Lac, best known in the form of shell-lac, is a coloured resinous substance of great importance in the arts. It is produced by a small insect—from th to th of an inch in length—called Coccus lacca (Carteria lacca of Signoret), belonging to the sub-order Homoptera of the Hemiptera, or Bugs. Lac is found in India, Burma, Siam, China, and in some of the islands of the Eastern Archipelago. The lac-insect lives upon the young branches of many different species of trees, but the best lac is collected from two or three species of fig, Zizyphus and Butea frondosa.
As soon as the young are hatched they crawl about in search of sappy twigs. To these they fix themselves by their proboscides, and immediately begin to form their lac cells or cocoons. These have one anal aperture and two others for the admission of air, and in their cocoons the insects remain in a lethargic state for two and a half months. The females, which greatly outnumber the males, never leave the spot to which they attach themselves, but the males escape by a ventral opening in the cocoon. After impregnation the female feeds voraciously on the juice of the twig to which it is fixed, increases in size, and continues to form lac. The lac surrounds all parts of the insect except the mouth and the three apertures already mentioned. When the young are perfectly formed they issue by the anal opening in the lac incrustation. Naturalists are divided in opinion as to whether the lac is secreted by the insect itself, or whether it is not merely the resinous juice of the trees altered in character by the insect while making its puncture, or just after it is made.
The appearance of the incrustation varies. It often takes the form of coalesced rounded prominences, at some places surrounding, at others scattered over, the branches; but in other instances it looks, superficially, more like a thick, irregular outer layer of bark roughened on the surface. The incrustation is cellular, each cell indicating the position of the insect which formed it. Stick-lac is the name given to it when the incrustation is still attached to the twigs, which are usually cut into pieces from three to six inches long. The next step is to remove the lac by a roller moving on a platform, the detached fragments being afterwards placed in tubs of water, and beaten by pestles or trodden by men. It is now in the state of scdd-lac. The water left in the tubs is coloured red by the bodies of the insects, and, after this water is evaporated, the red substance is made into cakes, forming the lac-dye of commerce (see DYEING). After drying the seed-lac is melted in cylindrical cotton bags before charcoal fires, and, by twisting the bags, forced through the pores of the cloth. It drops into troughs, and is either allowed to spread out itself, or is spread by a strip of leaf upon a roller, into a thin sheet. After the impurities are broken out, the flakes are packed in bags, forming the shell-lac of commerce. Another form is button-lac, which is made by letting the melted lac drop into rounded pieces from 1-1½ inches in diameter.
In India a good deal has been done in the cultivation of lac by transporting the encrusted branches to suitable forests about a fortnight before the young insects begin to move about. The twigs with the insects in the larval state are tied on branches of trees which have been selected for the rearing of future broods. The inland trade for the year 1889 was valued at 101 lakhs of rupees.
Lac has many industrial applications. Shell-lac varnish is more extensively employed than any other spirit varnish. One variety of it is French Polish (q.v.) for furniture (see also LACQUER). Lac applied as an alcoholic solution is used to stiffen the calico frame of silk hats (see HAT). In fine sealing-wax it is the most important ingredient, and either alone or mixed with other bodies it forms a good Cement (q.v.). Personal ornaments, such as chains and bracelets, are largely made of lac in India, and, when mixed with sulphur and some colouring matter, it is used there for coating wooden toys. Another mixture of lac with vermilion, closely resembling red sealing-wax in appearance, is applied by the Chinese to the surface decoration of boxes, trays, vases, and other small articles.