CHAPTER III.
CUTTING AND BENDING GLASS—FORMING GLASS APPARATUS BEFORE THE BLOW-PIPE—MAKING AND GRINDING STOPPERS TO APPARATUS, ETC.
In the later pages of this Chapter it will be assumed that the operations first described have been mastered. The beginner should therefore practise each operation until he finds himself able to perform it with some degree of certainty. Generally speaking, however, after the failure of two or three attempts to perform any operation, it is best to give up for a few hours, and proceed to the work next described, returning to that upon which you have failed subsequently. If, unfortunately, it should happen that the work next in order involves the performance of the operation in which the failure has occurred, it is best to pass on to some later work which does not demand this particular accomplishment, or to rest a while, and re-attack the difficulty when refreshed.
Cutting Glass Tubes.—The simplest method of cutting a glass tube is to make a sharp scratch with a file across the glass at the point where it is desired to cut it, and on pulling apart the two ends, it will break clean off. It is important that the file be sharp. In pulling apart the ends the scratch should be held upwards, and the pull should have a downward direction, which will tend to open out the scratch. In the case of a large tube, a scratch will not ensure its breaking clean across. The tube must be filed to some depth, half-way, or even all round it. A good way of breaking a tube is to place the file in the table after scratching the glass, to hold the glass tube above its edge with one hand on each side of the scratch, and to strike the under side of the tube a sharp blow upon the edge of the file, directly beneath the scratch. In this way very even fractures of large and moderately thin tubes may be made. It answers particularly well for removing short ends of tube, not long enough to hold; the tube is held firmly upon the file, and a sharp blow given to the short end with a piece of large tube or a key.
A file whose faces have been ground till they are nearly smooth, so as to leave very finely-serrated edges, will be found useful for cutting glass tubes. Such a file should be used almost as a knife is used for cutting a pencil in halves.
The simple methods just described are too violent to be applied to delicate apparatus, too tedious when employed upon the largest tubes, and very difficult to apply when the tube to be cut is very thin, or too short to permit the operator to get a good grip of it on either side of the file mark. In such cases, one or other of the following methods will be useful:—
1. Make a scratch with a file, and touch it with the end of a very small piece of glass drawn out and heated at the tip to its melting point. It is important that the heated point of glass be very small, or the fracture is likely to be uneven, or to spread in several directions. Also, it is best to use hot soda glass for starting cracks in tubes of soda glass, and lead glass for doing so in lead glass tubes. If the crack does not pass quite round the tube, you may pull it asunder, as previously described, or you may bring the heated piece of glass with which the crack was started to one end of the crack, and slowly move it (nearly touching the glass) in the required direction; the crack will extend, following the movements of the hot glass. Instead of hot glass, pastils of charcoal are sometimes employed for this purpose. They continue to burn when once lighted, and there is no need to re-heat them from time to time. They should be brought as close to the glass as is possible without touching it, and, when no longer needed, should be extinguished by placing the lighted end under sand, or some other incombustible powder, for they must not be wetted.