Copal, improperly called Gum copal, is a resinous substance obtained from the concrete juice of the Rhus copallinum, or narrow-leaved sumach, a native plant of North America, known there by the name of Beach-sumach. This resin is imported in irregular masses, some of which are transparent, of a yellowish or brown colour, others are whitish and semi-transparent. It possesses a more agreeable odour than frankincense, but is, unlike other gums and resins, neither soluble in water nor in spirit of wine. By these properties it resembles amber ; which has induced some to consider it a mineral bitumen similar to that substance. It yields on distilla-tion an oil, which, like mineral pe-trolea, is indissoluble in spirit of wine.
Copal itself is soluble in the essential oils, particularly in that of lavender, but not easily in those obtained by expression. It may, however, be dissolved in linseed-oil, by digestion, with a heat very little less than is sufficient to boil it.
A correspondent in the 17th vol. of the Transactions for the Encouragement of Arts, etc. informs us, that copal may be dissolve d in spirit of turpentine by the following process : Having prepared a glass vessel, of sufficient capacity to contain at least four times the quantity intended to be dissolved, and which should be high in proportion to its breadth, reduce two ounces of copal to small pieces, and put them into the vessel. Mix a pint of spirit of turpentine with one-eighth of spirit of sal ammoniac ; shake them well together; pour them on the powder, cork the glass, and lie it over with a string or wire, making a small hole through the cork. set the glass in a sand heat, so regulated as to make the contents boil as quickly as possible, but so gently that the bubbles may be counted as they ascend from the bottom. The same heat must be kept up exactly till the solution is complete. It requires the most accurate attention to succeed in this operation; for, if the heat abate, or the spirits boil quicker than is directed, the solution will be impeded, and it will afterwards be in vain to proceed with the same materials; but, if properly managed, the spirit of sal ammoniac will be seen gradually to descend from the mixture, and attack the copal, which swells and dissolves, except a very small quantity. It. is of much consequence that the vessel should not be opened till some time after the liquid has become perfectly cold ; as it frequently happens that the whole of the contents are blown with violence against the cieling. - The spirit of turpentine should be of the best quality.
The method of dissolving copal in. alkohol, is as follows: To half an ounce of camphor, add a pint of alkohol; put it in a spherical glass, with four ounces of copal in small pieces ; set it in a sand heat, so regulated that the bubbles may be counted as they rise from the bottom, and continue the same heat, till tine solution is completed.
Camphor acts more powerfully upon copal than any other sub-stance. If the latter be finely powdered, and a small quantity of dry camphor rubbed with it in the mortar, the whole becomes in a few minutes a tough coherent mass. The process here described, will dissolve more copal than the menstruum will retain, when cold. Hence, the most economical method will be, to let the vessel which contains the solution, stand for a few days ; and, when it is perfectly settled, to pour oft' the clear varnish, and leave the residuum for a future process.
Copal will dissolve in spirit of turpentine, by the addition of camphor, with equai facility, though not in the same quantity, as in al - kohol. - The vehicle employed in dissolving this resin dries very quickly, and is therefore, in some cases, really attended with disadvantage ; but this objection may be removed by the following pro-cess : Take a pint of nut, or poppy oil, put it into a large earthen vessel ; let it boil slowly over a moderate fire; add by degrees two ounces of white lead, and stir it continually, till the whole is dissolved. Prepare a pint of the copal-oil varnish heated in a separate vessel; pour this gradually into the hot oil, and stir them together till all the spirit of turpentine is dissipated ; let it stand till cold, when it will be fit for use.
It is obvious that, as this is a compound of the copal-varnish with the least exceptionable of the drying oils, it will partake of the properties of each. Although it imparts less brightness and durability to colours than the varnish, yet it may be used by painters in the same manner as any other drying oil, on account of the greater lustre and permanency which such colours derive from it, than are obtained from the common oil varnishes. Notwithstanding we have mentioned specific quantities of the ingredients, it must be obvious that the relative proportions may be varied, accordingly as it is re-quired to dry faster, or slower. It should also be remarked, that both the ingredients must be hot; because, if either of them be cold, the mixture will become turbid, and a part, nay, often the whole of the copal, be precipitated : but this inconvenience may be avoided, by mixing a:.d boiling them toge-ther, in the manner above direct-ed. Yet, as after some time, a spontaneous alteration takes place, which diminishes, and at length destroys, the drying quality of this mixture; it will be advisable to use it fresh, or at least not to employ it, after it has been prepared longer than a month or six weeks. litis varnish is also applied to snuff-boxes, tea-boards, and other articles. It preserves and gives lustre to paintings, and greatly restores the faded colours of old pictures, by filling up the cracks, and rendering the surfaces capable of reflecting light more uniformly.