A well-known transparent and brittle factitious substance, of which the basis is silica, brought into complete fusion by the addition of one of the fixed alkalies. There are several different kinds of glass, adapted to different uses. The best and most beautiful, are the flint and the plate glass; these, when well made, are perfectly transparent and colourless, heavy and brilliant. They are composed of fixed alkali, pure silicious sand, calcined flints, and litharge, in different proportions. The flint glass contains, likewise, a large quantity of oxide of lead, which, by certain processes, is easily separated.
Crown glass is that used for windows, and is made without lead, chiefly of fixed alkali fused with silicious sand, to which is added some black oxide of manganese, which is apt to give the glass a tinge of purple.
Bottle glass is the coarsest and cheapest kind; into this little or no fixed alkali enters the composition: in this country it is composed of sand, and the refuse of the soap boiler, which consists of the lime employed to render his alkali caustic, and of the earthy matters with which the alkali was contaminated. The most fusible is flint glass, and the least fusible is bottle glass; flint glass melting at the temperature of 10o Wedgwood, crown glass at 30o, and bottle glass at 47°. Although glass when cold is exceedingly brittle, when heated to redness it becomes one of the most ductile bodies known, and may be drawn into threads so very delicate as to become almost invisible to the human eye: it is extremely elastic, and one of the most sonorous of bodies. In making glass, the materials undergo a preparatory process called fritting, which consists in mixing them in the proper proportions, and submitting them to a moderate heat for six hours, by which they are reduced to a pasty consistence, and form what is called frit, which is cut into squares, and stored up for use; and as the quality of the glass depends upon the age of the frit, the principal manufacturers endeavour always to keep a considerable stock of frit on hand.
This frit is introduced into large pots made of prepared clay, and in these is exposed to a heat sufficient 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 very quickly; hence they are put into a furnace, that the heat may pass off very gradually, and this is called "annealing." Having said thus much to give a general idea of the process, we shall now proceed to describe, somewhat in detail, the manual operations in the manufacture of glass; observing that, owing to the excise laws, the different branches of the manufacture, viz. crown, flint, plate, and bottle glass, are not carried on at the same works, but require to be in separate establishments; we shall, therefore, select for description the manufacture of flint glass, by which the various utensils in glass are produced.
The other branches differ principally in the number and arrangement of the fur-naces; but in all of them, with the exception of plate glass, the glass is brought into the desired form principally by means of a blowing pipe: 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 it on the end of the tube; 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, and by varied management of these globes while in a soft state, and with the aid of a few simple tools, they are reduced into the forms of the different vessels in common domestic use. The first thing to be described is the furnace: it consists of two large domes set one over the other; the lower one stands over a long grating, which is on a level with the ground; on this grating the fuel is laid, and beneath it is a large arch, by which the air is admitted, and by which the ashes may be removed.
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 reverberates the flame from the roof down upon 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 brickwork; there is a small chimney opens from the top of the lower dome into the middle of the floor of the upper one, which conveys the smoke away from it, and a flue from the upper dome leads it completely 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 leads up to each.
The implements employed in the formation of glass vessels are few and simple; the following are the principal: a blowing-pipe, which is simply a tube of wrought iron about three feet long, and covered with twine towards the mouth-piece, pliers and calliper compasses, a pair of common shears to cut the glass, a very coarse flat file, and several small iron rods; there is also a bench or stool with two arms, another stool or table with a smooth cast-iron plate upon it, and upon the ground behind this stool is another plate of iron.
We shall now proceed to describe, somewhat more minutely, the manufacture of flint glass. The melting pots are charged with frit, thrown in by shovelsful from time to time, allowing each portion to melt before a fresh quantity is added. When the whole is converted into a clear transparent glass, and is become perfectly pure and free from particles of sand or bubbles of air, the gatherers and blowers commence operations, and continue working night and day until the batch is exhausted. The process of blowing is varied accord' ing to the form of the piece to be manufactured; to illustrate it, it will be sufficient to describe the method in which a wine glass is formed. When the blower has received from the gatherer the blowing pipe charged with a sufficient quantity of metal, he seats himself in a chair provided with two arms or elbows, one of which is plated with iron, and placing the blow pipe across the elbows, 80 that the heated end may rest on the iron plate, having first formed the glass into a hollow ball, he rolls the pipe backwards and forwards, and laying hold of the glass on the farther side of the ball with a small pair of pliers, draws it out to form the stalk of the glass, while the part next the blow pipe is fashioned into the bowl.