The moulds are usually made of iron or gun-metal. When the shape of an article to be pressed necessitates the division of the mould, the several pieces are so hinged together, that the article can be liberated with the greatest ease, and the joints are so fitted, that the glass may be as little marked by them as possible.
In addition to the saving effected by the use of the press, and by the substitution of unskilled for highly trained labour, a considerable reduction in coat has been brought about by the introduc tion of cheap substitutes for the raw materials originally used. The manufacturer of pressed-glass aim at the production of a glass rivalling flint-glass in clearness and whiteness, and surpassing it in softness, and in the power of retaining heat. Originally the same ingredients were used for pressed-glass as for flint-glass, the flint-glass for pressing being softened by the addition of borax in considerable quantity.
Flint-glass is expensive, owing to the large proportion of red-lead used in its composition. A substitute for red-lead has been found in certain of the salts of barium. The following recipe is the subject of a patent, and the resultant glass is said to be one-half less expensive than flint-glass, but to be equal in transparency, clearness, and brilliancy :—Sand, 17 ; carbonate of soda, 4; carbonate of barium, 6; borax, 2. Another raw material used for pressed-glass is cryolite, a compound of fluoric acid, water, soda, and alumina. If 4 parts of cryolite be added to 1 of oxide of zinc, and 10 of sand, a milk-white opal will be produced, transparent for light rays, but cutting off the red rays. If a smaller quantity of cryolite be added, a white transparent glass will be the result, of great brilliancy, strength, and refractive power. If, on the other hand, more than 4 parts of cryolite be used, an opaque white mass will be obtained, which, in appearance, closely resembles china or glazed earthenware. This opaque substance is now pressed into a great variety of useful and ornamental articles. It can be coloured by various metallic oxides.
Toughened discovery of a process by which either flat or shaped glass may be rendered less liable to breakage is due to M. de la Bastie. His process is to heat glass to the point
of plasticity, and immediately to plunge it into a heated bath of molten fat. The temperature of the bath must be adjusted to the chemical nature of the glass. The temperature for a soft glass is 68°-75° F.). For the bath, mutton-fat is preferred ; before being fit for use, it requires to be melted for at least 12 hours. The simplest form of the process is that used in the treatment of open-shaped vessels, such as tumblers and finger-basins. These are treated in the course of manufacture, being dropped into the bath, instead of being sent to an annealing-oven. For this purpose, a bath, Fig. 796, heated to the necessary temperature, either by a small gas-stove or by the insertion of hot metal, and containing a lining of wire net, is placed as near to the workman as possible. The heat of the bath, once acquired, is maintained by the heat of the vessels immersed in it. When the wire net is full, the bath is allowed to cool down to about 45° (113° F.); the glasses are taken out in the net, and arranged on sieves in an iron closet, which can be heated. The temperature is raised to about 70° (158° F.), causing the fat, which still adheres to the glasses, to drip through the sieves into a tank beneath, where it is collected for future use. From the heated closet, the glasses are removed to a tank containing caustic soda in solution, which is also slightly heated ; and thence to a bath of warm water. ' One of the first and most serious obstacles encountered in the appli cation of the discovery to vessels of general utility was the difficulty of expelling air, contained in bottles and other utensils with narrow apertures, sufficiently quickly to allow the interior and exterior surfaces of the glass to be simultaneously affected by the liquid. The manner in which this difficulty has been overcome, in the case both of large and of small vessels, will be understood by reference to Figs. 797 and 798. In Fig. 797, A represents the bath, B a bent tube, supported on guides at P, and having a pocket M to receive any of the liquid which may enter the end of the shorter arm ; E is the surface of the liquid.