The process may be regarded as twofold—first, the development of the lump into a sphere ; and secondly, the resolution of a sphere into a circular table. Constant rotation of the glass, while in a yielding state, is an essential element of success, as, if allowed to remain motionless, its symmetry would be immediately destroyed.
Sheet-Glass.—As before, a lump of glass is collected upon the end of the gatherer's pipe. For the metallic table or marver, is now substituted a block of wood, so hollowed out as to allow the lump, when placed upon it, to be expanded by the blower to the diameter ultimately required. The block, during this operation, is sprinkled with water, to prevent the wood from being burnt, and from scratching the glass. From the block, the glass is carried to the blowing-furnace, which is accessible through a number of holes or openings, each hole being allotted to a single blower. In front of the furnace corresponding to each opening, is a stage or frame of wood, erected over a large pit or well, about 10 ft. deep. and these parallel stages are sufficiently apart to enable each blower to swing his pipe to and fro in a vertical plane, that the glass may run freely out, as the phrase is, to the required length. When the glass has been sufficiently heated in the blowing-furnace, it is brought out, and swung round in a vertical plane, and also backwards and forwards, and the blower, at the same time, by blowing down the pipe, constantly keeps the lengthening cylinder full of air.
Uniformity of substance and diameter is chiefly secured by the skill of the workman, who when he finds the metal running out too freely, holds the cylinder vertically above his head, still keeping it well filled with air. These operations are continued until the cylinder has reached the required length. The diameter of the cylinder was determined by the wooden block, and remains the same throughout. The next stage of the process is opening the end of the cylinder. The thinner kinds of glass are all opened by submitting the end of the cylinder to the fire, at the same time forcing in air through the pipe, and stopping up its aperture. The air is expanded by the heat of tho firo, and bursts open the cylinder at the end, this being the hottest and most yielding part. The aperture thus made is widened out to the diameter of the cylinder, by subsequently turning the cylinder to and fro with the opening downwards. The thicker kinds are opened by attaching a
lump of hot glass to the end of the cylinder, which thus becomes the hottest and weakest part ; the air forced in by the blower as before bursts it open. The opening is then enlarged by cutting round it with scissors. If opened in the furnace, as in the first case, the ends of the thicker cylinders would be so thinned out that a considerable portion would be wasted.
The cylinder is now laid on a wooden rest, or " clievalet," and is easily detached from the pipe, by the application of a piece of cold iron or steel to the neck of the glass near the pipe-nose; the neck, being hot, suddenly contracts externally, and breaks away from the cylinder. There still remains the cap or end of the cylinder, which is easily taken off, by wrapping round the end of the cylinder a thread of hot glass, removing the thread, and applying a piece of cold iron to any point which the thread covered. The cylinder, as it lies upon the chevalet, ie in weight little more than two thirds of the lump of glass which the gatherer collected on his pipe. The quantity left upon the pipe nose, with that which formed the cap of the cylinder, are nearly equivalent in weight to one-half the cylinder.
The finished cylinder is now split open by a diamond, which, attached to a long handle, and guided by a wooden rule, is drawn along the inside length of the cylinder, and should pass through, or in the neighbourhood of, sonic notable defect, if such be present, for defects thus brought to the edges of the subsequent plate of glass are of lees injury to its value. An object to be avoided is the black mark, which the cylinder sometimes receives from the charring of the wooden "ehevalet," on which, while hot, it rested in the glass-house. If the diamond passes inside the cylinder, over tho space occupied by this mark upon tho outside, the cylinder will fly to pieces. The reason of this is not difficult to understand. The chevalet prevented, by its non-conducting tendency, that portion of the glass with which it was in contact, from cooling equably with the rest, and the particles at that point remained in a state of tension. A careful blower will never place any large defect in his cylinder in contact with the ehevalet, being aware of the probable result.