=159. Single- and Double-Thick Common Glass.=—In the manufacture of the ordinary window glass used in building construction, the glass is first blown in cylindrical form and then cut and flattened, after which it is reduced to salable sizes for shipment. Owing to this method of manufacture, the percentage of first-quality glass is small, the larger proportion of the product being second or third quality. All glass of this kind is wavy or uneven and also has a slight bow, or curve, to its length that is impossible to overcome when making. The only difference in the qualities of common glass is in the amount of the imperfections that it contains. These imperfections consist of bull’s-eyes, blisters, scratches, and streaks that distort objects seen through the glass. The better grade of common glass is manufactured in what are called continuous tanks, which are heated by natural gas. This glass is put up in boxes containing 50 square feet. =Single-thick glass= runs from thirteen to fifteen lights to 1 inch of thickness, and =double thick= from eight to ten lights to the inch. Single-thick glass above 20 in. × 24 in. in size should not be used except for work of a speculative nature. The ability to distinguish single and double thick and the various qualities of glass can only be acquired by observation. In judging glass, it is well to remember that small lights free from defects or uneven surfaces may be obtained much more easily than large ones. =160. Ground, or Sand-Blast Glass.=—Common or plate glass, the surface of which is made obscure by the process of grinding, is called =ground glass=. This treatment, however, does not injure the diffusing qualities of the glass. The =sand-blast process= of treating glass produces results similar to grinding. As the name implies, sand is blown against the glass under high pressure and thus makes numerous small cuttings on its surface, producing the appearance of ground glass. [Illustration: FIG. 159] =161. Chipped Glass.=—The glass illustrated in Fig. 159 is called =chipped glass=, and is manufactured in the following manner: The glass is first given a coat of glue and is then placed in a heated receptacle; this process tends to contract and curl the glue, so that when it dislodges itself from the glass, the cohesion will draw or chip off particles of glass, leaving the surface uneven and showing a design similar to that on a frosted window light. Chipped glass can be procured in either the single or the double process (i. e., with a single or a double chipping); and these processes are adaptable to either sheet or plate glass, white or colored. [Illustration: FIG. 160]