f. (Green.)—α. From pale shell-lac, dissolved in alcohol (lacquer), and tinged green by dissolving verdigris or acetate of copper in it.
β. From sesquiferrocyanide of iron and bichromate of potassa, 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 mass 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, dissolved in spirit of hartshorn, or in a weak solution of salt of tartar, and a little gum (dissolved) added.
h. (Ruby.)—α. From lake or carmine, ground in isinglass.
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.
i. (Yellow.)—α. Various shades of yellow may be produced by 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 isinglass 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 glass 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 possible by patient grinding, as without this precaution transparent and beautiful shades cannot be formed. The palest and cleanest mastic and lac, dissolved 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 possible, at once, as no part should be crossed, 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 possible.
FOMENTA′TION. Syn. Fomentatio, FOMENTUM, Fotus, L. A liquid, either simple or medicated, used for local bathing. Fomentations are distinguished from lotions chiefly in being applied in a heated state, and in larger quantities, and for a longer period at a time.