Sept. 1, 1864.] THE TECHNOLOGIST. 



THE MANUFACTUHE OP PLATE 3LASS. 65 



area of surface was required in the ovens in order to lay them all sepa- 

 rately, some of the ovens being as much as 50 ft. long for the purpose 

 of annealing six large plates of glass at a time ; the ovens were well 

 designed for uniformity of heat in all parts, notwithstanding their 

 great size. 



Mr. R Pilkington observed that the great cost of importing foreign 

 sand for making plate glass was a heavy expense in the manufacture. 

 The French sand cost about 21s. per ton, as compared with only 3s. per 

 ton for English sand, including cleansing by washing ; but the latter 

 when washed clean of impurities was good enough for the manufacture 

 of sheet glass. 



Mr. J. Silvester inquired whether the use of iron plates laid upon the 

 upper surface of the glass had been tried for flattening the glass in the 

 annealing oven : these were used successfully for flattening sheet steel, 

 which was rendered necessary by the tendency of the sheet to buckle 

 in hardening, but if made perfectly flat during the process of tempering 

 it remained so afterwards, and he thought the same plan might answer 

 for flattening plates of glass. 



Mr. F. J. Bramwell thought there would be a good deal of difficulty 

 in employing iron plates as covers for keeping the plates of glass flat in 

 the annealing oven, on account of the large size of plates that would be 

 required, 180 in. long, by 80 to 100 in. wide. He remarked that in 

 grinding the emery that was used for smoothing and polishing the plates 

 of glass, it had formerly been customary to grind it dry under edge 

 runners ; but recently a valuable improvement had been made by 

 grinding it in a stream of water, the whole apparatus being otherwise 

 the same. 



Mr. W. E. Newton remarked that for separating substances having 

 different sizes of particles there were two methods that might be em- 

 ployed, the wet and the dry. The former had already been described in 

 the case of separating the particles of emery by streams of water 

 running at different velocities ; in the dry method the separation was 

 effected by a blast of air. The latter plan was devised and employed 

 by Mr. Bentall, of Weybridge, for separating into different degrees of 

 fineness the coal dust which he used in his foundry for making castings, 

 whereby he obtained castings much superior to those generally produced 

 for agricultural purposes. The coal was crushed by edge runners to a 

 great degree of fineness, and an air blast from a fan blew the dust into 

 a long covered box or chamber about 30 ft. long, the bottom of which 

 was divided into four lengths or compartments : the finest dust was 

 carried to the extreme end of the chamber and deposited in the furthest 

 compartment, while the coarser and heavier particles fell into the nearer 

 compartments, according to their respective sizes, the coarsest falling- 

 nearest to the grinding apparatus. The process was found most satis- 

 factory in producing a distinct and accurate separation of the different 

 sorts of coal dust ; and he had himself examined with a microscope the 



vol. v. 



