are practically colourless, sand, limestone and sulphate and
carbonate of soda are used. The following is a typical analysis
of high quality bottle-glass: SiO2, 69.15%; Na2O, 13.00%;
CaO, 15.00%; Al2O3, 2.20%; and Fe2O3, 0.65%. For the
commoner grades of dark-coloured bottles the glass mixture
Fig. 18.—Tool for moulding the inside and outside of the neck of a bottle.
C, Bottle.
A, Conical piece of
iron to form the
inside of the
neck.
B, B, Shaped pieces
of iron, which can
be pressed upon
the outside of
the neck by the
spring-handle H.is cheapened by substituting common salt for part of the sulphate
of soda, and by the addition of felspar, granite, granulite,
furnace slag and other substances fusible at a high temperature.
Bottle moulds are made of cast iron, either in two pieces, hinged
together at the base or at one side, or in three pieces, one
forming the body and two pieces forming the neck.
A bottle gang or “shop” consists of five persons. The “gatherer” gathers the glass from the tank furnace on the end of the blowing-iron, rolls it on a slab of iron or stone, slightly expands the glass by blowing, and hands the blowing iron and glass to the “blower.” The blower places the glass in the mould, closes the mould by pressing a lever with his foot, and either blows down the blowing iron or attaches it to a tube connected with a supply of compressed air. When the air has forced the glass to take the form of the mould, the mould is opened and the blower gives the blowing iron with the bottle attached to it to the “wetter off.” The wetter off touches the top of the neck of the bottle with a moistened piece of iron and by tapping the blowing iron detaches the bottle and drops it into a wooden trough. He then grips the body of the bottle with a four-pronged clip, attached to an iron rod, and passes it to the “bottle maker.” The bottle maker heats the fractured neck of the bottle, binds a band of molten glass round the end of it and simultaneously shapes the inside and the outside of the neck by using the tool shown in fig. 18. The finished bottle is taken by the “taker in” to the annealing furnace. The bottles are stacked in iron trucks, which, when full, are moved slowly away from a constant source of heat.
The processes of manipulation which have been described, although in practice they are very rapidly performed, are destined to be replaced by the automatic working of a machine. Bottle-making machines, based on Ashley’s original patent, are already being largely used. They ensure absolute regularity in form and save both time and labour. A bottle-making machine combines the process of pressing with a plunger with that of blowing by compressed air. The neck of the bottle is first formed by the plunger, and the body is subsequently blown by compressed air admitted through the plunger. A sufficient weight of molten glass to form a bottle is gathered and placed in a funnel-shaped vessel which serves as a measure, and gives access to the mould which shapes the outside of the neck. A plunger is forced upwards into the glass in the neck-mould and forms the neck. The funnel is removed, and the plunger, neck-mould and the mass of molten glass attached to the neck are inverted. A bottle mould rises and envelops the mass of molten glass. Compressed air admitted through the plunger forces the molten glass to take the form of the bottle mould and completes the bottle.
In the case of the machine patented by Michael Owens of Toledo, U.S.A., for making tumblers, lamp-chimneys, and other goods of similar character, the manual operations required are (1) gathering the molten glass at the end of a blowing iron; (2) placing the blowing iron with the glass attached to it in the machine; (3) removing the blowing iron with the blown vessel attached. Each machine (fig. 19) consists of a revolving table carrying five or six moulds. The moulds are opened and closed by cams actuated by compressed air. As soon as a blowing iron is in connexion with an air jet, the sections of the mould close upon the molten glass, and the compressed air forces the glass to take the form of the mould. After removal from the machine, the tumbler is severed from the blowing iron, and its fractured edge is trimmed.
Compressed air or steam is also used for fashioning very large vessels, baths, dishes and reservoirs by the “Sievert” process. Molten glass is spread upon a large iron plate of the required shape and dimensions. The flattened mass of glass is held by a rim, connected to the edge of the plate. The plate with the glass attached to it is inverted, and compressed air or steam is introduced through openings in the plate. The mass of glass, yielding to its own weight and the pressure of air or steam, sinks downwards and adapts itself to any mould or receptacle beneath it.
The processes employed in the manufacture of the glass bulbs for incandescent electric lamps, are similar to the old-fashioned processes of bottle making. The mould is in two pieces hinged together; it is heated and the inner surface is rubbed over with finely powdered plumbago. When the glass is being blown in the mould the blowing iron is twisted round and round so that the finished bulb may not be marked by the joint of the mould.
Fig. 19.—Owens’s Glass-blowing Machine. g,g,g, Blowing-irons.
III. Mechanically Pressed Glass. (A) Plate-glass.—The glass popularly known as “plate-glass” is made by casting and rolling. The following are typical analyses:
| SiO2. | CaO. | Na2O. | Al2O3. | Fe2O3. | |
| French English |
71.80 70.64 |
15.70 16.27 |
11.10 11.47 |
1.26 0.70 |
0.14% 0.49% |
The raw materials for the production of plate-glass are chosen with great care so as to secure a product as free from colour as possible, since the relatively great thickness of the sheets
