2. A method much practised by the makers of sheet glass, and suitable for large objects, is to wrap a thread of hot glass round the tube, at once removing it, and touching any point of the glass which the thread covered with water or a cold iron, when a crack will be started and will pass round the glass where it was heated by the thread.
3. Tubes which are large and slightly conical may have a ring of red-hot iron passed over them till it comes into contact with the glass, then, the iron being removed, and a point on the heated glass being at once touched with cold iron as before, it will break as desired. Or a string, moistened with turpentine, may be loosely twisted round the tube, and the turpentine ignited, afterwards the application of sudden cold to any point on the zone of hot glass will usually start a crack, which, if necessary, may be continued in the usual manner. The last three methods are chiefly useful in dealing with the largest and thickest tubes, and with bottles.
A fairly stout copper wire, bent into the form of a bow so that it can be applied when hot to a considerable surface of a glass tube, will be found superior to the point of hot glass or metal usually employed, for leading cracks in glass tubes. With such a wire a tube can be cut so that the cross section of the end is at any desired angle to the axis of the tube, with considerable precision. I am indebted for this suggestion to Mr. Vernon Boys and Dr. Ebert.
Fig. 6.
Bending Glass Tubes.—The blow-pipe flame is not a suitable source of heat for bending tubes, except in certain cases which will be mentioned in a subsequent paragraph. For small tubes, and those of moderate size, a fish-tail burner, such as is used for purposes of illumination, will answer best. Use a flame from one to two inches in breadth—from A to A ([Fig. 6]), according to the size of the tube which is to be bent. If the length of tube that is heated be less than this, the bend will probably buckle on its concave side.
The tube to be heated should be held in the position shown in [Fig. 6], supported by the hands on each side. It should be constantly rotated in the flame, that it may be equally heated on all sides. In the figure the hands are represented above the tube, with their backs upwards. A tube can be held equally well from below, the backs of the hands being then directed downwards, and this, I think, is the more frequent habit. It is difficult to say which position of the hands is to be preferred. I lately observed how a tube was held by three skilful amateurs and by a professional glass-blower. All the former held the tube with the hands below it. The latter, however, held it from above, as in [Fig. 6]. He, however, was working with a rather heavy piece of tube, and I am inclined myself to recommend that position in such cases. During a long spell of work, the wrist may be rested from time to time by changing the position of the hands.
When the tube has softened, remove it from the flame, and gently bend it to the desired angle. The side of the tube last exposed to the flame will be slightly hotter, and therefore softer, than that which is opposite to it. This hotter side should form the concave side of the bent tube.
