This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
Shellac is a resinous exudation that encrusts the bodies of Coccus (Tachardia) Lacca, Kerr (Order Hemiptera, Family Coccidce).
These minute insects live upon plant juices sucked up by a proboscis that penetrates the succulent tissues of the host. Twice or sometimes three times in the year the larvae emerge from the dead bodies of the females, crawl away and establish themselves in new situations. They are minute creatures, about 0.5 mm. long, of an orange-red colour with fully formed feelers and powerful legs, but no visible separation of the body into head, thorax, or abdomen. They puncture the young twigs with their probosces and suck up the juices; they become fixed, and their legs, being no longer required, drop off; a resinous secretion forms around their bodies and gradually more or less encrusts the twigs. At the period of maturity the males, which undergo a complete metamorphosis, escape from their pupae, impregnate the females and shortly afterwards die. The females rapidly increase in size, assume a bright red colour, develop viviparous larvae and die. New colonies are established by cutting off twigs with the gravid females and tying them on to various trees.
The principal trees visited by the lac insects are Aleurites laccifera, Willdenow (N.O. Euphorbiaceoe), Ficus religiosa, Linne (N.O. Urticaceoe), Schleichera trijuga, Willdenow (N.O. Sapindaceoe), Butea frondosa, Roxburgh (N.O. Leguminosoe). Acacia arabica, Willdenow (N.O. Leguminosoe), and Cajanus indica, Sprengel (N.O. Leguminosai), are cultivated for that purpose.
Whether the resin is secreted by the insects or produced by the plant as a result of the irritation caused by the insects is at present not definitely known. The resin shows, in its chemical composition, a variation from vegetable resins, and this seems to indicate that it owes its origin, in part at least, to a change effected by the insects in the constituents of the plant.
After the secretion of the resin the twigs are broken off, and form without further preparation the stick lac of commerce. Stick lac therefore consists of the twigs of the trees coated with a granular brownish or reddish resin which is frequently 7 mm. thick. Enclosed in this resin are the bodies of the insects, which contain a valuable colouring matter. The latter is extracted by breaking the lac from the twigs, crushing it, and exhausting it with water and sometimes subsequently with dilute soda solution. The aqueous solution contains a red colouring matter, laccaic acid, allied to, but not identical with, carminic acid; it is evaporated to dryness, and the residue, pressed into cakes, constitutes the lac dye of commerce. After the resin has been crushed and freed from colouring matter, it is spread out on floors to dry and bleach; it then forms brownish grains which are known as seed lac. This still contains various debris of the insects, etc.
It is melted with a little orpiment and resin (2 to 5 per cent.) and pressed whilst hot through a cloth. The strained lac thus obtained is spread into sheets about 3 mm. thick which are softened by heat and stretched to thin sheets. These sheets, broken up and sorted, constitute the shellac of commerce. Garnet lac is a dark coloured variety, and button lac is in the form of thin discs.
Lac is prepared in various parts of India, particularly in Bengal and Assam, and is exported chiefly from Calcutta.
Shellac usually occurs in thin, brittle, translucent, leafy flakes devoid of odour and taste. The colour varies from brownish yellow (orange shellac) to deep reddish brown (garnet shellac), the palest being considered the best. It melts when heated, evolving a characteristic odour.
A recent investigation of stick lac (Farner, 1899) resulted in the separation of the following constituents: -
Colouring matter (laccaic acid) ..
Residue (sand, vegetable and animal debris)
Moisture and loss ....
Of the resin 35 per cent., composed principally of fatty acids, was soluble in ether containing alcohol. The part insoluble in ether (65 per cent.) consisted of a resinotannol combined with aleuritic acid, a crystalline acid belonging to the fat acid series. A colouring matter erythrolaccin, crystallising in golden yellow crystals, is also present.
Shellac consists principally of the resin (90 per cent.) and wax and other substances (10 per cent.) Orpiment, although used in its preparation, seldom occurs in shellac. Adulteration with colophony may be detected by dissolving in alcohol, precipitating the resin with water, filtering, drying, and digesting with petroleum spirit; if colophony is present the petroleum spirit solution will assume an emerald-green colour when shaken with a 0.1 per cent, aqueous solution of cupric acetate. The presence of colophony is also indicated by the iodine value which should not exceed 12 in pure shellac (colophony 136 to 180).
Bleached shellac is made by dissolving shellac in hot solution of sodium carbonate and adding chlorinated lime, which forms chlorinated soda and deposits calcium carbonate. The resin is then precipitated, collected, ' pulled' under water, twisted into sticks, and kept under water.
Shellac is largely used in varnishes, but is not employed medicinally.
The following resins are largely used for various technical purposes:
Zanzibar copal (gum animi); from Trachylobium Hornemannianum, Hayne, (N.O. Leguminosoe); a fossil resin dug up on the East Coast of Africa; varies much in size; pale yellow to deep reddish- brown or greenish-red; transparent or semi-transparent; surface warty or longitudinally striated; consists chiefly of trachylolic acid (80 per cent.) isotrachylolic acid and resene.
American copal, from Hymenoea Courbaril (Brazil), Linne; pale brown transparent and brittle, of agreeable odour.
Australian copal (gum Kauri); from Agathis australis, Stender; mostly fossil, pale yellow or greenish yellow, conchoidal vitreous fracture, balsamic odour.
West African copal; from Copaifera Guibourtiana, Benth.
Manilla copal (=East Indian Dammar); from Dammora orientalis, Lambert (N.O. Coniferoe); fragments of large masses; yellow, slight but distinctly aromatic odour.
The copals are entirely soluble in alcohol, but only partially in benzene, chloroform, etc.
Dammar, probably from a species of Shorea (N.O. Dipterocarpeoe); imported from Singapore; pale yellow, transparent or translucent nodules or stalactitic masses coated with white powder; fracture vitreous, often exhibiting air bubbles.
Gum accroides (grass-tree gum; black-boy gum), from Xanthorrhoea hastilis, Robert Brown, and X. australis, Robert Brown (N.O. Liliaceoe, Australia). The resin of the former is yellow and contains xanthore-sinotannol combined with paracumaric acid, and also free para-cumaric and cinnamic acids; that of the latter is red and contains erythroresinotannol combined with paracumaric acid, free paracumaric acid but no cinnamic acid. The resin is a natural exudation covering the stem and leaf bases of the plant.