LAC. 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 fiat cakes. The first, encrusting the 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 'lee fibr use the natives of India put it into long canvas which they heat over a charcoal fire until the resin melts; a of the lac then exudes through the bags, which are subsequently • , 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 larankhi; 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 oxymmiate the to be manufactured in the large way, the two processes, that of g the salt and of bleaching the resin, might be advantageously combined. " Dissolve," (says Dr. Hare,) " in as 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 twos, Mr. George Field, and Mr. Henry Luning. The Society, upon a 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 solu tion becomes slightly coloured. Of this bleaching liquor add one or two ounces to the spmtnoua 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 bleach ing liquor is now to be added, made by diluting muriatic acid with thrice its bulk of water, and dropping into it pulverised red lead, till the last added portions do not become white. Of this acid bleaching liquor, smell quantities at a time are to be added to the half bleached lac solution, allowing the effer vescence, 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,tes from the liquor. The supernatant fluid is now to be poured away, lac is to be i well washed in repeated water', and finally wrung as dry as possible in a cloth.