly dried, is very hard and durable. It preserves and gives lustre to paintings, and greatly restores the decayed colours of old pictures, by filling up the cracks, and rendering the surfaces of reflected light more uniform. It has been observed by Mr. Sheldrake, that if powdered copal be triturated with a little camphor, it softens and becomes a coherent mass; and camphor, added either to alcohol or oil of turpentine, renders it a solvent of copal. Half an ounce of camphor is sufficient for a quart of oil of turpentine, which should be of good quality; and the copal, about the size of a walnut, should be broken into small pieces, but not to powder.
The mixture should be set on a fire, so brisk as to make it boil almost immediately.
The vessel should be of metal, strong, with a long neck, and capable of holding about two quarts. The mouth should be stopped with a cork, having a notch cut in it to prevent its bursting. Mr. Cornelius Varley (who has bestowed much judicious attention to the preparation of artists' materials), states that a good copal varnish may be prepared by pouring upon the purest lumps of copal, reduced to a fine mass in a mortar, colourless spirits of turpentine, to about one-third higher than the copal, and triturating the mixture occasionally in the course of the day. Next morning it may be poured off into a bottle for use. Successive portions of oil of turpentine may thus be worked with the same. Oil of lavender is stated to be alone a solvent of copal, which is probably owing to the camphor contained in the former. Camphorated oil of turpentine will also dissolve copal, but a mixture of camphor and alcohol effects it more readily.