FOILS

These are thin leaves of polished metal, placed under precious stones and pastes, to heighten their brilliancy, or to vary the effect. Foils were formerly made of copper, tinned copper, tin, and silvered copper, but the last is the one wholly used for superior work at the present day. Foils are of two descriptions:—white, for diamonds and mock diamonds, and—coloured, for the coloured gems. The latter are prepared by varnishing or lacquering the former. By their judicious use the colour of a stone may often be modified and improved. Thus, by placing a yellow foil under a green stone that turns too much on the blue, or a red one under a stone turning too much on the crimson, the hues will be brightened and enriched in proportion. Prep. 1. ( Crystal, Diamond , or White foil .)— a. This is made by coating a plate of copper with a layer of silver, and then rolling it into sheets in the flatting mill. The foil is then highly polished, or covered with crystal varnish. b. The inside of the socket in which the stone or paste is to be set is covered with tin foil, by means of a little stiff gum or size; when dry, the surface is polished and the socket heated, and whilst it is warm, filled with quicksilver; after repose for two or three minutes the fluid metal is poured out, and the stone gently fitted in its place; lastly, the work is well fitted round the stone, to prevent the alloy being shaken out. c. The bottom of the stone is coated with a film of real silver, by precipitating it from a solution of the nitrate in spirit of ammonia, by means of the oils of ca ia and cloves. [318] This method vastly increases the brilliancy both of real and factitious gems, and the work is very permanent. [318] See Silvering . 2. ( Coloured foils. ) The following formulæ produce beautiful coloured effects, when judiciously employed:— a. (Amethyst.) Lake and Pru ian blue, finely ground in pale drying oil. b. (Blue.) Pru ian blue (preferably Turn bull’s), ground with pale, quick-drying oil. Used to deepen the colour of sapphires. c. (Eagle marine.) Verdigris tempered in shell-lac varnish (alcoholic), with a little Pru ian blue. d. (Garnet.) Dragon’s blood di olved in rectified spirit of wine. e. (Vinegar garnet.) Orange lake finely tempered with shell-lac varnish. f. (Green.)—α. From pale shell-lac, di olved in alcohol (lacquer), and tinged green by di olving verdigris or acetate of copper in it. β. From sesquiferrocyanide of iron and bichromate of pota a, of each 1 ⁄ 2 oz.; ground to an impalpable powder, first alone, and then with gum mastic (clean and also in fine powder), 2 oz.; a little pyroxilic spirit is next added, gradually, and the whole again ground until the ma becomes homogeneous and of a fine transparent green. The beauty increases with the length of the grinding. The predominance of the bichromate turns it on the yellowish green; that of the salt of iron on the bluish green. For use it is to be thinned with pyroxilic spirit. (‘Chem.,’ iii, 231.) Used for emeralds. g. (Red.) Carmine, di olved in spirit of hartshorn, or in a weak solution of salt of tartar, and a little gum (di olved) added. h. (Ruby.)—α. From lake or carmine, ground in isingla . β. Lake ground in shell-lac varnish. Both are used when the colour turns on the purple. γ. From bright lake ground in oil. Used when the colour turns on the scarlet or orange. tinging a weak alcoholic solution of shell-lac or mastic, by digesting turmeric, annotta, saffron, or socotrine aloes in it. The former is the brightest, and is used for topazes. β. From hay saffron digested in 5 or 6 times its weight of boiling water until the latter becomes sufficiently coloured, and a little solution of gum or isingla added to the filtered liquor. When dry, a coating of spirit varnish is applied. Obs. By the skilful use of the above varnishes, good imitations of the gems may be cheaply made from transparent white gla or paste; and by applying them to foils set under coloured pastes ( FACTITIOUS GEMS ), a superior effect may be produced. The pigments employed must be reduced to the finest state po ible by patient grinding, as without this precaution transparent and beautiful shades cannot be formed. The palest and cleanest mastic and lac, di olved in alcohol, and also the palest and quickest drying oil should alone be used when these substances are ordered. In every case the colour must be laid on the foil with a broad soft brush; and the operation should be performed, if po ible, at once, as no part should be cro ed, or twice gone over, whilst wet. If the colour turns out too pale, a second coat may be given when the first one has become quite dry, but this practice should be avoided if po ible.