Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/681

Rh MANUFACTURE] section. A is the melting cou1part111ent, B the reﬁning compartment, and C‘ the working-out compartment. The compartment A is fed with raw material (or batch) through the door D at the back end of the furnace; it is separated from the compartment B by the ﬂoating bridge E, under which the partially melted glass passes to reach the latter. In the compartment B the metal, by the inﬂuence of the higher temperature maintained on its surface, is com- pletely puriﬁed, and sinks to ﬂow under the bridge F in a complete workable condition. Suitable provision is made, by means of air—passages, to keep the sides of the tank of the requisite temperature to prevent any egress of glass through them, and the ﬂoating bridges E and F are renewed as often as necessary. The regenerative gas furnace is em- ployed, and the gas and air ports H H’, leading from the regenerators I I’ respectively, are arranged along each side of the tank, so as to cause the ﬂames to play across the furnace. The temperature of the diﬁerent parts is regulated according to the various stages of preparation of the glass iii the several compartments, this regulation being effected by constructing the gas and air ports of larger dimensions, or increasing their number, where the greatest heat is re- quired; it is also facilitated by means of division walls (not shown in the ﬁgures) which may be built over the ﬂoating bridges to separate the compartments. The temperature of the working—ont compartment C is controlled by regulat- ing the draught of the furnace chimney, by diminishing which more or less ﬂame must necessarily pass from B over the ﬂoating bridge F into C, and through the working holes M. The principal advantages resulting from the use of the continuous melting furnace are the following :— 1. An increased power of production, as the full melting heat may be employed without interruption, whilst with the old method of melting nearly half time is lost by cooling and settling the metal, the working it out, and the re-heating of the furnace. 2. An economy in working, as only half the number of men are roquired for the melting operations. 3. A greater durability of the tank and furnace, owing to the uniform ti,-mperature to which they are subjected. 4. - much greater regularity of working, and more uniform quality of the product than in other furnaces. 5. For the manufacture of window glass, the compartment C may be so arranged that the blowers can work without interfering with the gather_ers; this would do away with the separate blowing fur- nace IlO' 111 USC. ‘ Although the Siemens regenerative gas ﬁring is generally used with tank furnaces, that system is not essential to the successful working of tanks. Mr Archibald Stevenson of Glasgow has patented a tank furnace ﬁred by common coal from one end, with working holes on the other three sides, and furnaces on this principle are worked in a perfectly satisfactory manner with much economy of coal and working room. Tank furnaces are used principally in bottle works and in the manufacture of rolled plate. The following statement showsthe extent to which Siemens furnaces and tanks have been introduced by manufacturers. Furnaccs using Pals. Window and Plate Glass. Bottle Glam Flint Glass Great Britain .......... .. 5 5 2 France. ................ .. 7 4 11 Belgium .............. ... 4 1 1 Other countries ..... .. 6 10 15 Furnaces with Trmlcs. Great Britain ....................................... .. 6 Furnaces 'u:ith Continuous Tanlcs. Great Britain ........................................ .. 4 France ................................................. ..10 Belgium. ............................................. .. 1 Other countries ..................................... .. 3 Formerly it was the habit that to the sides or wings of the main melting furnace there were attached calcar or fritting arches, annealing ovens, pot-ﬁring arches, and other sub- Esidiary furnaces required in certain stages of glass manu- GLASS 659 facture. These were heated by ﬂues leading from the ﬁre- space of the principal furnace; but such a practice is 11ow generally abandoned, and distinct furnaces or ovens, arranged and ﬁred according to the necessities of the case, are provided instead. In the manufacture of common bottle glass, however, for which highly impure materials are used, it is still the practice to prepare a frit in a side arch occasionally attached to the melting furnace. The whole of the pots in a common furnace are charged or ﬁlled with the prepared “batch” at the same time. Immediately the heat is forced, and the stoking must thereafter be carefully regulated to maintain the high temperature. As the mass begins to fuse it settles down and occupies considerably less space in the pot, to which thereon a second quantity of material is added, and generally a third portion is subsequently ﬁlled in so as to have at the close of the melting process as large a quantity of metal as possible. When the fusion is complete a scum composed of uncombined salts, and known as glass gall or sandiver, rises and collects on the surface. It consists almost entirely of sulphate of soda, with sulphate of lime, and a small percentage of chloride of sodium. This scum of glass-gall is carefully removed with a perforated scoop, and the heat of the furnace is then forced to the most intense degree with the view of rendering the metal as ﬂuid and limpid as possible, so as to free it from all included gaseous bubbles which it still contains. This process of “ﬁning,” “reﬁning,” or hot-stoking, as it is indifferently termed, involves a temperature which is esti- mated in certain cases to reach from 10,000° to 1"_’,00O° Fahr. ; and the operation is sometimes assisted by stirring the molten mass with a pole of wood, in a manner analogous to the poling of copper i11 the reﬁning of that metal. Throughout the operation of melting, test pieces are periodi- cally withdrawn from the pots for the purpose of observing the progress and condition of the glass. When it is found that the vitriﬁcation is complete and the object of reﬁning fully accomplished, the heat of the furnace is considerably reduced, so that the glass may be brought into that condition of viscosity in which it is capable of being worked. Tn contradistinction to the reﬁning or hot—stoking period, this is known as colcl—stoki11g. Glass W07-In'n‘r/.——The means by which melted glass is caused to assume its varied forms for use are (1) by blow- ing ; by casting; and (3) by pressing in moulds——an operation i11 which the other two processes may be partly combined. Minor manipulative processes which do not fall under any of these heads are called into action ; but these are for the most part merely subsidiary to the others, which reallyzcomprehend all the lines along which the formation of glass proceeds. Having regard principally to the forms into which glass is worked and the uses to which it may be applied, the following classiﬁcation embraces the principal departments of the glass-making industry. 1. Flat glass. Crown glass. Sheet glass. Plate glass. II. Hollow glass. Flint glass, blown. Bohemian glass. Venetian glass. Bottle glass. Slag glass (Britten’s). Tube and gauge glass. III. Pressed and massive glass. Flint glass. Optical glass. Strass. Rod glass, marbles, and beads. _ _ IV. Coloured, opar 11c, and enamel glass, including mosaics and hot cast porcelain, &c. glass