The property of glass to display a variety of tints by the addition of metallic oxides, is made use of for the production of artificial gems. The percentage composition of the base used is SiO2 38.10; K20, 7 - 90; PbO, 53.0; A1202,Fe2O2, 1 - 00. (Powell.)
10 lb. broken glass is melted, and to it are added 0.15 lb. copper oxide, 0.15 lb. each of the oxides of chromium and of manganese, 0.02 lb. each of oxide of cobalt and nitrate of silver, 0.01 lb. oxide of uranium, 0.4 lb. red argols, 0.3 lb. bone meal. Each oxide is added alone and at intervals of 10 minutes. After heating the mixture for an hour, 0.3 or 0.4 lb. of fine soot is put in.
Artificial turquoises are made in Paris and Vienna that cannot be distinguished by external appearances from the natural product, and when artistically made can only be distinguished by means of the file, being usually softer. They are made from alumina phosphate and copper phosphate mixed together and subjected to hydraulic pressure. Even in chemical composition they resemble the natural mineral, which is a hy - drated alumina phosphate with 2 Per cent. of copper oxide. (Diamant.)
The primary materials from which the different kinds of artificial stones are made on the Continent is as follows:-
Pare pulverized quartz ..
Pure dry soda carbonate ..
22 . 8
Calcined borax .....
7 . 6
Pure red - lead (minium)
11 . 8
These substances are thoroughly mixed together, introduced into a Hessian crucible, and heated to vivid redness in a charcoal fire. When the mixture has been thoroughly melted, the product is a transparent crystal glass of very great brilliancy. In order to imitate the various precious stones, cer tain oxides or carbonates are added to the above ingredients in the following proportions by weight: -
According to Donault, a magnificent description of artificial ruby can be obtained in the following manner: -Of the mixture given above to produce the white transparent basis of all these stones, 31 parts by weight are taken, which, finely pulverized, are intimately mixed with 2.30 parts antimony glass and 0.05 purple of Cassius. On cooling this mixture, after being well melted, it sometimes gives a transparent mass, and at others an opaque product. When transparent, it is a brilliant imitation of the topaz; when opaque, it forms a splendid imitation ruby by melting 1 part of it with 8 of the primary material (1st formula given above). The product when taken from the crucible is in the form of a brilliant yellow crystal, but before the blow - pipe it is transformed into a vivid red crystal, having exactly the tint of the oriental ruby. (Eisner.)
(1) Annealed glass vessels may be subjected to a variety of processes after they have become cold. The mark of fracture left at the base of a blown - glass vessel by the working - iron, is removed by pressing it upon the edge of a swiftly - revolving stone wheel. After the inequality is removed, the roughness is polished away by substituting a wooden wheel for the stone one. Cutting and engraving are modified forms of the same process. The difference of effect lies in the greater depth of incision produced in cutting. In either process, lathes are used, in which the glass is pressed against the fitting - tools; these are wheels revolving rapidly in a perpendicular plane. In cutting, the lathes are driven by steam, and the cutting - wheels are of considerable dimensions. The actual cutting is performed by iron wheels supplied from hoppers with sand and water. The incisions produced by iron wheels are smoothed by stone wheels supplied with water, and are polished by wooden wheels supplied with water and emery - powder, putty - powder, pum - ice, or rouge. For engraving, the lathes are usually worked by foot - treadles, and the wheels are of copper, and in some cases do nut measure more than 1/2 in. in diameter.
In engraving, it is customary to leave the pattern rough and the ground clear; this arrangement, however, may be reversed, by polishing the pattern with leaden wheels supplied with oil and rouge, and by previously roughening the ground. (Powell.)
(2) Glass can be cut under water, with great ease, to almost any shape, by simply using a pair of shears or strong scissors. In order to ensure success, 2 points must be attended to - first and most important, the glass must be kept quite level in the water while the scissors are applied; and secondly, to avoid risk, it is better to begin the cutting by taking off small pieces at the corners and along the edges, and so reduce the shape gradually to that required, as if any attempt is made to cut the glass all at once to the shape, as we should cut a piece of cardboard, it will most likely break just where it is not wanted. Some kinds of glass cut much better than others, the softer glasses being the best for this purpose.
The scissors need not be at all sharp, as their action does not appear to depend on the state of the edge presented to the glass. When the operation goes on well, the glass breaks away from the scissors in small pieces in a straight line with the blades. This method of cutting glass has often been of service when a diamond has not been at hand, for cutting ovals and segments, and though the edges are not so smooth as might be desired for some purposes, yet it will answer in a great many cases.
(1) For drilling holes in glass a common steel drill, well made and well tempered, is the best tool. The steel should bo forged at a low temperature, so as to be sure not to burn it, and then tempered as hard as possible in a bath of salt water that has been well boiled. Such a drill will go through glass very rapidly if kept well moistened with turpentine in which some camphor has been dissolved. Dilute sulphuric acid is equally good, if not better. It is stated that at Berlin glass castings for pump - barrels are drilled, planed and bored like iron ones, and in the same lathes and machines, by the aid of sulphuric acid. A little practice with these different plans will enable the operator to cut and work glass as easily as brass or iron.