Lac. Lacca. Resina (Gummi) Lacca. Laque, Gomme lacque, Fr. Lack, Gummilack, G.—A resinous substance obtained from several trees growing in the East Indies, particularly from Croton laccifera L. (Fam. Euphorbiaceae); several species of Ficus, especially F. religiosa L. and F. indica L. (Fam. Artocarpaceae), and, according to Valentine Ball, Schleichera trijuga Willd. (Fam. Sapindaceae), Butea frondosa Roxb., of the Fam. Leguminosae, and Zizyphus jujuba Lam. (Fam. Rhamnaceae). Stillman states that Acacia Greggii A. Gray (Fam. Leguminosae) and Covillea tridentata (DC.) Vail. (Larrea mexicana Moric.) (Fam. Zygophyllaceae), plants growing in Arizona, Colorado, and the Western territories, furnish both shellac and lac dye. (A. J. P., 1880, 409.) Lac is found in the form of a crust, surrounding the twigs or extreme branches, and is generally supposed to be an exudation from the bark, owing to the puncture of an insect belonging to the genus Coccus, and denominated C. lacca. By some it is thought to be an exudation from the bodies of the insects themselves, which collect in great numbers upon the twigs, and are embedded in the concreted juice, through which the young insects eat a passage and escape. Several varieties are known in commerce. The most common are stick-lac, seed-lac, and shellac.
Stick-lac is the resin as taken from the tree, still encrusting the small twigs around which it originally concreted. It is of a deep reddish-brown color, of a shining fracture, translucent at the edges, inodorous, and of an astringent, slightly bitterish taste. Its external surface is perforated with numerous minute pores, as if made by a needle, and when broken it exhibits many oblong cells, often containing the dead insect. When chewed it colors the saliva red, and when burnt, diffuses a strong, agreeable odor. It is in great measure soluble in alcohol.
Seed-lac consists of minute irregular fragments, broken from the twigs and partially exhausted by water. It is of a light or dark brown color, inclining to red or yellow, feebly shining, almost tasteless and capable of imparting to water less color than the stick-lac, sometimes scarcely coloring it at all. It is occasionally mixed with small fragments of the twigs.
Shellac is prepared by melting the stick-lac or seed-lac, previously deprived of its soluble coloring matter, straining it, and pouring it upon a flat smooth surface to harden, or pulling it out by hand into large thin sheets, which are broken up after cooling.
A variety of lac is mentioned by writers, in the form of cakes, called cake-lac or lump-lac (lacca in placentis); but this is at present rare in commerce.
According to John, lac consists of resin, coloring matter, a peculiar principle insoluble in alcohol, ether, or water, called laccin, a little wax, and various saline matters in small proportion. The resin, according to Unverdorben, consists of several distinct resinous principles differing in their solubility in alcohol and ether. The laccin is nearly or quite wanting in shellac, which also contains scarcely any of the coloring principle. Hatchett found in stick-lac 68 per cent. of resin, and 10 of coloring matter; in seed-lac 88.5 per cent. of resin, and 2.5 of coloring matter; in shellac 90.9 per cent. of-resin, and 0.5 of coloring matter. The other constituents, according to this chemist, are wax and gluten, besides foreign matters. R. E. Schmidt (Ber. d. Chem. Ges., 1887, 1285-1303) has prepared the lac dye in a pure crystallized state. He gives it the formula C16H12O8, and calls it laccaic acid. It was obtained crystallized from ethereal solution. Caustic alkalies dissolve it, giving a magenta color. Baryta water forms a violet lake. Laccaic acid shows some analogy to carminic acid, but the colors they give on wool and silk are different. Laccaic acid is decomposed on heating with concentrated hydrochloric acid to 180° C. (356° F.), as well as on fusing with caustic potash.
Japanese lac from Rhus vernicifera was examined by A. B. Stevens. (See Proc. A. Ph. A., 1905, 311.)
The importations of gum shellac for the year 1914 was 16,824,130 lbs., valued at $2,686,480; and for 1915, 24,308,145 lbs., valued at $3,018,296.
Lac in its crude state is slightly astringent, and was formerly used in medicine, but at present it is not employed. Shellac is wholly inert. It is sometimes used in alcoholic solution as a glaze for confectionery. The use of such insoluble resinous glaze for a food product is prohibited in some states partly on account of its indigestibility per se and partly because in its preparation for varnish or pigment purposes arsenical compounds are used to heighten the color and many instances are on record where confectionery which had been coated with such glaze contained appreciable amounts of arsenic. A bleached form of arsenic-free lac has recently appeared in commerce, which would seem to be free from many of the objections previously urged against lac in coating confectionery. Stick-lac and seed-lac are used on account of the coloring principle which they contain. Shellac, as well as the other varieties, deprived of their coloring matter, is applied to numerous purposes in the arts. It is the chief constituent of sealing wax. The best red sealing wax is made by melting together, with a very gentle heat, 48 parts of shellac, 19 of Venice turpentine, and 1 of balsam of Peru, and mixing with the melted mass 32 parts of finely powdered cinnabar. But common rosin is often substituted in part for the lac, and a mixture of red lead and chalk for the cinnabar. For methods of detecting adulteration with rosin see J. Soc. Chem. Ind., 24, 1905; also Chem. Ztg., 1910, 991. The best black sealing wax consists of 60 parts of lac, 10 of turpentine, and 30 of levigated bone black; the best yellow sealing wax, of 60 parts of lac, 12 of turpentine, and 24 of lead chromate. (Berzelius.) Lac is also used as a varnish, and forms an excellent cement for broken porcelain and earthenware. It may be dissolved in alcohol, oil of turpentine, benzin, or naphtha. For a method of preparing a colorless varnish from lac the redder is referred to P. J., 1864, 338. Lac has been highly recommended as an adhesive material for the dressing of wounds, ulcers, etc. It is prepared for use by dissolving, with the aid of a gentle heat, in alcohol contained in a bottle, sufficient lac to give it a gelatinous consistence, and then closing the bottle. It is used by spreading it on the bandages. For details of the preparation of shellac, see U. S. D., 19th ed., p. 1543.
The Dispensatory of the United States of America, 1918, was edited by Joseph P. Remington, Horatio C. Wood and others.