GLASS 663 



floor of the halle. Plates of cast iron form the thresholds of these openings, and 

 facilitate the ingress and egress of the cuvettes. The apertures are arched at the top, 

 with hewn stone like the tunnels, and are 18 inches wide when the cuvettes are 16 

 inches broad. 



The upper and smaller apertures, or the higher ouvreaux, called the lading holes, be- 

 cause they serve for transvasing the liquid glass, are three in number, and are placed 

 31 or 32 inches above the surface of the sieges. As the pots are only 30 inches high, 

 it becomes easy to work through these openings either in the pots or the cuvettes. The 

 pots stand opposite to the two pillars which separate the openings, so that a space is left 

 between them for one or more cuvettes according to the size- of the latter. It is obvious 

 that if the tunnels and ouvreaux were left open, the furnace would not draw or take the 

 requisite founding heat. Hence the openings are shut by means of fire-tiles. These 

 are put in their places, and removed by means of two holes left in them in correspon- 

 dence with the two prongs of a large iron fork supported by an axle and two iron 

 wheels, and terminated by two handles which the workmen lay hold of when they 

 wish to move the tile. 



The closing of the tunnel is more complex. When it is shut or ready for the firing, 

 the aperture appears built up with bricks and mortar from the top of the arch to the 

 middle of the tunnel. The remainder of the doorway is closed, 1. on the two sides 

 down to the bottom, by a small upright wall, likewise of bricks, and 8 inches broad, 

 called walls of the glaye ; 2, by an assemblage of pieces called pieces of the glaye, be- 

 cause the whole of the closure of the tunnel bears the name of glaye. The upper hole, 

 4 inches square, is called the tisar, through which the billets of wood are tossed into the 

 fire. Fuel is also introduced into the posterior openings. The fire is always kept up 

 on the hearth of the tunnel, which is, on this account, 4 inches higher than the furnace- 

 hearth, in order that the glass which may accidentally fall down on it, and which does 

 not flow off by the bottom-hole, may not impede the combustion. Should a body of 

 glass, however, at any time obstruct the grate, it must be removed with rakes, by open- 

 ing the tunnel and dismounting the fire-tile stoppers of the glaye. 



Formerly wood-fuel alone was employed for heating the melting-furnaces of the 

 mirror-plate manufactory of St. Gobain ; but within these few years, the director of 

 the works makes use with nearly equal advantage of pit-coal. In the same establish- 

 ment, two melting-furnaces may be seen, one of which is fired with wood, and the other 

 with coals, without any difference being perceptible in the quality of the glass furnished 

 by either. It is not true, as has been stated, that the introduction of pit-coal has made it 

 necessary to work with covered pots in order to avoid the discolouration of the materials, 

 or that more alkali was required to compensate for the diminished heat in the covered 

 pots. They are not now covered when pit-coal is used, and the same success is ob- 

 tained as heretofore by leaving the materials two or three hours longer in the pots and 

 the cuvettes. The construction of the furnaces in which coal is burned is the same 

 as that with wood, with slight modifications. Instead of the close-bottomed hearth of 

 the wood-furnace, there is an iron grate in the coal-hearth through which the air 

 enters, and the waste ashes descend. 



When billets of wood were used as fuel, they were well dried beforehand, by being 

 placed a few days on a frame- work or wood called the wheel, placed two feet above 

 the furnace and its arches, and supported on four pillars at some distance from the 

 angles of the building. 



The progress of chemistry, the discovery of a good process for the manufacture of 

 soda from sea-salt, which furnishes a pure alkali of uniform power, and the certain 

 methods of ascertaining its purity, have rendered this department of glass-making 

 far more certain than formerly. At St. Gobain no alkali is employed, except artificial 

 crystals of soda, prepared at the manufactory of Chauny, subsidiary to that estab- 

 lishment. The first crop of soda-crystals is reserved for the plate-glass manufac- 

 ture ; the other crystals and the mother-water salts are sold to the makers of inferior 

 glass. 



If glass contains much lead it has a yellow tint. If manganese is present it changes 

 by the action of light to a pale rose. Iron imparts a dull greenish tint ; therefore the 

 proportions of all those materials should be adjusted with great care. 



At the mirror-plate works of Kavenhead, near St. Helen's in Lancashire, soda-crys- 

 tals, from the decomposition of the sulphate of soda by chalk and coal, have been also 

 tried, but without equal success as at St. Gobain ; the failure being unquestionably 

 due to the impurity of the alkali. Hence, in the English establishment, the soda is 

 obtained by treating sea-salt with pearlash, whence carbonate of soda and chloride of 

 potassium result. The latter salt is crystallised out of the mingled solution, by evapora- 

 tion at a moderate heat, for the carbonate of soda does not readily crystallise till the 

 temperature of the solution falls below 60 Fahr. When the chloride of potassium is 

 thus removed, the alkaline carbonate is evaporated to dryness. 



