Spirit-varnish resins.—(1) Lac (p. 1668), which stands at the head of varnish-making resins, having no rival amongst them for hardness and toughnees. (2) Certain true copals from Africa are entirely soluble in etrong alcohol (methylated spirit). (3) Soft Manilla copal (so called). (4) Sandarach, which hae for several hundred years been the stock resin for pale spirit-varnishes, being the only pale dry resin then known soluble in spirit. (5) The turpentines, rosins, and nsturally-dried turpentines (" thus") of nearly all the pines, firs, and larches. (6) The Pistacia resins.
Many of the copale and other resins, which are not entirely soluble in strong alcohol, dissolve completely in a mixture of methylated epirit with some simple hydrooasbon, e.g. benzol, toluol, turps, lavender-oil, &c., or with ohloroform, acetic ether, or ac,etone, making excellent varnishes.
To the above resins, may be added a few others, witioh are only chosen for their colour in lacquers and "changing-varnishes,' or to give a more pleasant odour to furniture and fancy varnishes. They are :— Gamboge (pp. 1551, 1650-1), a bright-yellow gum-resin, yielding its colour to alcohol, turps, ether, benzol, &c., gives the brightest of all yellows for pale brass lacquers.
Dragon's-blood (p. 1648), yields a rich orange-red coloured resin to alcohol, ether, chloroform, and borizol, hut not to turps nor petroleum.
Gum Aearoides and Black-boy gum (p. 1693) in solution vary in colour from brownish-yellow to brownish-red. They are insoluble in chloroform, benzol, turps, and petroleum. . They furnish resins hard enough to be used alone as spirit lacquers, for deep brass or bronze.
Aloes (p. 791-3). The inspissated juice of various species of Al, i! was much used in the 17th and previous centuries, for colouring oil-laequers and changing-varn6lies, used over tinfoil to imitate gilding, in deeoration, and is siill used in pale brass spirit-laequers.
Benzoin (p. 1637). That which is of tbe greyest colour, and containing most and largest white tears, will usually give the palest solution in spirit. It ia sometimes used alone as a last coat or " finish " to French-polish and other spirit-varnishes, to give scent and brilliancy. lt is soluble in all the solvents commonly used, excepting benzol and petroleum, which only dissolve a portion of it.
Asphaltum is of such very important use in all black oil-varnishes and japans, as also in etching-grounds and some black essenee-varnishes, and is so liable to adulteration, that great care is requisite in choosing it of good quality. It should be entirely (excepting perhaps 4-5 per cent.
of earthy impurities) soluble in chloroform, toluol and its higher homologues, rosin-spirit, and turps, and quite insoluble in alcohol, aud in a mixture of equal parts of alcohol and chloroform. It should break with a conchoidal fracture and brilliant resinous lustre ; the rubbed surfaces and angles are usually covered with a bright-brown powder. It should not flow like wood-pitch when laid by for some time on a horizontal surface, and an angular fragment or thin chip should retain its shape and the sharpness of its angles in boiling water, and only begin to flow at a temperature of 150°-200° (300°-400° F.). When adulterated with coal-pitch, it is much less brilliant on the surface of fracture, which then has a lustre rather adamantine or sub-metallic than resinous, and when fused, has a granular, pasty appeartnee and feel, instead of being srnooth and homogeneous, and will not draw out into even and transparent brown threads like pure asphaltum. This is the moot important adulteration, a email percentage of coul-pitch spoiling it utterly for varnish-making, or etching-grounds. OSee Asphalt, p. 341.) Gums, used in water-varnishes and glazes.—(1) Gelatine (pp. 620-2). That used for glazing paper and fancy articles of cardboard, light wood, &c., should be of the best quality, particularly in damp climates. It should especially be freo from saline impurities, and overboiled poi tioos, which make it liable to get damp and sticky in wet weather, besides diminishing its lustre. Parehment-size (p. 622) is one of the best. Of commercial glues and gelatines, those should be chosen which give the stiffest jelly with the same proportion of water.
(2) Gluten of cereals, especially wheat (see pp. 622-3, 1821-9), after washing away the starch, has been used as a varnish when dissolved in weak alcohol, in which a considerable portion is said to be soluble.
(3) Albumen, Caseine, Legumine, &c.—The first Iwo are regularly manufactured as articles of oommerce (see pp. 19]-2, 1304). Egg-albumen is the cleanest and palest, but reqiiires long-continued " whipping " to thoroughly break down its organized structure, before its glairy character is removed, and the albumen is fit for varnishes. Blood albumen is now manufactured of such excellent quality ns to supersede that from eggs wherever large quantities are wanted.