[Illustration: FIG. 13.--Later stage in sheet glass blowing.]
The completed cylinder, still attached to the pipe, is now carried away from the blowing-stage and laid upon a wooden rack; then the blower takes up a piece of cold iron, and placing it against the neck of glass attaching the cylinder to the pipe, produces a crack; a short jerk then serves to completely sever the pipe from the cylinder. A boy now takes the pipe to a stand where it is allowed to cool and where the adhering glass cracks off from it prior to passing it back to the pipe-warmer for fresh use.
On the wooden rack the cylinder of glass is allowed to cool to a certain extent, and then the remaining portion of the neck and shoulder (see Fig. 13) are removed. This is done by a boy who passes a thread of soft, hot glass around the cylinder at the point where it is to be cut off; the thread of hot glass merely serves to produce intense local heating, for as soon as it has become stiff, the thread of glass is pushed off and a cold or moist iron is applied to the cylinder at the point where it had been heated by the thread. As a rule a crack immediately runs completely round the cylinder along the line of the thread, and the “cap” is thus removed. The glass is now in the form of a uniform cylinder open at both ends, but it must be opened out into a flat sheet before it can assume the familiar form of sheet-glass.
The first stage in the opening-out process is that of splitting. For this purpose the cylinders are carried to a special stand, upon which they are laid in a horizontal position, and here a crack or cut is made along one of the generating-lines of the cylinder. This may be done either by the application of a hot iron, followed, if necessary, by slight moistening, or by the aid of a cut from a heavy diamond drawn skilfully down the inside of the cylinder. It will be seen from the account of the process so far given, that the glass has as yet undergone no real annealing, although the blower is expected to “anneal” his cylinder during the blowing process, as far as possible, by never allowing it to cool too suddenly, and this degree of annealing is usually sufficient to save the cylinder from breaking under its internal stresses when left to cool on the racks. The surface of the glass, however, is left in a decidedly hardened condition, especially on the outside, which has necessarily been most rapidly cooled. For this reason--among others--the splitting cut is always made on the inside of the cylinder. The difference between the rates of cooling of the outside and inside of the cylinder has a further effect, which becomes evident as soon as the cylinder is split. The outside having become hard while the inside was still relatively soft, the outer layers of glass are in a state of compression and the inner layers in a state of tension in the cold cylinder. As soon as the cylinder is split, however, these stresses are to some extent relieved, the inner layers being then free to contract and the outer layers to expand; the result is an increase in the curvature of the cylinder, which slightly decreases in diameter, the cut edges overlapping. If the cylinder has been cooled rather too quickly, or if the glass itself has a high coefficient of expansion, this release of internal stresses at the moment of splitting becomes very marked, and each cylinder splits with the sound of a small explosion, while if the internal stresses are still more severe, the cylinders may even fly to pieces as soon as they are cut.