GLASS 



661 



1101 



the arched grid b a t built of fire-bricks, whoso apertures are wide enough to let the 

 flames rise freely, and strike the bottom and sides of the vessel. From 1 to 2 feet 

 under that arch, the fuel grate c d is placed. B c are the 

 two working openings for introducing the materials and 

 inspecting the progress of the fusion ; they must be closed 

 with fire-tiles and luted with fire-clay at the beginning of 

 the process. At the back of the furnace, opposite the mouth 

 of the fire-place, there is a doorway, which is bricked up, 

 except upon occasion of putting in and taking out the pot. 

 The draught is regulated by means of a slide-plate, upon 

 the mouth of the ash-pit /. The pot being heated to the 

 proper pitch, some purified pearl-ash, mixed with fully 

 twice its weight of colourless quartz-sand, is to be thrown 

 into it, and after the complete fusion of this mixture, the 

 remaining part of the sand, along with the oxide of lead 

 (fine litharge), is to be strewn upon the surface. These 

 siliceous particles in their descent serve to extricate the air 

 from the mass. Whenever the whole is fused, the heat must 

 be strongly urged to insure a complete uniformity of com- 

 bination by the internal motions of the particles. As soon 

 as the gas has been found by making test phials to be per- 

 fectly fine, the fire must be withdrawn, the two working- 

 holes must be opened, as well as the mouths of the fire-place 

 and ash-pit to admit free ingress to cooling currents of air, 

 so as to congeal the liquid mass as quickly as possible ; a 

 condition essential to the uniformity of the glass. It may 

 be worth while to stir it a little with the pottery rod at 

 the commencement of the cooling process. The solidified glass may be afterwards 

 detached, by a hammer, in conchoidal discs, which after chipping off their edges, are to 

 be placed in proper porcelain or stone-ware dishes, and exposed to a softening heat, in 

 order to give them a lenticular shape. Great care must be taken that the heat thus 

 applied by the muffle furnace be very equable, for otherwise wreathes might be very 

 readily produced in the discs. A small oven upon the plan of a baker's is best fitted 

 for this purpose, which being heated to dull redness, and then extinguished, is ready 

 to soften and afterwards anneal the conchoidal pieces. 



Guinand's dense optical flint-glass of specific gravity 3'616, consists, by analysis, of 

 oxide of lead, 43'05 ; silica, 44 - 3 ; and potash, 11-75 ; but requires for its formation the 

 following ingredients : 100 pounds of ground quartz ; 100 pounds of fine red lead ; 35 

 pounds of purified potash ; and from 2 to 4 pounds of saltpetre. As this species of 

 glass is injured by an excess of potash, it should be compounded with rather a defect 

 of it, and melted by a proportionably higher or longer heat. A good optical glass 

 has been made in Germany with 7 parts of pure red lead, 3 parts of finely-ground 

 quartz, and 2 parts of calcined borax. 



5. Plate-glass. This, like English crown-glass, has a soda flux, whereas flint-glass 

 requires potash, and is never of good quality when made with soda. We shall distri- 

 bute our account of this manufacture under two heads : 



1. The different furnaces and principal machines, without whose knowledge it would 

 be impossible to understand the several processes of a plate-glass factory. 



2. The materials which enter into the composition of this kind of glass, and the 

 series of operations which they undergo ; devoting our chief attention to the changes 

 and improvements which long experience, enlightened by modern chemistry, has in- 

 troduced into the great manufactory of St. G-obain in France, under the direction of 

 M. Tassaert. It may, however, be remarked that the English plate-glass manufacture 

 derives peculiar advantages from the excellence of its grinding and polishing 

 machinery. 



The following description given by Dr. Ure refers almost entirely to the manufac- 

 ture of plate-glass in France. It is retained in nearly its original form, and is, in 



free from lime and iron, and 



all respects, applicable to the manufacture of plate-glass in this country. 



The clay for making the bricks and pots should be free from lime i 

 very refractory. It is mixed with the powder of old pots passed through a silk sieve. 

 If the clay be very plastic it will bear its own weight of the powder, but if shorter in 

 quality, it will take only three-fifths. But before mingling it with the cement of old 

 pots, it must be dried, bruised, then picked, ground, and finally elutriated by agita- 

 tion with water, decantation through a hair-sieve, and subsidence. The clay fluid 

 after passing the sieve is called slip (coulis). 



The furnace is built of dry bricks, cemented with slip, and has at each of its four 

 angles a peculiar annealing arch, which communicates with the furnace interiorly, and 



