FURNACES AND FUEL. The furnaces used for melting glass are of two general types, pot fur naces and tank furnaces. The traditional fur nace, still the type of pot furnace for flint-glass melting, is round, with from eight to twelve `monkey pots' in a circle around the central fire, at the base of an enormous chimney. The monkey pot is an oval cylinder with a round top, open only on the upper part of one side. Each one is inclosed in an arch of fire-brick, with its mouth only visible outside, and when it cracks the heat must be lowered and the arch torn down, while the red-hot crucible is dragged out and another substituted for it. The melting-pots play a very important part, and demand the most careful preparation. The slightest flaw is discovered by mens, and this type of furnace is in wide use in Europe and' America.
the intense heat, and the precious contents are emptied into the well below the furnace. The .clay is most carefully selected, but after months of labor in its construction, the monkey not In these furnaces the gas is generally produced from coal outside of the furnace, mixed with the air, on the principle of the Bunsen burner. In the United States natural gas, and later on pe troleum, have been largely used as a fuel. In stead of melting-pots there is a tank, constructed from pot-clay, covering the whole area of the furnace, and divided by floating partitions into compartments. The melting compartment at the rear receives the raw material through the doors. As this melts it flows into the refining compartment, where a higher temperature puri fies it till it flows under the second partition into the gathering compartment, and there a lower temperature thickens it for the blower. . A re cent improvement dispenses with the floating partitions by the use of floating vessels, which gather the molten glass at the lowest depths in the tank and raise it to the surface to be com pletely refined in a special compartment, whence, as it sinks in perfect fusion (the best glass being the heaviest), it can only flow into the working out compartment. The depth of the floaters is usually one-fourth the depth of the tank. These tank furnaces may be worked continuously, with no change in temperature, and no discoloration from smoke, and on a colossal scale.
Improvements in the methods of glass manu facture have all been in the direction of substi tuting gaseous for solid fuel. Even in pot furnaces gas is preferred, and for a tank furnace it is indispensable. Natural gas is the ideal fuel, but is very limited in its occurrence, and when it cannot be obtained gas is manufactured from other fuel. Its advantages are its freedom from ashes and other dirt, and the ease with which the flame may be applied and controlled. A com mercial classification of glass and of the prin cipal types of furnaces producing it may be given as follows: (1) Polished plate embraces all glass cast upon a smooth table, rolled to the required thickness with a roller, annealed, and then ground and polished. Under this head comes thin plate, a recently developed process. (2) Rough plate includes all glass cast as above, but not ground and polished. The principal varieties are ribbed plate, colored cathedral, rough plate. wire glass, heavy rough plate for sky lights. (3) Window - glass embraces all glass blown in cylinders and afterwards cut and flat tened out and polished while hot. Chiefly used for pictures and mirrors. (4) Crown glass is glass blown in spherical form and flattened to a disk shape by centrifugal motion of blowpipe. A little is now made for decorative purposes. (5) Green glass. All the common kinds of glass, and not necessarily green in color. Used in manufacture of bottles, fruit-jars, etc. (6) Lime flint includes the finer kinds of bottle-glass, cer tain lines of pressed tableware, and many novel ties. (7) Lead flint embraces all the finest prod ucts of glass-making, such as cut glass, fine table ware, artificial gems, and optical glass.
The principal furnaces are: (1) Open-pot fur naces for bottles and window-glass; (2) open pot furnaces for plate glass; (3) covered-pot furnaces for flint glass; (4) day-tanks, which are practically open-pot furnaces, gas-fired, and with a single tank or pot; (5) regenerative continuous tank furnaces for window-glass; (6) regenerative continuous tank furnaces for bot tles; (7) recuperative continuous tank furnaces for bottles.