Types of Window Glass for Glazing
The different types of glass for window glazing.
Window glass is of two sorts: plate glass and sheet or cylinder glass; the latter is the more common.
Sheet glass is made by blowing a cylinder of glass which for the larger sizes has to be about fifteen inches in diameter and seven or eight feet long; when cold the ends are cut off, the cylinder is cut open lengthwise and put into an oven; when hot it can be opened and spread out into a sheet; such a sheet will afford a piece of glass 40 by 60 inches, which is the largest regular size. Smaller sizes are cut out of large sheets. "Single thick" glass is about one-sixteenth of an inch thick, and "double thick" is about twice this thickness. The products of different mills differ in thickness as in everything else, so there is no exact uniformity of thickness. All glass is imperfect, showing streaks of irregular thickness, bits of dirt, and bubbles; it is all inspected at the factory and graded. Foreign glassmakers have each his own marks for quality, so there are no grades corresponding to ours; but American makers classify sheet glass as AA, A, B, and "stock sheets." The sizes for single-thick glass run up to 28 by 34, and of double thick to 40 by 60 inches.
Plate glass is not blown, but is cast in plates on iron tables fifteen by twenty-five feet; on these hot tables it is compacted and made of uniform thickness by a roller, which leaves it half or five-eighths of an inch thick; it is annealed first of all, and then ground with sand and emery to a thickness of five-sixteenths to a quarter of an inch thick, and then polished. There is, however, a thinner sort, about like double-thick sheet glass, called crystal plate, used for car windows and the like. Selected sheets of plate glass are reserved for silvering and are called mirror plate.
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