Glass is a combination of silica with two or more alkalies, alkaline earths or other oxides, as lime and soda. In its ordinary state glass is a solid body with a luster, is more or less brittle, and is commonly transparent. When softened by heat, it may be easily molded into any shape; it welds when red hot; and at a lower heat it may be cut with knives and scissors. But molten glass can be rapidly drawn out into threads hundreds of feet long, and these when cooled can be woven into a beautiful silky fabric. The different kinds of glass are made of slightly different materials. Window-glass including the crown, sheet and plate varieties, is made of silica, soda and lime; flint glass, often called crystal glass, of silica, potash and lead; and bottle glass, of silica, lime and alumina, with sometimes other materials added. Any of these kinds may be colored by the use of certain oxides of metals. Silica is found in its purest form in rock crystals; but all quartz, flint, sandstone and sand are made of it. Pure white sand is now generally used, the best in the world being found in Berkshire County, Mass.
The materials are sifted and mixed together, and then melted in large pots set into a furnace. These pots are made of clay, and about ten of them are placed in a furnace. Opposite each pot is a door through which the workman can fill it with material, and the pots are not taken out until used up, which happens after one or two months. The furnaces also, which are built of fire-proof bricks, are never allowed to cool until, after a year or two, they are worn out. To the other materials already in the pots about one fourth of their weight of broken glass is added; the furnace doors are closed and the fires raised to a white heat, which gradually melts the contents of the pots into liquid glass in about 24 hours. The heat is then allowed to go down until the glass becomes about as thick as paste, in which state it is kept while it is being used by the workmen.
If bottle glass is to be made, a workman now dips a long iron tube called a blowpipe into a pot, and takes up a “gathering” or enough material to make a bottle. Another workman brings this into a pear shape by slightly blowing and turning it on a stone or iron table, called a marver. The glass is now placed into a mold and made to fill it by blowing down the tube; the bottom is pushed up, and the ring around the mouth is made by the addition of a strip of metal. This process was formerly done by hand. When the glassblower has finished a bottle, it is immediately taken to the annealing oven, where it remains about 36 hours, while it cools very gradually to the ordinary temperature of the air. Articles made in this way, as wine-glasses, tumblers etc., are often subjected to the process of glass-cutting, which is really grinding. The glass is held against a cast-iron wheel upon which a thin stream of sand and water is dripping. The sand leaves rough marks on the glass, which are smoothed out on other wheels and finally polished on a wooden wheel with a soft powder made of the rust of tin and lead. Glass is sometimes engraved by means of the sand-blast. For making plate-glass, crown-glass and sheet or cylinder glass different processes are required. Stained glass, seen mostly in church windows, is made by painting the surface of clear glass with various materials, chiefly the oxides of metals mixed with oil of turpentine. The pictures are painted with brushes, and the glass is then heated until the colors become stained into it. Another kind, called mosaic-glass, is made of a great number of small pieces set in lead frames.
The invention of glass dates from the earliest times, and several nations claim the honor of its discovery. But it probably is due to the Egyptians, among whom glass is mentioned as early as 3300 B. C. They generally made it into beads, vases, small figures, etc. Next came the Phœnicians, and from these two centers the art spread into neighboring centers. Glass making was introduced among the Romans about the beginning of the empire, though glass had long been imported. As it declined in Rome with the decay of the empire, it was transferred to Constantinople, where it flourished during the dark ages. Next the Arabs obtained a knowledge of glass making, and the glass of Damascus became celebrated during the middle ages. Venetian glass also early became famous, and mirrors, goblets and cups from Venice were sent all over the world. Extensive glass works were soon in operation in Germany, Prance and Great Britain. The first attempt at the manufacture in the United States was at Jamestown, Va., in 1608, and others soon followed. The chief centers of the manufacture in the United States are Pennsylvania, New Jersey and New York. A large part of Pennsylvania glass comes from Pittsburg. Pressed glass is an American invention. With a few exceptions the United States is fully up to Europe in this industry. The production of all kinds in 1909 was $92,185,693. See Sanzay's Marvels of Glass-Making and Wallace-Dunlop's Glass in the Old World.