GLASS BLOWING. There are three principal kinds of glass—window glass, plate glass and bottles and jars or hollow ware.
Separate factories, or at least separate depart ments of factories, are devoted to each of these classifications. A glass furnace is fitted up for the kind of work it is designed to do. Previous to 1889, the furnace was almost al ways a great circular structure in the centre of a more or less circular building, with aper tures on several sides, so that a number of men could work around it. The bottom of such a furnace is made of clay and is termed a pot. The fuel may be wood, coal, oil, natural gas, almost anything, but natural gas is the cheapest, and the glass industry thrives where gas is plentiful and cheap. In 1889 the continuous tank furnace was introduced, each tank having a capacity of about 15 large pot furnaces. In making window glass, which is also termed cylinder glass and sheet glass, a quantity of molten glass is taken up at the end of a long tube called a ponty. This the blower dips into the furnace, and by adroit manipulation brings out a sizable mass of semi-molten glass nearly white hot. Putting the other end of the ponty in his mouth, the glassblower uses all his lung power in rapid blowing, causing the glass at the other end to swell out very much as a child blows a soap bubble. He stands over a small pit, and blows and whirls and swings his ponty, with the hot glass bubble growing bigger and bigger, and by reciprocating it vigorously in the pit, the bubble elongates and soon assumes the form of a cylinder. When it is sufficiently large and has also sufficiently cooled, this cylinder of hot glass may be swung on to a flat table, slit longitudinally with a knife and will settle down in a flat sheet. It may later be annealed and cut into required sizes.
Hollow Bottle glass, jars, tumblers and hollow ware generally are also made by blowing, taking the molten glass from the furnace on a ponty, but only a little glass is taken at a time, sufficient for one bottle or whatever is to be blown. After the blower has formed this into a hot bulb perhaps half the size of the finished bottle, he puts it into a mold, which he closes with his foot, and blows until the glass fills the mold, which de termines its outer form. This is the hand
method of blowing bottles. The machine method was evolved about 1896, and has gradu ally come into extended use, until now a large proportion of hollow ware is mechanically blown. Machine-blown bottles can usually be distinguished from hand-blown by the wide mouths which are essential to the process. The glass bottle-making machine has a combina tion of molds mounted on a rotary table. The molds may be individual or double, but each mold has an outer blow-section, with a ring in which the neck of the bottle is pressed, and a telescopic press-section rising within the blow section and receiving the glass, forming, with the neck of the blow-section, a pressure-mold. The molten glass is dropped into the combined mold when in this pressmold position, and the table rotated to a point where the mold is un der a plunger, which enters and presses the neck and wind-cavity into the dependent mass of hot glass. The plunger is then withdrawn, the telescopic section of the combined mold is dropped, and by another rotation the table brings the mold tinder a blow-stem, exposing the glass blank within the blow section. A bottom plate being iaserted, air under pressure is admitted to expand the glass blank to the form of the mold. At the next rotation 'of the table the finished article is taken off by an attendant and sent to the annealing oven. Such is the system. In practice, the machines handle from four to eight bottles at a time, and can thus make many more bottles than a man, so that the production of a glass furnace is greatly increased. Only a limited number of blowers, as eight, can work around a pot furnace, as each has to be in a position to reach the molten glass. Therefore, when a machine doing four men's work is put in the place of each man, the output of the furnace is increased four times.