Open main menu

Page:Popular Science Monthly Volume 34.djvu/630

This page has been proofread, but needs to be validated.
612
THE POPULAR SCIENCE MONTHLY.

flattened into something the shape of an enormous decanter. The bottom is very flat, and has the bullion-point exactly in the center. The pipe and its ungainly burden are now permitted to rest horizontally on two iron supports. In the mean time a second workman has gathered a small lump of glass on the end of his iron rod, or "ponty," and by pressing it against an iron point forms it into the shape of a tiny cup. This is fitted over the bullion-point of the glass, and, as they are both hot, soon becomes firmly attached to it. The glass has now two handles, but one of these, the blow-pipe, is speedily separated from it by means of a sharp blow. The open neck which is thus exposed is known, in the glass-maker's parlance, as the "nose." It gives its name to the furnace where it is subsequently reheated. During this operation the ponty is constantly and rapidly revolved. The nose gradually expands under the combined action of heat and centrifugal force. The opening grows larger and larger until the glass assumes the shape of a typical crown. This appearance, however, remains but an instant. One sees in its place a brilliant circular plate of glass whose shape is only maintained by continuing the rotation of the ponty until the plate, or table, as it is now called, can be placed upon a flat support. The ponty is then detached from the bullion-point by means of shears. The mark that is left is known as the bull's-eye. The tables vary in size from a few inches in diameter up to six feet, but this latter dimension is extreme. After annealing they are cut into panes by means of a diamond. The loss involved in the operation more than counterbalances the admirable brilliancy of the material. At the present time, the circular tables, just as they come from the annealing oven, are being used in decorative windows with the most excellent effect. Frequently the glass is tinted, or else it is left colorless itself, and the bull's-eye is either shaded or opalescent.

The window-pane of the rich is commonly plate glass. Of the three varieties, this is by far the most desirable in everything except, it must be added, the price. Though similar in composition to the sheet and the crown glass, its fabrication is carried out upon a totally different principle. Instead of being, like them, the result of the blower's breath, the plate glass is cast into a flat sheet and then ground and polished, a process of manufacture which at once accounts for its expensiveness. The best practice to be seen anywhere in America, if not in the world, is at Creighton, some twenty miles north of Pittsburgh. It is near the wellknown natural-gas district of Tarentum. There are a number of large establishments in this country where plate glass is manufactured, but the Creighton plant enjoys the reputation of possessing the most favorable economic conditions as well as the best equipment.