Among the hard resins, damar is usually met with in the form of clear pale yellow nodules (melting point about r 4o° C), although varieties of various colours are collected. It is obtained from coniferous trees of the Dipterocarpaceae family in the Malay States. Damar varnish is prepared as a solution in turpentine, in which about 3o% of the resin dissolves. It can be used as a paper varnish on account of its lustre and pale colour. North African and Australian cypress pines are the source of sandarac, a hard resin from which a colourless spirit varnish can be produced, suitable for coating paper labels, leather, wood and metals. It melts between 135° C and 145° C, after previous softening at about ioo° C. It is completely soluble in ether and methylated spirit, but only slightly soluble in turpentine, benzene and petrol.
Shellac requires an animal intermediary for its production, thus differentiating this resin from other natural resins. Insects (Tac chardia lacca), belonging to the Coccidae or scale insects, attach themselves for feeding purposes to certain Indian trees of the aca cia family, and produce a scaly covering consisting of an amber like material, which is the basis of lac or shellac. This resinous secretion is ground, washed and filtered, by squeezing while hot and molten through a cotton bag, on to hot plates or water-cooled rollers from which it is removed in the familiar flaky form. Lac is one of the most widely used resins in industry, providing an electrical insulating material, a stiffening agent for felt hats, etc., a preservative coating for wood and metal, and a binding agent for moulding compositions. The gramophone record industry absorbs about 6o% of the output of shellac. It is marketed in various grades and under various descriptions, e.g., stick lac, seed lac, button lac and garnet lac. The bulk of the shellac manufac tured bears the mark "T.N.," a term the origin of which is lost in antiquity, although presumably the letters are the initials of an early manufacturer. The melting point lies between 77° C and 82° C. Chemically, shellac consists largely of esters of aleuritic acid, together with a red colouring matter, wax and moisture. It dissolves in methylated spirit forming a cloudy solution which clarifies on standing. For hat-stiffening, shellac is dissolved in a water solution of borax. A white shellac can be obtained by a bleaching process, from which colourless transparent lacquers can be made.
The Oil-soluble Resins.—Useful varnishes can be made with rosin and China wood oil, an oil of somewhat similar character to linseed oil. Soap and paper-making are also responsible for the consumption of considerable quantities of rosin. Compounds of rosin with certain metals known as resinates provide the important ingredients in paint and varnish called "driers" (see VARNISH). Rosin as the sole resin basis of a varnish is not desirable on ac count of its susceptibility to the action of air and moisture. It is used as a flux in melting the harder resins and preparing them for use in varnishes.
French rosin is obtained from the Pinus maritima. The flow of oleo-resin is stimulated by cutting the bark in a manner sys tematically controlled as to size of incision, position on the tree trunk, and season at which the cutting takes place. One tree can be made to yield rosin for three or four years, after which sev eral years' rest are given. The annual production in France of
rosin is about ioo,000 tons, and turpentine 25,00o tons. American products are obtained from various kinds of pine grown in the south-eastern portions of the United States. The long leaf pine, Pinus palustris, is the most important.
Abietic acid, melting at i66° C, is the chief constitu ent of rosin, together with certain inert substances known as resenes. Current chemical opinion attributes a phenanthrene sion") to resin acids of the abietic acid type. The properties of rosin as a varnish resin may be modified by heating with metallic oxides, such as lime or zinc oxide, or with glycerine, the products being known respectively as hardened rosin or ester gum. Rosin modified in these ways is present in spar, boat and other varnishes.
The copals form an important group of varnish resins (Spanish, from Mexican copalli, incense). They vary from soft to hard ac cording to their age. Soft Manila copal collected direct from the tree is easily soluble in methylated spirit and oils, whilst the hard fossil varieties such as Pontianak require heat treatment before they can be mixed with linseed oil. Oil varnishes containing Ma nila resin are used for interior work. Kauri copal, melting at about i5o° C, a valuable varnish resin from a New Zealand pine, is usually obtained as a fossil about 4 ft. underground, but some times is found buried as deep as 20 feet. The cheaper Congo copal, also a fossil resin, melting at about 200° C, is becoming a rival of Kauri. Sierra Leone copal, obtained by tapping the tree, and consequently not so hard as the fossil Congo copal, finds a use in making good pale-coloured varnishes. Zanzibar copal is one of the hardest resins used by the varnish maker, and is dug up now mostly from the mainland opposite the island of Zanzibar. Nodules of this resin, marked with distinctive "goose-flesh" mark ings, are found as a rule about 3 ft. in the ground. The melting point may be as high as 36o° C.
Amber, the hardest resin known, is little used in the varnish industry. It is found in the ground in East Prussia, in the Baltic region, and efforts are being made in Germany to develop more fully the uses of this natural product.
Japanese and Chinese lacquer are obtained chiefly from the Rhus vernicifera (Urushi No-ki or Tsi-chou, varnish tree). The sap collected is a grey-brown viscous fluid, darkening in the air and forming a tough skin. A remarkably protective coating is obtained by using this fluid as a varnish. It is peculiar in requiring a moist atmosphere to enable the hardening process to take place. Chemically, the lacquer contains compounds of a phenolic char