=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]
=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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: FIG. 164]
[Illustration: FIG. 165]
[Illustration: FIG. 166]
[Illustration: 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.