Concluding remarks. It is absolutely necessary for the successful application of the preceding formulæ that the substances employed should be perfectly free from impurities, more particularly those of a mineral kind. The litharge, oxide of lead, and carbonate of lead, above all things, must be entirely free from oxide of tin, as the smallest particle of that substance may impart a ‘milkiness’ to the paste. All the ingredients must be separately reduced to powder, and, after being mixed, sifted through lawn. The fusion must be carefully conducted and continuous, and the melted mass should be allowed to cool very slowly, after having been left in the fire from 24 to 30 hours, at the least. Hessian crucibles are preferred for this purpose, and the heat of an ordinary pottery or porcelain kiln is sufficient in most cases; but a small wind-furnace, devoted exclusively to the purpose, is, in general, more convenient. It is found that the more tranquil, continuous, and uniform the fusion, the denser and clearer is the paste, and the greater its refractive power and beauty.
All the coloured vitreous compounds noticed under Glass may be worked up as ornamental stones, in the same way as those just referred to.
The following method of obtaining artificial rubies and emeralds, first pointed out by Boëttger, is exceedingly simple and inexpensive, and deserves the serious attention of those interested in this ingenious art:—Recently precipitated and well-washed hydrate of aluminum is moistened with a few drops of neutral chromate of potassium, and kneaded so that the mass assumes a tinge scarcely perceptible; it is then rolled up into small sticks, about the thickness of a finger, and slowly dried, taking the precaution to fill the fissures (if any) that form during desiccation with fresh hydrate of aluminum. When perfectly dry, and after having been submitted to a gentle heat, one end of these sticks is brought into the termination of the flame of an oxyhydrogen blowpipe, until a portion of the mass is fused into a small globule. After the lapse of a few minutes, several minute balls form, having a diameter of some millimètres, and of such intense hardness that quartz, glass, topaz and granite, may be easily and perceptibly scratched with them. These, when cut and polished, appear, however, slightly opaque. By employing nitrate of nickel in lieu of chromate of potassium, green-coloured globules, closely resembling the emerald, are obtained.
By the substitution of oxide of chromium for chromate of potassium, Mr Cooley produced factitious gems of considerable hardness and beauty, though slightly opaque in some portion of the mass. The addition of a very little silica prevented, in a great measure, this tendency to opacity.
It may be observed that the beauty of
pastes of factitious gems, and especially the brilliancy of mock diamonds, is greatly depending upon the cutting, setting up, and the skilful arrangement of the foil or tinsel behind them. See Enamel, Foils, Gems, Glass, &c.
PAS′TELS. [Fr.] Coloured crayons.
PAS′TIL. Syn. Pastille; Pastillus, Pastillum, L. A lozenge or confection. The pastilles (PASTILLI) of French pharmacy, are merely ‘confectionery drops’ aromatised or medicated. The name is also given to mixtures or odorous substances made up into small cones and burnt as incense. (See below).
Pastils, Explo′sive. Fumigating pastilles, containing a little gunpowder. Used to produce diversion, but they often prove far from harmless.
Pastils, Fumiga′ting. Syn. Aromatic pastilles, Incense p.; Pastilli fumantes, P. odorati, L. Prep. 1. Benzoin, 4 oz.; cascarilla, 1⁄2 oz.; nitre and gum Arabic, of each 3 dr.; myrrh, 1 dr.; oils of nutmeg and cloves, of each 25 drops; charcoal, 7 oz.; all in fine powder; beat them to a smooth ductile mass with cold water, q. s.; form it into small cones with a tripod base, and dry them in the air.