In this state the glass is removed from the fire, and the neck is cracked off, by resting the tube across the rod and turning it round, whilst the neck is touched with it piece of iron wetted in cold water. This produces a cir cular crack round the neck, which is separated from the tube, by laying the glass upon a table, shewn in Fig. 4, which is composed of two planes d, inclining towards each other, so as to receive the glass without danger of its roll ing off In front of the table is a sharp bridge or wedge a, over the edge of which the workman rests the glass at the place where it is cracked. In this situation, a slight stroke upon the rod breaks off the neck, and leaves the glass upon the table. The workman how dips the end of an iron rod o into the melted glass, and takes up as much as will make it adhere firmly to the promincace c, so as to serve for a spindle to turn the glass by. The glass be ing now thin, soon becomes so cool as to require heating, which is done h) presenting the open end to the flame, the workman resting the iod in the hook, and turning it slowly mound, that the glass may become equally heated. the open end is sufficiently softened, the workman retires from the fire, and supporting the rod over a rest or hook, he turns the glass steadily round, whilst a boy introduces an iron tool into the open end of the neck. The motion opens and enlarges the aperture to the form of Fig. 5 ; and they take great care, by turning with a regu lar motion, and pressing regularly with the iron tool with in the opening, to preserve the circular figure, whilst they form the glass into a sort of dish, such as is represented in Fig. 6. In this state, being carried to the furnace, and heated before a large mouth, whilst it is whirled rapid ly round, the centrifugal force throws it out into a flat circular plate a 1 7, of from 3 feet 6 inches to 4 feet dia meter.
When the plate is sufficiently cooled, the workman ap plies a cold iron, to crack the neck of the central projec tion which connects the plate with the rod, and he then lays the plate flat down upon a bed of ashes previously prepared. The rod or spindle is now separated by a gentle stroke, and the plate is taken up upon an iron fork, and conveyed to the annealing oven, where a great number are set up edgewise, and supported by iron frames to keep them flat. The fire of the oven is suffered to burn out, and the heat to diminish as slowly as possible, until the glasses become quite cold : They are then withdrawn from the furnace, by taking down a slight front wall built in the door or arch. The heat of the annealing oven, which is not sufficient to make the glass so soft as to bend by its weight, is continued for 48 hours, or longer.
Window plate, also called German plate, or table glass, is made by the same means of blowing and rolling ; but these are managed to produce a cylinder, which is cut open and spread flat upon a table.
In order to form glass plates by the extension of a cylin der, the workman blows the glass into the shape of a pear, as at Fig. 7. The length of this pear must be nearly equal to the length of the plate, and its diameter of such a size, that the circumference, when unfolded, will be equal to the breadth of the plate desired. He now supports the blowing iron over a stool or iron bar, whilst an assistant, with a pointed iron, pierces a hole into the extreme end of the pear, opposite the end of the blowing iron : This opening is enlarged by introducing the blade of a pair of spring tongs, as in Fig. 8. whilst the glass is turned round ; and, by a peculiar management of these, the end of the pear is at last opened out to a cylinder, as in Fig. The workman now mounts a stool, and holds the blowing-iron perpendicularly, whilst his assistant cuts open one sidle of the cylinder with a pair of cutting shears, as in Fig. 9. The blowing iron is next broken off, and a rod b, Fig. 10. applied to the end of the cylinder, to form a spindle to work the other end by. This rod has a flat circular plate upon the end of it, or three prongs c, c, c, which being dip ped in the melted glass, arc applied to the end of the cylinder. By this rod the glass is carried to the fire, and the end from which the tube was broken off is heated. It is then opened by the spring tongs, and reduced to a cylin der of the same size as that at the- other end. The shears are next used to cut the cylinder open from this end, and leave it in the state of a sheet of paper rolled up. The cylinder is now laid upon a smooth copper table, where the spindle is detached, and the glass spread into a flat sheet, and annealed.
Flint glass was formerly manufactured front flints cal cined, and afterwards gi mind, well washed, ruld to which was added pearl ashes, or an alkali of some kind, to serve as a flux, and a small quantity of arsenic.
The materials now used for the finest flint glass, ate, first, a fine white transparent sand, which is termed the body of glass ; this is found often mixed with clay, &c. from which it is freed by washing, until the water comes quite clear off ; it is then calcined, and afterwards sifted through a very fine sieve of from 40 to 50 passes to the inch. The second article is red lead, or litharge, which serves as a powerful flux, gives greater density to glass, renders it more ductile, less apt to crack front sudden changes of temperature, and imparts to it a high refrac tive and reflective power. Litharge is preferable to red lead, as it gives a purer glass, is less liable to be adul terated, and acts more powei fully as a flux. The third ingredient is an alkali, either soda or potash ; the finest pearl ashes dissolved, and the solution evaporated to dry ness, is considered the best, as it imparts no particular colour to the glass, whereas soda gives it a greenish-blue tinge.