Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/676

Rh GLASS 658 Plate V plate-glass furnaces, is shown i n Plate '., where ﬁg. 3 is a ground plan at the level of tlie siege of a common form of furnace, while in fig. 1 is seen a front elevation of the same furnace, l, 2, and 3 being the working holes, 4, 5, G, and T pipe-holes for heating the blowing pipes, and 8, 9, and If) foot-holes for mending the pots and sieges. The furnace Furnace. is covered with a low—roofed crownlor dome, and the whole structure is bound together with a system of iron bars. The materials used in the construction and lining of all furnaces must be selected with the utmost care, and built with special regard to the enormous temperature to which they are subjected. Formerly a ﬁne-grained purely siliceous sandstone was much used, but now the principal materials .._.._......__.é_J....._..,._..._...._.. . .ierarLa;* . J "1 -. ‘fl//"".“~r' ' I N '/,-"/f / [..i.'L'r.iC1‘ URE. are large moulded bricks or blocks of ﬁre-clay of the most infusible and refractory description. For the crown of the furnaces used in plate-glass melting Dinas silica blocks are employed. In laying the blocks and throwing the arches no mixture containing lime can be used, but only ﬁre-clay or Dinas sand, in as small quantity as possible. .$'honld any of the materials of the crown of the furnace gradually fuse under the inﬂuence of the heat, the dropping of the molten matter into the glass-pots is the cause of most serious annoyance and loss to the manufacturer. An English ﬂint-glass furnace furnishes the type of cir- cular furnaces. Usually a large number of pots, sometimes ten, are provided for in such a furnace, because, the objects made in ﬂint glass being in general of small size, the metal is worked off only slowly, and a large number of glass- blowers can be accommodated at the separate work-holes. The arrangements of the cave and ﬁre-grate are the same as in the case of square or oblong furnaces, but ﬂint-glass furnaces differ from the prevailing rule in others by being provided with a system of ﬂues and chimneys, one flue being placed between each pair of pots. The general appearance presented within a ﬂint—glass house is illustrated in Plate l:1I:Il<- VI. ﬁg. l; and the accompanying woodcut (ﬁg. 3) is a sec- tional illustration showing the construction and internal appearance of a seven-pot furnace. The furnace is com- posed of a double arch or vault springing from strong pillars or abutments M. The space 0, between the outer arch and the vault proper of the furnace J, is a common receptacle for the ﬂues Jff led from within the furnace, and the products of combustion escape by the chimney 2'. The work-holes are at la, and at that place the furnace wall is taken down when a pot requires to be removed and renewed. The “cave” or air canal is seen at In; 2: is the ﬁre-grate, stoked in this case from one side only ; I shows openings at which the blowing tubes are heated; on is an opening for cleaning the ﬂues; and a is the bank or siege with the position of the pots indicated. Fre-- quently instead of being arched the outer portion of the furnace is carried up in the form of a wide truncated cone or open chimney stalk, and in other cases short separate chimney stalks are built for each ﬂ11e terminating within the glass-house, which itself forms such an open-topped FIGS. 4 and 5.—Siemens’s Continuous Tank Furnace. cone or chimney. Of course in cases where such separate small chimneys are provided no second or outer vault is required. In the year 1861 Dr (.3. W. Siemens introduced a form of furnace in which the use of melting pots was altogether abandoned, and the batch was introduced into, melted in, and worked from a tank which occupied the whole bed of the furnace. This furnace he heated from the sides by means of his well-known regenerative gas system described under FURXACE and IP.o.'. In 1872 he effected a further development of the tank furnace by dividing the tank, on the principle of his melting pot, by means of two ﬂoating bridges or partitions into three compartments, and thus he elaborated what is termed Siemens’s patent con- tinuous melting furnace. Of this improved furnace ﬁg. 4 shows a longitudinal section, and ﬁg. 5 is a transverse