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.