Page:Popular Science Monthly Volume 37.djvu/172

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beauty, since the dense lead compounds have a tendency to separate from the lighter silicates, and, consequently, if present in too large amounts, they make the glass streaky and mottled. In general, lead glass for domestic uses has a specific gravity of from three to four—that is, it is from three to four times as heavy as an equal bulk of water. The brilliancy given to the glass by its increased density has attached the name crystal to this particular product.

It is essential that the several ingredients should be thoroughly mixed, and to this end the operation is carried out mechanically.PSM V37 D172 The process of engraving on glass.jpgFig. 2.—The Process of Engraving on Glass. The materials are fed into the upper end of a slowly revolving hopper, whose axis is slightly inclined to the horizontal, and are thoroughly mixed by the time they reach the discharging end. A dainty pink powder falls into the receiving bins. Its subsequent baptism by fire transforms the opaque into the transparent. The furnaces employed for this purpose are of the type common to other glass-melting processes—simply a circular and intensely heated chamber, surmounted by a stack, and provided with radial openings to permit the blowers to dip their blow-pipes into the molten contents of the fire-clay crucible-pots.

The scene around this industrial caldron is quite as busy as that which has its center in the bottle furnace, and is even more varied. The workers are fashioning objects of the most diverse shape and for the most unlike purposes. Some are blowing lamp-chimneys, others gaslight globes, or decanters or dishes. In the center of the apartment a large press, with engraved steel dies, is squeezing the plastic "metal"—for so the glass-blower designates his still fluid glass—into decorative panels for car-windows and transoms. As one passes from one end of the large room to the other, he will see almost every conceivable shape in glass, suited