This is then carried to an auxiliary heated furnace, which has a wide opening emitting great heat, and by resting the pontil upon a convenient support and rotating it quickly the action of centrifugal force and heat causes the glass to spread out at the opening, which becomes larger and larger until the glass finally opens out into a flat circular disc of fairly even thickness throughout, with the pontil still at the centre, forming a bullion point or slight swelling, due to the knob of glass used in affixing it (d).
Next, the workman, keeping the disc in rotation, brings it away from the furnace and allows the metal to stiffen and set by cooling, when it is carried to the annealing oven and detached from the pontil. The discs are then stacked up for annealing. When annealed, these are afterwards cut across in sections or squares of convenient size by using a glass cutter’s diamond.
FOUR STAGES IN CROWN GLASS-MAKING
It is evident that the centre portion, containing the bullion point or bull’s eye, is useless for plain window glazing, but occasionally these are sought after by glass decorators for use in coloured leaded lights for door panels, etc.
Sheet Glass is made in the form of thin, walled, hollow cylinders of glass, which are split along their length and round the cap and then opened out by heat and allowed to uncurl until each sheet lies out flat. The workman gathers a sufficiency of glass upon his blow-iron by repeated gatherings, and marvers it into a ball about as big as one’s head. This is blown out (a) and widened by rotating the blow-iron until he gets a mushroom shape (b), with a heavier bulk of glass at the extremity than at the sides.
SIX STAGES IN SHEET GLASS-MAKING
This extra thickness of glass at the extremity of the bulb tends to lengthen the bulb of glass as he swings it in a pendulum fashion, and by blowing and swinging it alternately he gets an extended form (c).
To permit the workman to swing the mass of glass out conveniently to the full length of the intended cylinder, a long, narrow pit or trench is provided below the floor level, and by standing alongside this trench the workman is enabled to swing the glass within the trench at arm’s length until the requisite length and width of cylinder are obtained. This work requires a high degree of skill and strength. The shape of the cylinder of glass is now as shown on page 91 (d).