FICTILE MANUFACTURES. 



or sucking surface, they are turned against the 

 other in a horizontal position, sand and water 

 being constantly supplied between them ; and by 

 means of the most ingenious machinery, the two 

 surfaces rapidly rub one against the other in all 

 directions, and are ground at the same time by 

 the sand.' The other sides are then subjected to 

 the same process. A very thin layer only of the 

 surface is rubbed off, the sheet lying so perfectly 

 flat. 



Plate-glass. 



The manufacture of plate-glass requires greater 

 care than either of the two preceding kinds, 

 and the process is different plate-glass being 

 moulded, and not blown, as is the case with other 

 kinds of glass-ware. The materials, generally 

 speaking, are the same, great purity, however, 

 being always aimed at, especially in the sand : a 

 small quantity of lime is added to the sand and 

 soda, and the English makers use a small quantity 

 of arsenic. In France, where the most perfect, but 

 also most expensive plate-glass is produced, the 

 makers use borax instead of arsenic : this renders 

 the glass so soft, that, although it makes better 

 mirrors, it is not so well suited for putting in 

 windows. 



For the melting of the materials, the pots used 

 are the same as those used in the manufacture of 

 crown and sheet glass ; but in addition to these 

 are other and similarly formed pots, called cisterns, 

 differing only from the melting-pots in having a 

 stout rib running around each one about its 

 middle. When the glass is properly melted and 

 scummed or refined, it is ladled out with a copper 

 ladle into the cistern, which stands in the furnace 

 beside the pot. The cisterns are suited in size to 

 the sheets which are to be cast. The metal is left 

 in the cistern for ten or twelve hours to fine, after 

 which it is lifted out by means of an enormous 

 pair of pincers, which are balanced on a fulcrum 

 resting on wheels, so that the cistern is laid hold 

 of, lifted out of the furnace, and wheeled to the 

 casting-table. Then it is lifted by a crane, which 

 places it over the casting - table a beautifully 

 smooth and polished plane of iron, 20 feet long, 

 by 10 or more in width. Upon this the metal 

 begins to flow rapidly in a glowing red-hot state. 

 Two bars of polished iron of the thickness of the 

 intended glass plate are laid one on each edge of 

 the table, and a polished iron cylinder of consider- 

 able weight, and long enough to rest on the edge- 

 bars, is lowered, and rolled along the bars : this 

 spreads out the liquid metal over the table ; the 

 bars preventing it from running over the sides, 

 and regulating its thickness. For a few seconds 

 after the rolling is finished, the appearance of the 

 sheet of glass is very beautiful : at a glowing red 

 heat, it parts irregularly with its heat, and hence 

 its surface, notwithstanding it has been rolled with 

 a polished roller on a polished iron table, is a minia- 

 ture sea covered with waves. This will be quite 

 understood by any one who has seen rough plate- 

 glass. 



In a very short time, the sheet is sufficiently 

 hardened to allow the operators to seize it on 

 either side with hand-pincers, and pull it forward 

 on to an endless band of wire-gauze, which is of 

 equal width with the table. This band travels on 

 rollers, and transfers the sheet of glass to the 

 annealing oven, which is of immense length, as 



the sheets must lie flat in it, being too large to be 

 tilted on edge, ^as in the crown and flint glass 

 kilns. After a sufficient annealing, the plates are 

 next polished. 



The polishing is done chiefly by machinery. In 

 the first process, the plate of glass is laid upon 

 a low table, and is imbedded in cement, so as 

 to leave only the upper surface exposed. Sus- 

 pended over the table by long rods of iron are the 

 rubbers, pieces of wood which rest on the surface 

 of the glass. To the iron rods are attached crank 

 arms, which are moved by steam-power, and 

 make the rubbers move rapidly in circles over the 

 surface of the glass, which is kept well supplied 

 with fine sand and water. When all the irregu- 

 larities are rubbed down, and the surface is per- 

 fectly level, the plate of glass is reversed, and the 

 other side dressed in the same way. The next 

 process is performed by women, who take large 

 fragments of glass, and apply them with emery 

 and water to the surface of the plate, and rub till 

 the whole is polished, and all the sand-marks are 

 removed. The final polishing is done by a series 

 of mechanical rubbers, which are covered with 

 leather, and supplied with minium, or red-lead, as 

 the polishing material. These rubbers move with 

 great rapidity backwards and forwards, upon a 

 large table, upon which the plate is laid, and they 

 are sufficiently numerous to act upon every portion 

 of the plate, and give it the beautiful lustrous 

 polish which makes this kind of glass so valuable. 



Plate and sheet glass are both made into 

 mirrors ; the latter only for those of a small size. 

 The ordinary plan is, to take tinfoil and lay it 

 over the surface of the glass, which is for the 

 purpose placed on a very strong table, with a 

 raised edge running round it, and with green 

 baize, or other cloth, between it and the glass. 

 Quicksilver is then poured over the tinfoil, with 

 which it forms an amalgam, and the whole is 

 covered over with smooth boards, upon which 

 very heavy weights are placed, which press the 

 amalgam, and squeeze out any free mercury. After 

 a sufficient time, the weights are removed, and 

 the mirror is finished. Several other methods 

 have been from time to time employed, all con- 

 sisting chiefly in precipitating silver from the solu- 

 tions of its salts upon the surface of the glass, but 

 hitherto they have not supplanted the old method, 

 which is that chiefly employed. 



Flint-glass. 



Flint-glass, or crystal, is composed of fine white 

 sand which is calcined, sifted, and washed for 

 the purpose red-lead, or litharge, and refined 

 pearl-ash. It was formerly made of calcined flint, 

 but the finest Lynn sand has been found to produce 

 a clearer glass, and is therefore preferred. 



A flint-glass furnace varies little from those 

 described for other kinds of glass, except that it 

 is round in the top. The pots in which the glass 

 is melted have their tops arched over, that no 

 dust may fall in, with a hole at the side near the 

 top, for the insertion of the tube (fig. 11). When 

 the glass is sufficiently melted, the tube is inserted, 

 and a quantity lifted out upon its point, in the 

 same manner as for crown-glass. After being 

 rolled upon the marver, the glass is blown out to 

 a globe-shape, when the punty-rod is attached, 

 and by means of an instrument resembling a pair 



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