Laboratory Manual of Glass-Blowing

by Francis C. Frary

Discussion.—Most beginners tend to err in the first steps of this operation, by blowing too hard and too long when blowing out the little bulb. The result is a large, very thin bulb, which breaks off in such a way as to leave a hole in the main tube, occupying nearly half the circumference of the tube at that point, instead of the neat side tube which they should have. It is not difficult to seal a tube on this side tube, but it is very difficult to seal a tube into a hole in another tube. Care should be taken here, as in the two previous exercises, that the lump obtained at the joint when the two tubes are put together is made as small as possible, and reduced if possible by gently drawing on the side tube as soon as the tubes have actually joined. It is much easier to prevent the formation of a lump at the joint than it is to remove the lump after it is formed. The remarks previously made about blowing quickly after removing the work from the flame apply here with especial force. A "tee" tube, from its very nature, is exposed to a good many strains, so care must be taken that the walls of the joint are of uniform thickness with the rest of the tube.

The beginner will find it easiest to make this tube out of two pieces of the same tube, about ¹⁄4 inch in diameter. Larger or smaller tubing is usually more difficult. If tubing much more than ¹⁄4 inch is used, the whole joint, including part of the main tube, must be heated nearly to the softening point at the close of the operation, and well annealed, as described in Chapter 1 (page 3) or it will be almost certain to crack. In the larger sizes of tube it will be necessary to heat the whole circumference of the main tube frequently during the operation, to prevent it from cracking.

In sealing a small tube on the side of a large one, it is usually advisable, after warming the spot where the joint is to be made, to attach a small drop of glass to the tube at that point, and direct the flame upon that, thus supplying at the same time both a definite point to be heated and an extra supply of glass for the little side tube which is desired. In this way it is also easier to blow out a side tube with a sufficiently small diameter. If the diameter of this tube should be much greater than that of the small tube, the latter may be enlarged with a carbon or a flanging tool.

EXERCISE NO. 4

To Join Two Tubes of Different Diameters

In this case the first method (Exercise No. 1) is to be used whenever possible, as it gives a much smoother joint than the second method. The directions given will describe the adaptation of this method to the problem: if the second method must be used on account of awkward shape, etc., of the work, the modifications required will be obvious to any one who has learned to make the joint by the first method.

After sealing or corking one end of the larger tube, the other end is drawn out to form a tail as described on page 9, taking care to have the tube uniformly heated, and to draw the tail rapidly enough so that the cone is short, as indicated in a, Fig. 8. The tube is now rotated, a small flame directed against the cone at right angles to an element of it, and it is allowed to shrink a little, as indicated in b, Fig. 8, so that its walls will thicken. When the tail is cut off, at the dotted line, the diameter of the opening and the thickness of the walls at that point should correspond with the dimensions of the tube to be sealed on. As the glass is hot, the scratch for cutting it must be made with a file (moisten the edge!), and it often will not break square across. Before proceeding to seal on the small tube, any large projections on the cut end are best removed, by warming the cut surface a little, directing the small flame upon each projection in turn and touching it with a warm scrap of glass. It will adhere to this and may then be removed by rotating this scrap a little so as to wind up the projection on it, and then drawing it off, while the flame is still playing on the spot. This must be done rapidly and care taken not to soften the main part of the cone.

Fig. 8.—Joining two tubes of different diameters.

The large tube is now taken in the left hand, the small one in the right, the ends heated and joined in the usual manner, taking care not to get any larger lump at the joint than necessary. A small flame is now directed on the cone at right angles to its elements as before, and the tube rotated so as to heat the whole circumference. The flame should be just large enough to heat the whole of the cone. As the latter shrinks, the lump at the joint is brought into the edge of the flame, and it and a very little of the small tube allowed to shrink with the cone.