UNBREAKABLE OR TOUGHENED GLASS. 
227 
'brought it into practical working order, rendering the process 
ns certain of success as any in use in the arts and manufactures 
in the present day. 
As already observed, M. de la Bastie is not a glass manufac- 
turer ; he therefore had to re-heat glass articles when toughen- 
ing them. It, however, by no means follows that the toughening 
process cannot be applied in the course of manufacture, thus 
avoiding re-heating. On the contrary, it not only can be, but 
has been, applied at glass-works to glass just made, and so 
■saves the costly and time-absorbing process of annealing. But, 
for reasons stated, M. de la Bastie had to apply the process to 
the manufactured article, and the method adopted, and the 
apparatus used in its application, next merit attention. In the 
first place, the glass to be toughened had to be raised to a very 
high temperature — the higher the temperature the better — the 
risk of breaking the glass being thereby reduced, and the 
shrinkage or condensation being increased. It was therefore 
advantageous, and often necessary, to heat the glass to the 
point of softening ; but in that condition glass articles readily 
lost their shape, and had to be plunged into the bath almost 
without being touched. Then came another difficulty — that of 
preventing an already highly heated combustible liquid taking 
fire upon the entrance of the still more highly heated glass. 
The latter difficulty was met by placing the tempering bath in 
direct communication with the heating oven, and enclosing it 
so as to prevent access of air ; and the former by allowing the 
heated glass articles to descend quickly, by gravitation, from the 
oven to the bath. 
The apparatus used by M. de la Bastie is shown in the accom- 
panying Plate CXXIL, in which fig. 1 is a front view, fig. 2 a 
vertical section, and fig. 3 a sectional plan of the oven and bath. 
The working oven, a, is heated by a furnace, 6 . The bottom of 
the oven, c, and the slope to the bath, are made in one piece of 
refractory material, and are very smooth on the surface. At 
the side of the oven is a preparatory oven, d , communicating 
by a passage, e, in the separating wall. In this oven the glass 
is partially heated before being placed in the main oven, a . 
The products of combustion are carried away in the direction of 
the arrows through the chimney. When the oven, a, is sufficiently 
heated, the ash-pit and fire-doors, //, are ■ closed, and rendered 
air-tight by luting, and the fire is maintained by small pieces 
of fuel introduced by a hole, < 7 , in the fire-door. The draught 
is then stopped by lowering the chimney cap, h , or closing the 
damper. The vertical damper, i, is then raised, so that the 
flame passes by the flue, j 9 to a second chimney, Jc, fig. 3, passing 
thus along the slope and heating it, and also opening communi- 
cation from the oven, a, to the bath, £, which is filled with the 
