A natural resin used in varnish and incense. Three types of Central and South American copal are marketed: black copal, white copal and golden copal.
COPAL, a resin which exudes spontaneously from two trees, the Rhus copallinum and the Elaeocarpus copalifer, the first of which grows in America, and the second in the East Indies. A third species of copal tree grows on the coasts of Guinea, especially on the banks of some rivers, among whose sands the resin is found. It occurs in lumps of various sizes and of various shades of colour, from the palest greenish yellow to darkish brown. I found its specific gravity to vary in different specimens from 1.059 to 1.071, being intermediate in density between its two kindred resins, animé and amber. Some rate its specific gravity so high as 1.139, which I should think one of the errors with which chemical compilations teem. Copal is too hard to be scratched by the nail, whence the excellence of its varnish. It has a conchoidal fracture, and is without smell or taste.
Copal varnish. - Hard copal, 300 parts; drying linseed or nut oil, from 125 to 250 parts; oil of turpentine, 500; these three substances are to be put into three separate vessels; the copal is to be fused by a somewhat sudden application of heat; the drying oil is to be heated to a temperature a little under ebullition, and it is to be added by small portions at a time to the melted copal. When this combination is made, and the heat a little abated, the essence of turpentine, likewise previously heated, is to be introduced by degrees: some of the volatile oil will be dissipated at first; but more being added, the union will take place. Great care must be taken to prevent the turpentine vapour from catching fire, which might occasion serious accidents to the operator. When the varnish is made, and has cooled down to about the 130th degree of Fahr., it may be strained through a filter, to separate the impuritie's and undissolved copal.
Almost all varnish makers think it indispensable to combine the drying oil with the copal, before adding the oil of turpentine; but in this they are mistaken. Boiling oil of turpentine combines very readily with fused copal; and, in some cases, it would probably be preferable to commence the operation with it, adding it in successive small quantities. Indeed, tbe whitest copal varnish can be made only in this way; for if the drying oil have been heated to nearly its boiling point, it becomes coloured, and darkens the varnish.
This varnish improves in clearness by keeping. Its consistence may be varied by varying the proportions of the ingredients, within moderate limits. Good varnish, applied in summer, should become so dry in 24 hours that the dust will not stick to it, nor receive an impression from the fingers. To render it sufficiently dry and hard for polishing, it must be subjected for several days to the heat of a stove.
Andrew Ure, M.D.
A Dictionary of Arts, Manufactures and Mines...
London: Longman, Orme, Brown, Green and Longmans, 1839.
An interesting paper on the use of copal in mesoamerica: www.utexas.edu/courses/stross/papers/copal.htm
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Last revised: 10 December 2011.