i8 



mencement, a few words may be said about the pots or 

 crucibles in which the materials are melted. 



Seeing that clay is composed chiefly of silica and alumina 

 (the former oxide being almost essentially, and the latter not 

 infrequently, a constituent of glass), it might be thought 

 that this material would be readily absorbed by the glass. 

 To some extent solution does actually take place; but no 

 other container has ever been employed with success on a 

 large scale, and, if clay of suitable composition is used, the 

 attack by the glass may be diminished to a comparatively 

 small amount. 



The clay is mixed with a proportion of burnt clay which 

 is ground to a suitable size, in order to diminish the contrac- 

 tion of the crucible in drying. If this burnt clay or " grog " 

 were not used, the drying of the pots, which in any case 

 takes some six months, would have to occup}' a longer time, 

 and there would be much greater danger of cracking in the 

 furnace. Innumerable minute fissures form as the clay con- 

 tracts round the particles of " grog," but when the pot is 

 slowly brought to a high temperature, before the materials 

 are added, the fine particles of clay sinter or weld together 

 into a hard mass, which is comparatively resistant to the 

 glass. 



The burning of the pots that is, the operation of bringing 

 the cold crucibles to a red heat previous to their introduction 

 into the glass-making furnace occupies at least a week and 

 must be very carefully carried out, particularly in the early 

 stages. 



The constituents from which the glass is made are care- 

 fully weighed out, and thoroughly mixed together, and 

 shovelled into the crucible. Successive fillings are necessary 

 to fill the pot; and when the mass is molten, it must be 

 allowed to stand at a high temperature for a sufficient time 

 to enable the bubbles of gas produced during the operation 

 to find their way to the surface. When a sample of the 

 glass, taken on a rod, shows it to be " plain " that is, free 

 from bubbles the stirring process is allowed to commence. 

 This operation has not been modified in principle since its 

 original discovery in 1800 by Guinand. 



The process is by no means simple. We are not dealing 

 here with a liquid contained in a vessel which is completely 

 insoluble in the liquid; the case, on the other hand, is more 

 nearly analogous to that of an acid solution in a metal vessel 

 which is slowly dissolving in the liquid, which, in addition, 

 attacks the stirrer used to produce complete admixture. 



