In making glass, the materials are com pletely fused together, and in this state the hot mixture is called frit. The frit is introduced into large pots made of pre pared clay, and exposed to a beat suffici ent to melt it completely. When the fusion has continued the proper time, the furnace is allowed to cool a little. In this state the glass is exceedingly ductile, and will assume any shape, according to the fancy of the workman. The vessels thus formed must not be permitted to cool too quickly, hence they are put into a hot fur nace, in order that the heat may pass off very gradually ; this is called annealing.
Glass is often tinged of various colours, which is performed by mixing with it, while in fusion, some one of the metallic oxides. Thus blue glass is formed by the oxide of cobalt ; green by the oxide of iron, or copper ; violet by the oxide of manganese ; red by a mixture of the ox ides of copper and iron ; purple by the oxide of gold; white by the oxide of ar senic and zinc ; and yellow by the oxide of silver, and by combustible bodies.
We shall give now a detail ofthe manu al operations in the manufacture of glass.
Glass-blowiiar, the art of forming ves sels of glass ; the term, however, is exclu sively applied to those vessels which are blown by the mouth. The operation is exceedingly simple ; the workman has a tube of iron, the end of which he dips into a pot of melted glass, and thus gathers a small quantity of glass on the end of it ; he then applies the other end of the tube to his mouth, and blows air through it; this air enters into the body of the fluid glass, and expands it out into a hollow globe, similar to the soap bladders blown from a tobacco pipe. Various methods are used to bring these hollow globes in to forms of the different utensils in com mon domestic use, and several tools ; the chief part of these are represented. in PLATE-GLASS MAKING.
The first and greatest of the glass-blow ers' implements is the furnace ; it consists of two large domes set one over the other, the lower one stands over a long grating, (on a level with the ground) on which the fuel is placed ; beneath the grate is the ash pit, and a large arch, leading to it, con veys air to the furnace. In the sides of the lower dome, as many holes or mouths are made as there are workmen to make use of the furnace, and before each mouth a pot of melted glass is placed ; the pots are very large, like , crucibles, and will hold from three to four hundred weight of liquid glass ; they are supported upon three small piers of brickwork, resting on the floor of the furnace. The form rever
berates the flame from the roof down up on the pots, and they are placed at some distance within the furnace, that the flame may get between the wall and the pots. The upper dome is built upon the other, and its floor made flat by filling up round the roof of the lower dome with brick work ; there is a small chimney opens from the top of the lower dome into the middle ofthe floor of th e upper one, which conveys the smoke away from it, and a flue from the upper dome leads it com pletely from the furnace.
The upper dome is used for annealing the glass, and is exactly similar to a large oven, it has three mouths, and in different parts a small flight of steps lead up to each. We now come to describe the smaller implements.
Fig. 1 and 2, is a bench or stool with two arms a b at its ends, which are a little inclined to the horizon ; the operator, when at work, sits upon the stool, and lays his blowing tube d across the arms, as shown in the figure.
Fig. 3, are a pair of shears, or rather pliers, formed of one piece of steel ; they have no sharp edges, and spring open when permitted ; the workman has seve ral of these of different sizes, which are hung upon hooks ate in the fig. 4.
Fig. 4, is a pair of compasses to measure the work, and ascertain when it is brought to the proper size ; the workmen should have three or four of these.
Fig. 5, a common pair of sheers, for cut ting the soft glass.
Fig. 6, a very coarse flat file.
Fg. 7, is the blowing pipe ; it is simply a wrought iron tube, about three feet long, at x it is covered with twine, to pre vent it burning the workman's hand.
Fig. 8, a small iron rod, of which there should be several.
Fig. 9, is a stool with a flat plate of cast iron laid upon it, and f is another flat plate upon the ground behind the stool.
To explain the use of these tools, we shall describe the manner of forming a lamp or urn of glass, Fig. 10, with a wide mouth at top and a small necks at bot tom, through which the candle is inserted, and which is fitted into a brass cap to sup port the lamp by.