Related entry: Coccus
Lac is a resinous substance formed by a scale insect, Tachardia Lacca, R. Blanchard, also known as Coccus Lacca, Kerr (Class Coccidae, Order Hemiptera), which lives on the juices of the stems of various plants. A resinous secretion forms around their bodies, and the twigs become encrusted. These encrusted twigs, collected from the trees during May, June, October, and November, are known as stick lac. The best known plants yielding it are Butea frondosa, Roxb.; Ficus religiosa, Linn.; Schleichera trijuga, Willd.; Shorea robusta, Gaertn.; Zizyphus Jujuba, Lam.; but there are also plants specially grown for the purpose, including Acacia arabica, Willd., and Cajanus indicus, Spreng. The washed and dried lac is melted with a certain proportion of orpiment and resin, in bags of special cloth, through which it is pressed. This molten lac is placed on a surface heated by means of hot water, and spread into a sheet or skin about 3 millimetres in thickness. This is further stretched out, and so made to yield a very thin paper-like sheet, shellac. It is then sorted according to colours. "Garnet lac," a dark-coloured variety, contains no arsenic as a rule, though it may have a high percentage of resin. Seed lac is stick lac which has been reduced to roundish pieces, and washed. When dried it is mixed with yellow arsenic (orpiment), or resin, or both. The former is said to impart a rich straw colour to the shellac, but its action is probably only mechanical. Resin is added to lower the melting-point, an essential condition for many industries, and it is frequently present to the extent of 2 to 5 per cent., few shellacs being entirely free from it. Most of the shellac of commerce comes from the neighbourhood of Mirzapore and Calcutta. It occurs in thin fragments or scales, varying in size, and often somewhat curved, reddish-orange to reddish-brown in colour, transparent, hard and brittle, odourless, and tasteless. The palest variety is the best, and is known as "orange" shellac. The darker varieties, known as "ruby," "garnet," etc., diminish in value in proportion to the depth of colour. Various impurities have been found in lac. Resin is detected by dissolving the sample in alcohol, pouring the solution into water, collecting and drying the precipitate, triturating it with petroleum ether, filtering the liquid, and shaking the filtrate with a small quantity of water containing 0.1 per cent. of copper acetate; if resin be present the petroleum ether layer will show an emerald-green colour. The specific gravity is of no value as a means of determining the purity of shellac. The iodine absorption is the most reliable test; the figure for genuine shellac lies between 4 and 10. Resin, on the other hand, has a fairly constant iodine value of 105 to 120. Bleached shellac is prepared by treating an alkaline solution with chlorine or sulphurous acid, when, on the addition of sulphuric acid, it is obtained as a precipitate, which is collected, washed, "pulled" under water, and finally twisted into sticks and thrown into cold water. It has a yellowish-white colour and a silky lustre. It is readily soluble in alcohol or wood naphtha when freshly prepared, or when preserved under water, but becomes insoluble on exposure to the air. Liquid shellac is formed by boiling shellac with solution of soda; on cooling, sulphuric acid is added, and the precipitated, viscous, liquid shellac can then be extracted with ether. It is a thick liquid, insoluble in water, but soluble in alcohol and in ether. It forms salts with lead, zinc, barium, and magnesium; the magnesium salt is amorphous, very soluble in water, precipitated on boiling, but redissolved on cooling.
Insoluble in water, but easily soluble in alcohol, especially on heating; soluble also in solution of potash, soda, and borax. Cold petroleum ether, free from water and alcohol, extracts only about 5 per cent., consisting chiefly of wax. Digested with ammonia in a closed vessel, shellac swells to a gelatinous mass.
Constituents.—The chief constituents of lac are about 6 per cent. of wax, about 6.5 per cent. of a pigment named laccin or laccaic acid, 70 to 85 per cent. of resinous matter, 65 per cent. of which is insoluble in ether, and 35 per cent. soluble in ether containing alcohol. The part insoluble in ether is probably a resinotannol ester of aleuritic acid, while the ether-soluble portion includes a yellow colouring matter called erythrolaccin.
Uses.—Shellac is of importance in the arts, but is not used in medicine. It is the chief constituent of sealing-wax, and an important ingredient in many varnishes and polishes. It is used as a cement for joining broken porcelain and earthenware.