A resinous substance, the product of an insect found on several different kinds of trees in the East Indies. These insects pierce the small branches of the trees on which they feed; and the juice that exudes from the wounds is formed by them into a kind of cells for their eggs. Lac is imported into this country adhering to the branches in small transparent grains, or in semi-transparent flat cakes. The first, encrusting the branches, is called stick-lac; the second 'are the grains picked off the branches, and called seed-lac; the third is that which has undergone a simple purification, as we shall presently notice. There is a fourth called lump-lac, made by melting the seed-lac, and forming it into lumps. To purify the lac for use the natives of India put it into long canvas bags, which they heat over a charcoal fire until the resin melts; a portion of the lac then exudes through the bags, which are subsequently twisted; or wrung by means of cross sticks at the ends of the bags, the surface of the latter being scraped at the same time to accelerate the process. The chief consumption of lac in this country is in the manufacture of sealing-wax and varnishes.
It has been a great desideratum among artists to render shell-lac colourless, as, with the exception of its dark brown hue, it possesses all the properties essential to a good spirit varnish in a higher degree than any other known resin. A premium of a gold medal, or thirty guineas, for " a varnish made from shell or seed-lac, equally hard, and as fit for use in the arts," as that at present prepared from other substances, was offered for some years by the Society of Arts. The editor of the Franklin Journal, of Philadelphia, observes, in reference to the foregoing, that "these ends are perfectly attained by the process given by Dr. Hare, which leaves nothing to desire, excepting on the score of economy." Were the oxymuriate of potash to be manufactured in the large way, the two processes, that of making the salt and of bleaching the resin, might be advantageously combined. "Dissolve," (says Dr. Hare,) "in an iron kettle, one part of pearl-ash in about eight parts of water; add one part of seed or shell-lac, and heat the whole to ebullition; when the lac is dissolved, cool the solution, and impregnate it with chlorine till the lac is all precipitated.
The precipitate is white, but its colour is deepened by washing and consolidation; dissolved in alcohol, lac bleached by the process above-mentioned yields a varnish which is as free from colour as any copal varnish." About the same period of time as the publication of the foregoing, the before-mentioned premium of the Society of Arts was claimed by two persons, Mr. George Field, and Mr. Henry Luning. The Society, upon a due examination of both of the processes and products, found them both to answer the intended purpose, and awarded the sum of twenty guineas to each of the candidates.
The following is Mr. Field's process: Six ounces of shell-lac, coarsely powdered, are to be dissolved by gentle heat in a pint of spirits of wine; to this is to be added a bleaching liquor, made by dissolving purified carbonate of potash, and then impregnating it with chlorine gas till the silica precipitates, and the solution becomes slightly coloured. Of this bleaching liquor add one or two ounces to the spirituous solution of lac, and stir the whole well together; effervescence takes place, and when this ceases, add more to the bleaching liquor, and thus proceed till the colour of the mixture has become pale. A second bleaching liquor is now to be added, made by diluting muriatic acid with thrice its bulk of water, and dropping into it pulverized red lead, till the last added portions do not become white. Of this acid bleaching liquor, small quantities at a time are to be added to the half bleached lac solution, allowing the effervescence, which takes place on each addition, to cease before a fresh portion is poured in. This is to be continued until the lac, now white, separates from the liquor.
The lac obtained in the foregoing process is to be dissolved in a pint of alcohol, more or less, according to the required strength of the varnish; and after standing for some time in a gentle heat, the clear liquor, which is the varnish, is to be poured off from the sediment Mr. Luning's process is as follows: - Dissolve five ounces of shell-lac in a quart of rectified spirits of wine; boil for a few minutes, with ten ounces of well-burnt and recently heated animal charcoal, when a small quantity of the solution should be drawn off and filtered; if not colourless, a little more charcoal must be added. When all colour is removed, press the liquor through silk, as linen absorbs more varnish, and afterwards filter it through fine blotting-paper. In cases where the wax found combined with the lac is objectionable, filter cold; if the wax be not injurious, filter while hot. This kind of varnish should be used in a temperature of not less than 60° Fahr.; it dries in a few minutes, and is not afterwards liable to chill or bloom; it is therefore particularly applicable to drawings and prints which have been sized, and may be advantageously used upon oil paintings which have been painted a sufficient time, as it bears out colour with the purest effect.
This quality prevents it from obscuring gilding, and renders it a valuable leather varnish to the book-binder, to whose use it has already been applied with happy effect, as it does not yield to the warmth of the hand, and resists damps, which subject bindings to mildew. Its useful applications are very numerous, indeed, to all the purposes of the best hard spirit varnishes: it is to be used under the same conditions, and with the same management. Common seed-lac varnish is usually made by digesting eight ounces of the bright, clear grained lac in a quart of spirits of wine, in a wide-mouthed bottle, putting it in a warm place for two or three days, and occasionally shaking it. When dissolved it may be strained through flannel into another bottle for use. In India, lac is fashioned into rings, beads, and other trinkets. Its colouring matter, which is soluble in water, is employed as a dye. The resinous portion is mixed with about three times its weight of finely powdered sand, to form polishing stones.
The lapidaries mix powder of corundum with it in a similar manner.