THE ANNEALING OF GLASS. 



By F. Twyman. 



Although owing to the small sizes of microscope lenses, and the fact 

 that they are usually cut out of comparatively massive pieces of glass, 

 want of annealing is very unlikely to cause the microscope maker any 

 trouble, yet a few remarks relative to the principles underlying the 

 annealing of optical glass may be of interest, if for no other reason than 

 that the same principles underlie the efficient moulding of glass (or 

 indeed of other materials), and there is no reason why an appreciable 

 saving in the cost of manufacture of microscope objectives might 

 not be effected by high quality moulding. 



When the Eesearch Laboratory of Adam Hilger, Limited, took up 

 the question of annealing glass in 1915, we were unable to discover that 

 any research work of a fundamental character had been done on the 

 subject. We were unable to find even a clear presentation of the 

 cause of faulty annealing. For various reasons we have not published 

 a full account of the work, but the principles involved and some 

 of the results as applied to glassware will be found sufficiently fully 

 described in a paper read before the Society of Glass Technology in 

 1917. C The Annealing of Glass," by F. Twyman. Trans. Soc. 

 Glass Technology, 1917. I. 61, e^ seq.) 



The phrase " badly annealed " when applied to glassware implies 

 the presence of internal stress. 



When glass is in a definitely molten condition there can be, of course, 

 no permanent internal stress. Moreover, it can be shown, by keeping 

 a suitable glass object under observation in a tube furnace, that even 

 when the glass is cool enough to be practically solid under such stresses 

 as are occasioned by its own weight, it may yet be mobile enough for 

 severe internal stresses to disappear in a few minutes. On the other 

 hand, at ordinary air temperatures glass is almost (though not quite) 

 perfectly elastic. 



But between this high temperature, w^iere the glass is so mobile that 

 internal stresses are evanescent in, at most, a few seconds, and the 

 low temperatures, where the glass behaves as an elastic solid, is a region 

 where internal stresses take, say, a minute, or an hour, or a few 

 hours, to die out. It is this range of temperature which is important 

 in annealing, and an accurate knowledge of the mechanical properties 

 of the glass throughout this region is necessary if we are to attain any 

 specified perfection of annealing in a minimum time, and without dis- 

 tortion of the articles. This region I call the annealing range. 



If it were possible to cool any glass object from the high temperature 

 down to ordinary air temperature in such a way that the temperature 

 remained uniform throughout the mass, then no matter how fast the 

 cooling the glass would be well annealed. 



What actually happens is that differences of temperature exist 

 while the glass is cooling. The stresses so caused are transient, so 

 long as the glass is within the annealing range ; but when eventually 

 it becomes cold and the temperature uniform, there are present 

 permanent stresses depending on the variations of temperature 

 throughout the mass which existed while the glass was cooling. 



