Glass Working Exercises 



99 



accumulated at the joint. Moreover, if the pieces cool before 

 joining, they frequently crack upon reheating. 



The joint being made, and proving airtight, it may be necessary 

 to improve its appearance, though when the skill has increased, 

 the joint will be completed in a single operation. Should this be 

 necessary, the whole of the tube at the junction may be heated, 

 and as it softens, gently blown out into a bulb, while the parts are 

 slightly pressed together. The thinner portions will blow out most, 

 and this must be guarded against, many reheatings and blowings 

 being necessary in order to coax the glass finally to an even thick- 

 ness. It should be mentioned here, that reheating/tfr/zVz//j> is of no 

 use the glass must attain the temperature at which it will flow, or 

 coalesce. Finally, the bulb so blown is fined down, the tube being 

 pulled out slightly at the same time as it is being blown into, in 

 order to reduce the diameter to that of the original tube. Tubes 

 joined in this manner should not show the joint at all (Fig. 87 (3) ). 



FIG. 88. Stages in the joining of two tubes of unequal diameter end to end. 



Two tubes of dissimilar diameters may be united similarly, by 

 opening the smaller one slightly as above, and drawing the wide 

 one down, cutting at a point where the diameter is slightly smaller 

 than that of the opened tube, in order to allow for the widening of 

 the aperture under the flame (Fig. 88 (4)). Reheating, blowing, and 

 moulding may again be necessary, and any trace of thinning of the 

 glass (indicated by a tendency to blow out more easily at one place 

 than at another), should be at once eliminated by heating strongly 

 about three times the area of the thin place, allowing it to collapse, 

 and finally restoring its shape by blowing. In joining two tubes in 

 this way, care should be taken not to pull the tubes apart, rather 

 to keep them pressed towards each other, maintaining the correct 

 shape by blowing into the tubes when necessary. 



Tubes of different diameters have walls of different thickness, 

 but, as explained in Exercise i, the wall of a drawn-out tube may 

 be made of any desired thickness by suitable treatment. It is well, 

 in joining tubes of unequal diameter, to arrange that the walls are 

 of similar thickness at the point of junction. 



