COPAL This substance„wLich deserves particular attention from its importance as a varnish, and which, at first sight, seems to belong to a distinct class from the resins, is obtained from the rims co pallinum, a tree which is a native of North America : but the best sort of copal is said to come from Spanish America, and to be the produce of different trees.
Copal is a beautiful white resinous sub stance, with a slight tint of brown. It is sometimes opaque, and sometimes almost perfectly -transparent. When heated it melts like other resins ; but it differs from them in not being soluble in alcohol, nor in oil of turpentine without peculiar management. Neither does it dissolve in the fixed oils with the same ease as the other resins. It resembles gum anim6 a little in appearance, but is easily dis tinguished by the solubility of this last in alcohol, and by its being brittle between the teeth, whereas anime softens in the mouth. The specific gravity of copal varies from 1.045 to 1.139. Mr. Hatchett found it soluble in alkalies and nitric acid with the usual phenomena, so that in this respect it agrees with the other resins.
When copal is dissolved in any volatile liquid, and spread thin upon wood, metal, paper, &c, so that.the volatile menstruum may evaporate, the copal remains per fectly transparent, and fbrms ,tine of the most beautiful and perfect varnishes that can well he conceived. The varnish thus formed is called copal varnish, from the chief ingredient in it. Copal varnish used by the English japanners is made as follows. Four parts by weight of co pal in powder are put into a glass ma trass and melted. The liquid is kept boiling till the fumes, condensed upon the point of a tube thrust into the ma trass, drop to the bottom of the liquid without occasioning any hissing noise, as water does. This is a proof that all the water is dissipated, and the copal has been long enough melted. One part of boiling hot linseed oil (previously boiled in a retort without any litharge) is now poured into it, and well mixed. The
matrass is then taken off the fire, and the liquid, while still hot, is mixed with about its own weight of oil of turpentine. The varnish thus made is transparent, but it has a tint of yellow, which the ja panners endeavour to conceal, by giving the white ground on which they apply it a shade of blue. It is with this varnish that the dial plates of clocks are covered after having been painted white.
Mr. Sheidrake has lately favoured the public with another and easier method of dissolving copal. This method is as fol lows : " Provide a strong vessel allude of tin or other metal ; it should be shaped like a wine bottle, and capable of hold ing two quarts ; it will be convenient to have a handle strongly rivetted to the neck ; the neck should be long, and have a cork fitted to the mouth, hut a notch or small hole should be made in the cork, that, when the spirit is expanded by heat, a small portion may force its way through the hole, and thus prevent the vessel from bursting. Dissolve half an ounce of camphor in a quart of spirit of turpentine, and put it into the vessel ; take a piece of copal the size of a large walnut, reduce it to a coarse powder or very small pieces, put them into the tin bottle, fasten the cork down with a wire, and set it, as quick as possible, upon a fire so brisk as to make the spirit boil al most immediately ; then keep it boiling very gently for about an hour, when so much of the copal will be dissolved as will make a very good varnish ; or, if the operation has been properly begun, but enough of copal has not been dissolved, it may be again put on the fire, and by boiling it slowly for a longer time, it may be at last brought to the consistence desired.