Glass Working Exercises 97 



Fig. 85 (2). This method is useful where quantitative results are 

 desired from closed tubes. 



Wide tubes cannot be treated in this way, the amount of glass 

 at the tip being too great for equal cooling and contraction, conse- 

 quently have first to be closed as in Fig. 85 (i) ; then reheated, and 

 either pressed vertically downwards upon a block of charcoal, or 

 " sucked " in, instead of being blown out, eventually producing an 

 end like that shown in Fig. 85 (3). Several reheatings may 

 be necessary in this case, the method adopted depending upon the 

 diameter of the tube dealt with. 



EXERCISE 3. To constrict the bore of a tube> while leaving 

 the outside diameter unchanged. 



This is a very simple exercise in tubes of small diameter, but 

 more difficult in larger cases, but as the practical application of this 

 process is made mainly in tubes of small diameter, it will be suffi- 

 cient to make use of tubes not greater than 10 mm. outside diameter. 



The tube is heated as regularly as possible along about 2 cm. 

 of its length where the constriction is desired, until the tube shows 

 signs of softening, when the temperature of one portion of the 

 heated part is raised, and the glass allowed to collapse slightly, 

 this being aided by pushing in the ends of the tube a little. A 

 further portion of the hot glass is then softened, and permitted 

 to go through the same processes, until the length to be con- 

 stricted is dealt with. The whole of this is then reheated just 

 to softening temperature, and gently pulled out until the outside 

 diameter of the constricted part is equal to that of the original 

 tube. This must be done carefully, and if the tube shows any 

 tendency to become too thin in places, the drawing out must 

 instantly cease, and the thin part be built up again. The tube 

 will thin out when that particular portion has been heated too 

 strongly, and it is built up again by reheating, blowing a rough 

 bulb with all the thin glass, and collapsing the bulb before a large 

 flame, while pressing the ends of the tube together. Gradually the 

 glass will return to its original diameter, but a bulge may be thus 

 caused at the point last worked ; if so, by pulling gently just as the 

 glass is solidifying, the tube may be brought down to the desired 

 diameter. 



Another method of producing the constriction is to use a very 

 small flame, and heat a small circle of the tube, rotating and gently 

 compressing the tube meanwhile, passing on to the adjoining part 

 when this small piece is completed. 



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