Figs. 750, 751, and 752 show respectively a section on the line 2 2 of Fig. 748; a section on the line 4 4 of Fig. 749, and a sectional plan on the line 5 5 of Fig. 749, of another form of truck kiln. In this last, the truck B has no charging openings at its front end, but is charged tbrough the openings M in the end wall of the stove. It is provided with double sides B' (of which the inner may he perforated), with an intervening air space, by which the glass articles are prevented from becoming unequally cooled by direct contact with the outer skin, which is cooled by the atmo sphere. The gas- and air ports A' in this arrange ment are at the front end of the kiln, J and K being respectively the gas- and air-flues leading to them, and the chimney-flue A' is at the back end. The other parts are similar to those previously de scribed, and are indicated by similar letters of re ference. The results of Dr. Siemens' arrange ment may be rendered still more satisfactory, espeoially for heavy solid goods, by completely covering the glass, when placed in tho annealing oven, with heated sand. The strength of glass thus treated surpasses that of " toughened " or " hardened " glass.
The glasses whose percentage compositions have been given, may be regarded as mixtures of the sodio and calcic silicates, and may be generally represented by the formula CaO, It is true that the English, German, and Venetian plate-glass contain, as shown above, considerable proportions of potash; but the French plate-glass, which, until quite recently, has sur passed every other kind of plate-glass in quality, is strictly a mixture of the sodic and calcic sili cates, and it will be more convenient to con sider plate - glass with crown- and sheet-glass, than with Bohemian glass, with which, except for the presence of potash, it has little in common.
The materials used in the composition of these glasses are :—(1) white sand, as free as possible from oxide of iron ; (2) sodic carbonate, or sodic sulphate. If the latter be used, it must be mixed with one - tenth of its weight of carbon, in the form of charcoal or coal dust ; the carbonate is supposed to produce a better coloured glass than the sulphate, and for this reason, its use has, until quite recently, been re tained in the manufac ture of plate-glass ; (3) slaked lime, or chalk ; (4) arsenic trioxide, the use of which is preferred to manganese dioxide. The ingredients, having been thoroughly incorpo rated, are, without further preparation, introduced into the crucibles in which they are to be melted. When harilla, kelp, and other forms of crude alkali were in use, it was the custom to sub ject the materials to pre liminary fusion in a reverberatory furnace, thereby effecting partial decomposition, and the burning off of carbona ceous impurities. The
introduction of the use of the carbonate or sulphate of soda has rendered this treatment super fluous.
The crucibles are open reservoirs, and vary in form and size according to the purposes for which they are intended. Plate-glass crucibles have either grooves or projections on their outer surface, to afford a firm grip to the iron " elaspers," or forceps, by which they are raised for the purpose of casting their contents. Fig. 753 is a sectional view of a gas-furnace, arranged for melting sheet or crown-glass: C are crucibles ; G, G', gas-ports ; R, R', regenerators; A, air-ports. The fumy:* is oblong. The crucibles stand on the furnace-bed, and immediately within arches, which are temporarily closed. Through those arches, new crucibles are introduced, and injured ones withdrawn, and, in plate-glass works, the crucibles are re moved and restored respectively before and after each casting. In crown- and sheet-glass manu factories, when the crucible is in position, the opening of the arch is closed up to a point level with the upper edge of the crucible. Above this point, one or more spaces are left for the convenience of working, and are so arranged as to be readily closed by movable flre-clay slabs or stoppers. Plate-glass cruci bles are enclosed by movable doors, by the removal or replace ment of which, the entire arches can be quickly laid open or closed. With the exception of the manner in which the arches are closed, and of the absence of grooves on the outside of the crucibles, Fig. 753 represents also one of the best forms of plate-glass furnace.
Plate-Glass.—The great thickness of plate-glass, and the uses to which it is applied, viz. the regular transmission and reflection of light, necessitate the utmost care in the selection and manipulation of the raw materials. The manufacture of plate-glass depends upon the fluidity of molten glass, and the readiness with which, in the viscous condition, glass acquires the form of any resisting surface, and, at the same time, yields both to the superimposed pressure of an advancing roller, and to any resisting boundaries which may be set to regulate its extent and thickness. After the complete fusion of the glass, there are seven processes :—(1) the pouring of the molten glass upon a flat iron table; (2) the passage of the roller over the molten glass; (3) the annealing of the glass ; and (4, 5, 6, 7) the trimming, grinding, smoothing, and polishing of the glass.