The punt, with the flattened globe of glass at its end, is then rapidly whirled round in a manner nearly resembling that in which a mop is twirled. By this motion, the globe becomes more and more flattened and extended in diameter, until at length, not being able longer to retain its shape, it bursts open, and spreads out in the form of a flat circular sheet of glass three or four feet in diameter. There is perhaps nothing in the whole range of the mechanical arts more astonishing to a spectator than this process, and there are few that require, from the workman, more of that dexterity of hand which can only be acquired by long practice. The workman continues to whirl the sheet of glass round,—gradually receding from the furnace,—until it is sufficiently set or solidified to retain its form. The punt is then, by a dextrous movement, detached from the centre of the sheet, leaving that bulb which is known as the “bull’s eye,” or the “knot.” The sheet is placed in an annealing oven, the temperature of which is lowered by slow degrees until cold; for it is found that glass is less brittle when it has been allowed to cool gradually than when the cooling has been rapid. Considerable care is required to regulate the temperature of the annealing oven; if the heat be too great the softened glass will bend: if the heat be insufficient the plates are liable to crack, or they prove so brittle that when they come to be used, the glazier will not be able to divide the glass so as to suit his purposes. Indeed, the management of the heat in the manufacture of crown glass requires so much care and skill that few workmen produce an article of the same value, even though working at the same furnace; hence crown glass is known in the market as firsts, seconds, thirds, and fourths; the fourth quality producing less than one-half of the price of the first.
We have not interrupted this description, to refer to engravings; but we may now illustrate it by the following cuts representing the glass in eight different stages of its formation.
1st. The melted glass attached to the tube, and worked on a board.
2nd. The workman blowing through the tube, to expand the glass.
3rd. Whirling it rapidly at the mouth of the furnace.
4th. Transferring it from the hollow tube to the solid punt.
