Laboratory Manual of Glass-Blowing

by Francis C. Frary

The method described in Exercise No. 1 is very satisfactory for joining short lengths of straight tubing, but becomes inconvenient or impossible when the pieces are long or bent, on account of the difficulty in uniformly rotating such work. In such cases, this second method is used. It does not usually give as smooth and pretty a joint as the first method, and takes a little longer.

The joint is begun exactly as in the first method, and the manipulation is the same until after the preliminary tight joint (b, Fig. 6) is made. The flame is reduced as usual, but instead of rotating the tube in the flame, only one part of the circumference is heated, and this is allowed to shrink thoroughly before blowing. It is then blown gently so that it becomes a slight swelling on the tube, and the operation repeated on an adjoining part of the joint. Three or four repetitions of the operation will usually cover the whole circumference of the joint, in a small tube, the result being a swelling roughly similar to the first thick bulb in the first method (d, Fig. 6). If all the lumps of the original joint have not been removed by this operation, it may now be repeated upon such parts as may require it. The thickness of the wall in the bulb should be about the same as that in the original tube. The whole of the expanded joint is now heated as uniformly as may be until soft enough so that it begins to shrink a little, and the swelling is gently drawn down to the same diameter as the main tube, as in the first case. Any irregularities in the finished joint may be corrected by local reheating, shrinking or blowing as required.

Discussion.—In using this method, especially with larger sizes of tubing, it is very important to keep the whole circumference of the joint hot enough during the operation so that it does not crack apart at the part which has not yet been worked. For that reason the first heating, shrinking and blowing should be performed as quickly as possible, leaving the resulting irregularities to be corrected later, rather than attempting to reblow the same part of the joint several times in succession until it is satisfactory. Care must be taken in this as in the first method that the blowing follows immediately upon the completion of the shrinking and removal of the object from the flame: delay in blowing will cause shrunken places where the joint meets the original tubes, on account of the cooling and setting of the glass before it was blown. Most beginners err in being afraid to shrink the part of the joint enough before blowing it. On small tubing, the shrinkage may often extend so far that the inner surface of the shrunken part reaches the center of the tube. Insufficient shrinking results in failure to remove the lump formed at the original joint. It is often of advantage, after blowing out part of the joint, to allow that part a few seconds to set before going on with the rest, keeping the whole joint warm meanwhile in or near the smoky flame. This helps to prevent the twisting of the joint, or other distortion incident to the handling of a piece of work of awkward shape.

In making a joint on a very long or heavy piece by this method, it is often advantageous to attach a piece of rubber tubing to the open end, hold the other end of this tubing in the mouth during the process, and blow through it, rather than attempt to bring the end of the glass up to the mouth. This enables one to keep closer watch on the joint, and avoid drawing it out or distorting it in handling. On the other hand, the rubber tube is an inconvenience on account of its weight and the consequent pull on the end of the apparatus, and makes rotation difficult.

EXERCISE NO. 3

The "Tee" Tube

The operations involved are two: the blowing of a short side tube on a piece of tubing, and sealing another piece of tubing on this, by what is essentially the second method as just described.

Fig. 7.—The "tee" tube.

The two pieces of tubing to be used each have one end cut square and the other sealed in the usual manner. The longer of the two is now heated at the point at which the joint is to be made, until it begins to color the flame. A small flame is used, and the tube rotated until the flame begins to be colored, when the rotation is stopped, and only one spot heated until a spot the diameter of the tube to be sealed on has become red hot and begun to shrink. This is now gently blown out into a small bulb, as in a, Fig. 7, and it will be noted that this bulb will have walls tapering from the thick walls of the tube to a very thin wall at the top. The sides of this bulb, below the dotted line, are to form the small side tube to which the main side tube is to be sealed. The top of the bulb is now softened by directing a small flame directly upon it, and as soon as it shrinks to the level indicated by the dotted line, it is removed from the flame and quickly blown out to form a thin bulb, as indicated in b, Fig. 7. This will usually be so very thin that a stroke of the file or glass-knife will break it off at the dotted line, leaving the side tube, to which the short piece of tubing is now sealed according to the second method (Exercise No 2). In doing this, care is taken to direct the flame partly on the main tube in the two crotches, so that both tubes blow out a little and give space for the gases to turn in, as indicated in c, Fig. 7, and at the same time increase the mechanical strength of the job. On the other hand, care is taken not to deform the main tube, and not to produce such a bulge or bulb at the joint as will prevent the finished tube from lying flat on a table.