Laboratory Manual of Glass-Blowing

by Francis C. Frary

Drawing Out a Tube.—Most students learn this the first day of their laboratory work in chemistry, but few take pains to do it well. The tube should be heated in the flame of a Bunsen burner, or blast lamp (preferably the latter) until it is very soft. During this time it must be continuously rotated about its axis, and so held that the edges of the heated zone are sharply defined; i.e., it should not be allowed to move back and forth along its own axis. When so hot that it cannot longer be held in shape, the tube is removed from the flame, and the ends slowly and regularly drawn apart, continuing the rotation of the tube about its axis. By regulating the rate of drawing and the length of tube heated, the desired length and diameter of capillary may be obtained. The tube should always be rotated and kept in a straight line until the glass has set, so that the capillary may have the same axis as the main tube. This capillary or "tail" is often a very necessary handle in glass-blowing, and if it is not straight and true, will continually make trouble.

In drawing out very large tubing, say from one to two inches in diameter, it is often necessary to draw the tube in the flame, proceeding very slowly and at a lower temperature than would be used with small tubing. This is partly on account of the difficulty of heating large tubing uniformly to a high temperature, and partly in order to prevent making the conical part of the tube too thin for subsequent operations.

Constricting a Tube.—Where a constriction is to be made in a tube, the above method must be modified, as the strength of the tube must be maintained, and the constricted portion is usually short. Small tubes are often constricted without materially changing their outside diameter, by a process of thickening the walls. The tube is heated before the blast lamp, rotating it about its axis as later described, and as it softens is gradually pushed together so as to thicken the walls at the heated point, as in a, Fig. 1. When this operation has proceeded far enough, the tube is removed from the flame, and the ends cautiously and gently drawn apart, continuing the rotation of the tube about its axis and taking care not to draw too rapidly at first. The resulting tube should have a uniform exterior diameter, as shown in b, Fig. 1.

Fig. 1.—Constricting a tube.

This method of constriction is not suited to tubes much over ¹⁄4 inch in diameter, since the mass of glass in the constricted part becomes so thick as to be difficult to handle when hot, and likely to crack on cooling. Larger tubes are therefore constricted by heating in a narrow flame, with constant rotation, and when soft, alternately gently pulling the ends apart and pushing them together, each motion being so regulated that the diameter of a short section of the tube is gradually reduced, while the thickness of the wall of the reduced portion remains the same as that of the rest of the tube, or increases only slightly. This pulling and pushing of the glass takes place in the flame, while the rotation is being continued regularly. The result may appear as indicated in c, Fig. 1. The strength of the work depends upon the thickness of the walls of the constricted portion, which should never be less than that in the main tube, and usually a little greater. This operation is most successful with tubing having a relatively thin wall.

Flanging a Tube.—This operation produces the characteristic flange seen on test-tubes, necks of flasks, etc., the object being twofold: to finish the end neatly and to strengthen it so that a cork may be inserted without breaking it. This flanging may be done in several ways. In any case the first operation is to cut the tube to a square end, and then heat this end so that the extreme sixteenth or eighth of an inch of it is soft and begins to shrink. The tube is of course rotated during this heating, which should take place in a flame of slightly greater diameter than the tube, if possible. The flange is now produced by expanding this softened part with some suitable tool. A cone of charcoal has been recommended for this purpose, and works fairly well, if made so its height is about equal to the diameter of its base. The tube is rotated and the cone, held in the other hand, is pressed into the open end until the flange is formed. A pyramid with eight or ten sides would probably be better than the cone.

Fig. 2.—Flanging tool.

A better flanging tool is made from a triangular piece of copper or brass, about ¹⁄16 inch thick, and mounted in a suitable handle. Such a tool is shown in Fig. 2, being cut from a sheet of copper and provided with a handle made by wrapping asbestos paper moistened with sodium silicate solution about the shank of the tool. It is well to have several sizes and shapes of these tools, for different sizes of tubing. The two sizes most used will be those having about the following dimensions: (1) a = 2 inches, b = 1 inch; (2) a = 1 inch, b = 1 inch. When the end of the tube is softened, the tool is inserted at an angle, as indicated in Fig. 3, and pressed against the soft part, while the tube is quickly rotated about its axis. If the flange is insufficient the operation may be repeated. The tool should always be warmed in the flame before use, and occasionally greased by touching it to a piece of wax or paraffin. After the flange is complete, the end must be heated again to the softening temperature and cooled slowly, to prevent it from cracking.