furnace; the impetus of the flame gases and their rapid expansion during combustion carry the flame out across the furnace, while the chimney draught ultimately sucks it back to the exit ports, the shape of the flame being shown in Fig. 5. [Illustration: FIG. 5.--Diagram of a furnace with “horse-shoe” flame.] In general arrangement, a tank furnace for glass melting resembles an open-hearth steel furnace. The tank or basin, as already indicated, is built up of a number of large fire-clay blocks, forming a bath varying in depth from 20 in. to 42 in. according to the design of the furnace and the kind of glass to be melted in it. The ports for entry of gas and air and for exit of the products of combustion are in most modern furnaces placed in the side walls of the furnace just above the level of the glass, the whole being covered by a vault built of silica brick. Figs. 6 and 7 show the general arrangement of a simple form of tank-furnace such as that used in the manufacture of rolled plate glass. The furnace indicated in the diagram is intended for regenerative working with alternating directions of flame; in recuperative furnaces the horse-shoe flame is always used in tanks, while this arrangement of ports is sometimes adopted for regenerative tanks also, particularly in the manufacture of bottles. For the production of sheet glass, tank furnaces are generally sub-divided into two compartments and are also provided with various constrictions intended to arrest impurities and to allow only clear glass to pass, but as regards the arrangement of flues and ports there is a very general similarity between various furnaces of this type. [Illustration: FIG. 6.--Longitudinal sectional diagram of tank furnace.] [Illustration: FIG. 7.--Transverse sectional diagram of tank furnace, showing regenerators and gas and air passages.] It is beyond the scope of this book to discuss the relative merits of tank and pot melting furnaces; wherever the former can be made to produce glass of adequate quality for the purpose desired, the great economy of the tank furnace inevitably carries all before it, so that bottle glass, for example, is now made exclusively in tanks, and the same applies also to rolled plate of the ordinary kind, and to the great majority of sheet glass. On the other hand, where special qualities of glass are required in relatively small quantities, or where the requirements as to quality are very extreme, the pot furnace remains indispensable. Optical glass and coloured glasses are examples of this kind, although some tinted glasses are used in sufficient quantity to justify the use of small tank furnaces for their production. The causes of the greater economy of the tank furnace are numerous, and complicated by the detailed requirements of each particular manufacture, but the most important factors in the question may be summed up thus:--