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Page:Popular Science Monthly Volume 34.djvu/631

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613
GLASS-MAKING.

In the selection of the crude materials great care is taken to secure purity. Tlie melting is carried out in large, open pots, the furnaces differing in their construction from those already described only in their greater size, and in the substitution of iron doors lined with tiles for the customary gathering-holes. When the melting is completed, the door nearest the pot to be emptied is opened, and a two-pronged fork, mounted on wheels, is inserted into the furnace. The prongs fit into depressions in the sides of the melting-pot, and thus secure it in a firm grasp. The pot of molten metal is then removed from the furnace and carried on a low truck to the casting table. At Creighton, the casting house, containing furnaces, tables, and annealing ovens, is 650 by 160 feet, about four times as large as the famous halle of Saint-Gobain in France, and nearly double the size of the British casting house at Ravenshead. The capacity of the American works has recently been greatly increased, and several new plants established in different sections of the natural-gas territory. The casting tables, the most important pieces of apparatus in a plate-glass works, are nineteen feet long, fourteen feet wide, and seven inches thick. Each is provided with an iron roller, thirty inches in diameter and fifteen feet long. Strips of iron on each side of the table afford a bearing for the rollers and determine the thickness of the plate of glass to be cast. The rough plate is commonly nine sixteenths of an inch in thickness; after polishing, it is reduced to six or seven sixteenths. The casting tables are mounted on wheels and run on a track that reaches every furnace and annealing oven in the building. The table having been wheeled as near as possible to the melting furnace, the pot of molten glass is lifted by means of a crane, and its contents quickly poured on the table. The heavy iron roller is then passed from end to end, spreading the glass into a layer of uniform thickness. The whole operation of casting scarcely occupies more time than it takes to describe it. Each movement is made with almost nervous rapidity. Few industries offer such fine scenic display as the pouring of the molten glass. One feels like crying Encore! it is so very brilliant.

In contact with the cold metal of the table, the glass cools rapidly. As soon as possible, the door of the annealing oven is opened, and the plate of glass introduced. The floor of the oven is on the same level as the casting table, so that the transfer can be conveniently and quickly made. When, after several days, the glass is taken out of the oven, its surface is found to be decidedly rough and uneven. A small quantity is used in this condition for sky-lights and other purposes where strength is required without transparency. It is known in the market as rough plate. The greater part of the glass, however, is ground, smoothed, and polished before it leaves the establishment. The work of the "hall