CHAPTER II
General Operations
Cutting the Glass.—For this purpose a "glass-knife" is preferred to a file, if the glass is cold: if it is hot a file must always be used, and its edge slightly moistened to prevent drawing the temper. The glass-knife is simply a flat piece of hard steel, with the edges ground sharp on an emery wheel. The bevel of the edge should be from 30 to 60 degrees. An old flat file can easily be ground into a suitable knife. The glass-knife makes a narrower scratch than the file but appears more likely to start the minute crack which is to cause the tube to break at that point, and the break is more likely to give a good square end. The scratch should be made by passing part of the knife or file once across the glass, never by "sawing" the tool back and forth. This latter procedure dulls the tool very quickly.
In breaking a piece of glass tubing, many persons forget that it is necessary to pull the ends apart, as well as to bend the tube very slightly in such a direction as to open up the minute crack started in the scratch. Care in breaking the tube is essential, as it is impossible to do as good work with uneven ends as with square ones.
When tubing of large diameter or thin wall is to be cut, it is often better not to attempt to break it in the usual way, but to heat a very small globule of glass (1⁄16 to 1⁄8 inch diameter) to red heat, and touch it to the scratch. This will usually start the crack around the tube; if it has not proceeded far enough, or has not gone in the desired direction, it may be led along with a hot point of glass. This is put a little beyond the end of the crack, and as the latter grows out toward it, moved along the path where the crack is desired. This point of glass is also very useful in breaking off very short ends of tubes, where there is not room to get a firm enough hold and sufficient leverage to break the tube in the ordinary way, and for breaking tubes attached to large or heavy objects, which would be likely to make trouble if treated in the ordinary way.
Another way of cutting large tubing, especially if it has rather thick walls, is to make a scratch in the usual way, and then turn on the smallest and sharpest possible flame of the blast lamp. The tube is next taken in both hands and held horizontally above the flame so that the scratch is exactly over it. The tubing is now rotated rapidly about its axis, and lowered so that the flame is just tangent to its lower side. After about ten seconds of heating, it is removed from the flame and the hot portion quickly breathed upon, when it will generally crack apart very nicely. Care must be taken to hold the tube at right angles to the flame during the heating, and to rotate it so that only a narrow strip of the circumference is heated, and the scratch should be in the center of this heated strip. By this means tubing as large as two inches in diameter is readily broken.
Griffin's glass cutter, which contains a hardened steel wheel, like that on any ordinary window-glass cutter, and a device by which this can be made to make a true cut clear around the tube, is a very handy article, especially for large tubing, and may be obtained from any dealers in chemical apparatus.
Bending Glass.—Inasmuch as this is one of the commonest operations in the laboratory, it is assumed that the reader knows how to perform it. However, it should be noted that in order to obtain the best results a broad (fish-tail burner) flame should generally be used, and the tube rotated on its axis during the heating, and allowed to bend mostly by its own weight. If large tubing is to be bent, one end must be stoppered and great care used. Whenever the tube shows signs of collapsing or becoming deformed, it must be gently blown out into shape, heating the desired spot locally if necessary. A blast-lamp is likely to be more useful here than the fish-tail burner.