GLASS AND GLAZING
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.
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.
Fig. 160
162. Figured Rolled Glass.—Glass known generally as figured rolled glass is manufactured for use in offices, public buildings, and private residences where a glass is required to intercept the vision and still diffuse the light. This glass has practically supplanted the varicolored cathedral glass previously used, because of its obscurity without reducing the quantity of light. Figured rolled glass is made in various artistic designs, several of which are illustrated in [Fig. 160], and is ⅛ and ³/₁₆ inch in thickness. This glass is inexpensive; the prices range from 15 to 25 cents per square foot.
163. Plate Glass.—The process of manufacturing plate glass is radically different from that of common sheet glass. The latter is blown, while the former is cast in large sheets and placed in annealing ovens to cool. When taken from the ovens the glass is rough and opaque, but it is afterwards ground and polished to make it transparent, the polishing being the most delicate process in its manufacture. Plate glass glistens like a mirror and reflects like one; objects seen through it are sharp and clean-cut; it has no imperfections or wavy effects, and does not distort and deform objects as does sheet glass.
164. Beveled Plate Glass.—The term beveled plate is applied to plate glass the edges of which are ground and polished to form a bevel, or border, around the glass. Plate glass finished in this manner is much used for glazing entrance doors and for ornamental work. The additional cost for beveling is slight compared with the effect resulting therefrom.
165. Floor and Skylight Glass.—Floor glass is made only from rough rolled or hammered glass in ½-, ¾-, and 1-inch thicknesses. The prices range from 30 cents to $1 per foot, according to the thickness.
Skylight glass is similar to floor glass, but can be procured in ⅛-, ³/₁₆-, ¼-, and ⅜-inch thicknesses, and in either ribbed or roughened surfaces. This glass is inexpensive, the prices ranging from 8 to 15 cents per square foot. Large quantities of skylight glass are used for mills, skylights, and various other purposes, but it is not used as a fire-retardant, as wire glass has entirely superseded it for this purpose.
166. Wire Glass.—Glass with wire embedded therein, as illustrated in [Fig. 161], is made either ribbed, rough-rolled, “maze” design, or clear, polished plate. The wire netting is embedded in its center during the process of manufacture, thus producing a very strong glass that is a good fire-retardant. The temperature at which the wire is embedded in the molten glass insures cohesion between the metallic netting and the glass, and the two materials become as one; thus, if the glass is broken by shock, by intense heat, or from some other cause, it remains practically intact. Wire glass combines the strength of the wire netting and the glass plate, and the wire is so thoroughly covered as to obviate the possibility of rust or corrosion. This glass possesses extraordinary strength; a piece ¼ inch thick is as strong as ordinary glass of twice that thickness. When wire glass is broken, it will not scatter like plate or skylight glass; consequently, it finds extensive use for overhead work, where falling glass due to accidental breakage would be a source of danger.
Fig. 161
Wire glass is made ¼, ⅜, and ½ inch thick, and up to 40 inches wide and 100 inches long. This glass is sold at from 30 to 40 cents per square foot for rough, ribbed, or maze pattern, according to the thickness, while polished wire glass costs either 60 cents, 95 cents, or $1.25 per square foot, according to the dimension of the light. Wire glass is practically burglar-proof and missile-proof, and when set in approved metallic frames forms a most efficient fire-retardant window. In its use as a fire-stop, it possesses advantage in that it does not hide the incipient blaze like steel-plate or tin-lined shutters. Windows glazed with wire glass require no shutters for protection, but an opening may be made in it by a blow from a fire-ax. Wire glass as a fire-retardant is approved by the various boards of fire underwriters, and it may be used for windows, skylights, and other exterior openings exposed to fire hazard.
Fig. 162
167. Prisms, or Prismatic Glass.—When a ray of light passes from air to glass, it undergoes the change in direction called refraction. The knowledge of this property of light is utilized in the design of prismatic glass, an example of which is illustrated in [Fig. 162]. Prisms are made of clear crystal glass, with the outer side usually plain. The inner side, however, is formed into prisms of a great variety of angles made to suit the several conditions under which the lights may be used. Prism plates are made in two styles: Pressed prism plates, which are composed of 4- or 5-inch pressed lenses that are glazed together with a drawn zinc or copper bar, the several bars forming a frame being brazed together electrically, and rolled, or sheet, prisms, which are first rolled in large sheets and then cut to size.
Under average conditions, the direct light from the sky entering a room through sheet or plate glass strikes the floor within a few feet of the window and is mostly absorbed. For this reason, the rear part of the room is dark. By the installation of prisms at the window openings, however, the direction of the entering light is changed and projected horizontally into the room. Prisms do not create light; they simply distribute and diffuse the excess light at the window. Under certain conditions the effective light in a room may be increased fifty times by the installation of prisms. In [Fig. 163], the arrows a show the direction of the lowest rays from the sky over the cornice of the opposite building, striking the prism plate at the floors. These arrows also indicate vertical rays, striking canopies and skylights, while the arrows b show the directions of the rays just mentioned after passing through the prism plate; also, the manner in which the light rays are projected into the building.
Fig. 163
168. Vault Lights for Basement, or Cellar, Lighting.—Where the sidewalk and the basement ceiling are nearly on a level, basement rooms can be successfully lighted by the installation of vault lights, or glass pavement tile. The ordinary method is to use plain, round, or square lenses, which are set in iron or concrete in iron frames, as illustrated in Figs. [164] and [165], though vault lights are now made with the supporting material of reinforced concrete. Where the ceiling is low or the room at all deep, the system in which plain lenses are used is unsatisfactory, as the direct light from the sky in passing through the lens strikes the floor immediately underneath.
Fig. 164
Fig. 165
Fig. 166
Fig. 167
The more modern method is to use prism tile set in the necessary frames. The prism receives the direct light from the sky on the upper face and turns, or refracts, it back into the basement, as illustrated in [Fig. 166]. Where the head-beam or other obstructions interfere with the deflection of light direct from the prism, it becomes necessary to use in connection with the skylight or canopy just described an independent vertical frame of window prisms of varying angles. These prisms intercept the light from the pavement tiles and project it into the basement, thus utilizing the light to the maximum advantage.
169. Glazier’s Points.—A necessary adjunct in glazing with putty is the sheet-zinc, triangular glazier’s points, as illustrated in [Fig. 167]. These points are made in six sizes, which range from No. 000, the largest, to No. 3, the smallest. They are also made in somewhat similar shape, called sharps, but this type is not so popular as the triangle point.
170. Grades of Putty.—Putty is usually graded as follows: Commercial, a very cheap grade; pure, a medium grade; and strictly pure, the best grade, which is made of pure whiting ground in linseed oil. The cheaper grades are made in the same manner, but with an amount of adulteration, either in oils or whiting, commensurate with the cost. Therefore, the cheaper the putty, the more it is adulterated, and none but strictly pure should be used when good results are desired.
White-lead putty is in a class by itself, and costs about twice as much as strictly pure whiting putty; but where permanent glazing is required, its use will show handsome returns on the investment.