THE beryl glass mentioned in the previous chapter marks the transition stage between manufactured stones which in all essential characters are identical with those found in nature, and artificial stones which resemble the corresponding natural stone in outward appearance only. In a sense both sorts may be styled artificial, but it would be misleading to confound them under the same appellation.
Common paste, which is met with in drapery goods and cheap ornaments in generalhat-pins, buckles, and so forth is composed of ordinary crown-glass or flint-glass, the refractive indices being about 1.53 and 1.63 respectively. The finest quality, which is used for imitations of brilliants, is called ‘ strass.’ It is a dense lead flint-glass of high refraction and strong colour – dispersion, consisting of 38.2 per cent. of silica, 53.3 red lead (oxide of lead), and 7.8 potassium carbon-ate, with small quantities of soda, alumina, and other substances. How admirable these imitations may be, a study of the windows of a shop devoted to such things will show. Unfortunately the addition of lead, which is necessary for imparting the requisite refraction and ` fire’ to the strass,
renders the stones exceedingly soft. All glass yields to the file, but strass stones are scratched even by ordinary window-glass. If worn in such a way that they are rubbed, they speedily lose the brilliance of their polish, and, moreover, they are susceptible to attack by the sulphurous fumes present in the smoky air of towns, and turn after a time a dirty brown in hue. When coloured stones are to be imitated, small quantities of a suitable metallic oxide are fused with the glass ; cobalt gives rise to a royal-blue tint, chromium a ruby red, and manganese a violet. Common paste is not highly refractive enough to give satisfactory results when cut as a brilliant, and the bases are therefore often coated with quicksilver, or, in the case of old jewellery, covered with foil in the setting, in order to secure more complete reflection from the interior. The fashioning of these imitation stones is easy and cheap. Being moulded, they do not require cutting, and the polishing of the facets thus formed- is soon done on account of the softness of the stones.
A test with a file readily differentiates paste stones from the natural stones they pretend to be. Being necessarily singly refractive, they are, of course, lacking in dichroism, and their refractivity seldom accords even approximately with that of the corresponding natural stone.
In order to meet the test for hardness the doublet was devised. Such a stone is composed of two partsthe crown consisting of colourless quartz or other inexpensive real and hard stone, and the base being made up of coloured glass. When the imitation, say of a sapphire, is intended to be more exact, the crown is made of a real sapphire, but one deficient in colour, the requisite tint being obtained from the paste forming the under part of the doublet. In case the base should also be tested for hardness the triplet has been devised. In this the base is made of a real stone also, and the coloured paste is confined to the girdle section, where it is hidden by the setting. Sapphires and emeralds of indifferent colour are sometimes slit across the girdle ; the interior surfaces are polished, and colouring matter is introduced with the cement, generally Canada balsam, which is used to re-unite the two portions of the stone together. All such imitations may be detected by placing the stone in oil, when the surfaces separating the portions of the composite _ stone will be visible, or the binding cement may be dissolved by immersing the stone, if unmounted, in boiling water, or in alcohol or chloroform, when the stone will fall to pieces.
The glass imitations of pearls, which have become very common in recent years, may, apart from their inferior iridescence, be detected by their greater hardness, or by the apparent doubling of, say, a spot of ink placed on the surface, owing to reflection from the inner surface of the glass shell. They are made of small hollow spheres formed by blowing. Next to the glass comes a lining of parchment size, and next the under lining, which is the most important part of the imitation, consisting of a preparation of fish scales called Essence d’ Orient. When the lining is dry, the globe is filled with hot wax to impart the necessary solidity. In cheap imitations the glass balls are not lined at all, but merely heated with hydrochloric acid to give an iridescence to the surface; sometimes they are coated with wax, which can be scraped off with a knife.