Glass-blowing spectacles

Devitrified glass should be pulled out of a joint with forceps or a glass rod, and if this leaves a thin-walled joint extra glass can be built on using thin rod. 

The aim in aU these operations is to produce a joint with the same bore as the tube and widi uniform wall thickness. Only practice will allow this to be achieved. However, with Pyrex, very bad-looking joints will hold together. 

There is very little basic difference in the procedures for joining tubes of different sizes and for joining tubes of the same size. The main difference lies in the preparation of the end of the larger tube the end of this tube should be drawn down to a spindle and the spindle cut off in the flame. The closed end so produced should be heated locally so that vslien it is blown out the hole produced will be of the same size as the bore of the smaller tube. But it is usually better to 

There are two basic methods of making these joints, (i) by the use of a bench torch and holding both tubes, (2) by the use of a hand torch and clamping the tubes (or one of them). 

Once again the first step is the adequate preparation of the surfaces to be joined. First the side arm should be prepared, as for butt-sealing two tubes, by blowing out the end (p. 59). A hole is then blown in the side of the tube at the position required (see p. 59 for details). Care should be taken not to make this hole too large. Both ends of the tube should then be blocked since it is best to blow through the side arm so that the whole joint can be seen while blowing. Some workers, however, prefer to blow through the cross-piece. 

---

Any increase in pressure, whatever the cause, may be sufficient to blow ground glass joints apart, or to cause some part of the line to explode, usually that part of the line with the greatest diameter of tube. Safety spectacles should always be worn by everyone in the vicinity of a glass vacuum line. When the chemicals handled in any vacuum line are known to be poisonous, the line should be in a fume cupboard. Pressure release devices such as mercury manometers with one limb open to the atmosphere, or "0"-ring cup joints, should be included in ail sections of a vacuum line which may be subject to excessive internal gas pressures. 

---