Laboratory Manual of Glass-Blowing

by Francis C. Frary

EXERCISE NO. 8

Sealing a Tube Through Another Tube

First Method—Making a Gas-washing Tube

This first method can be used whenever one can work through an open end opposite to the end of the tube where the joint is to be made. To illustrate it, take a piece of rather thin-walled tubing, about ³⁄4 inch in diameter, and some pieces of rather strong tubing a little less than ¹⁄4 inch in diameter. Draw off the large tube in a short cone, then draw off the tail as in the making of the bulb on the end of the tube, blow out the little lump slightly, shrink the whole cone a little and blow gently to form a rounded end like that on a test-tube, with walls about the thickness of those of the rest of the tube. Cut this tube to a suitable length, say about six inches, and provide two corks which will fit the open end of it. Now cut a piece of the small tubing of the proper length to form the piece which is to be inside the large tube. For practice purposes, this piece should be about an inch shorter than the large tube. Flange one end of this tube a little, and anneal the flange well in the smoky flame. Bore one of the corks so that a piece of the small tubing will fit it, and cut a couple of notches in the side of this cork so that air can pass between it and the glass. Pass a short piece of the small tubing through this cork, and attach the flanged piece of small tube to this by means of a short piece of rubber tubing, so that when the whole is inserted in the large tube it is arranged as in a, Fig. 11. The piece of glass tubing projecting out through the cork is now cut off so as to leave an end about ¹⁄2 inch long when the cork is firmly seated and the inner tube pushed into contact with the center of the end of the large tube, as shown in the drawing. Care should be taken that the little rubber tube which joins the two pieces is arranged as in the figure; i.e., most of it on the piece of tubing which passes through the cork, and very little on the other piece, so that when the cork is removed after the small tube has been sealed through the large one, the rubber tube may easily come with it. Select a short piece of the small tubing of suitable length for the piece which is to be on the outside of the large tube as a continuation of the piece inside, and another piece for the delivery tube. A small bulb may be blown in the latter at a point about 2-¹⁄2 inches from the closed end, and the open end cut off about 1-¹⁄2 inches from the bulb. A cork or cork-boring of suitable size to stopper the small tube is prepared, and laid ready with the other (unbored) cork for the large tube.

Fig. 11.—Gas-washing tube.

When everything is in readiness, the rounded end of the large tube is slowly heated until it softens and joins firmly to the small tube inside. After it has shrunk down well, it is blown out to its original size, placing the whole end of the large tube, cork and all, in the mouth. Now with a fine-pointed flame the glass covering the end of the small tube is heated to the softening temperature, and then is blown out to an excrescence by blowing on the end of the small tube which passes through the cork. The end of this excrescence is heated and blown off in the usual way, so as to leave the small tube sealed on the inside of the large one and opening through it into this short tube which has been blown out. The end of the small tube which passes through the cork is now closed with the cork prepared for it, and the short outer tube is joined to the tube that has just been blown out, so that the joint appears like b, Fig. 11. Use the first method (Exercise No. 1) for this joint. Reheat the whole of the end of the tube nearly to the softening temperature, anneal it a little, and allow to cool a few seconds until well set. Now remove the cork, short glass tube and rubber tube from the open end of the large tube and insert the solid cork in their place. Warm the joint and the whole of that end of the tube again carefully up to about the softening point, then seal on the side tube for the delivery of the gas in the usual way, taking care that the whole of the end and the joint are kept warm meanwhile. When thoroughly sealed, the delivery tube is bent up parallel to the tube through which the gas enters, and then out at right angles to it, as shown in c. The whole of the end of the tube is now cautiously reheated and then cooled slowly to anneal it.

The cork may now be removed from the open end of the large tube, this end heated in a large flame, caught together with a scrap of glass tubing and drawn off into a cone so that the base of the cone is about opposite the end of the inner tube. The lump of glass is drawn off the point of this cone and it is reblown to form a rounded end, as previously described.

After this cools, the tube through which the gas enters may be heated at the proper point and bent at right angles to form the finished apparatus as shown in d. The ends of the small tube are cut off square and fire-polished.

Discussion.—After the joint has once been made, great care must be taken that it is kept hot during all the subsequent manipulations, and if it becomes somewhat cool at any time it must be reheated very slowly. It is obvious that the rate of heating and cooling of the inner tube will be slower than that of the outer tube, and this will readily produce stresses which tend to crack the tube at the joint. The amount of heating and cooling which such a joint will stand depends upon its form. The beginner should examine such a joint on regular factory-made apparatus, and note the uniformity of wall-thickness and the "clean-cut" appearance of the joint, as a model for his imitation. A ragged joint, where the line of joining of the inner and outer tubes wavers instead of going squarely around the tube, is almost sure to crack during the cooling and heating unless extra precautions are taken with it. The presence of a small lump of glass at any point on the joint affords an excellent starting place for a crack, as do also the points on a ragged joint where the inner tube comes farther down on the outer tube than at other points.