KILN POTTERY. In modern practice, kilns as used in the clay industries may he described in three general types, up-draft, down draft and muffle kilns. The last named stands alone, as its distinctive feature lies not in the method of firing or the direction of the draft but in the fact that the kiln consists of a single laboratory closed to the direct entrance of flame and heated by radiation through the walls. This type of kiln is used in cases where it is necessary that the contained wares should be protected from dust and smoke, but where it is not convenient to enclose them in saggers. Large ware such as porcelain bath-tubs and sinks, heavy pieces of terra-cotta, etc., cannot be set in saggers as pottery is on account of weight and size. They are best burned in a muffled kiln, being set directly upon the brick floor. Muffled kilns are also used for burning painted wares. These are smaller than the kilns for heavy goods and are burned to a much lower temperature. Such kilns can be set, burned, cooled and drawn in 24 hours, while the large muffles cannot be turned in less than 10 days.
General kilns for pottery burning are of the open type. The wares are enclosed in saggers which are set over one another in tiers (bungs). Saggers are cases made of refractory clay and suited as to size to the wares they are to con tain. The flames and kiln gases pass freely round the saggers and the whole chamber is uniformly heated.
The usual form of kiln is cylindrical with a slightly domed crown. Outside this is the 'hovels or conical top familiar to the inhabit ants of pottery towns. The hovel serves the purpose of a chimney, and collects the smoke from a number of apertures in the kiln crown. The fire mouths range in number from 6 to 10 according to the size of the kiln. In the up-draft kiln the gases simply pass up between the bungs of saggers and find a free vent at the top. In the down-draft type the top of the
crown is closed and the gases after passing up the walls and under the crown are led down a centre stack and up again through flues ar ranged for the purpose. The structure of the down-draft kiln is more complicated than the up-draft, but a considerable economy of fuel is effected. The kiln also cools more rapidly.
Kilns for burning bricks follow the same general lines but vary in form. For this pur pdse the square kiln is among the most popular, being usually operated on the down-draft prin ciple.
Many plans hate been devised for the per fect utilization of heat, but none that is entirely satisfactory. One of the best is that of the con tinuous kiln. This has been applied in Germany to the burning of porcelain, and its use in America increases. The continuous kiln is a long low tunnel built in the form of a paral lelogram with rounded ends. This is divided into as many chambers as may be necessary, each chamber having an entrance at each side and two fire mouths. Paper partitions are used in order to secure the correct movem .nt of the draft, and as each successive chamber is filled with ware and the proper dampers opened the heat from the burning chambers is drawn through the unburned brick, bringing them up to a high temperature without any additional fuel. Meanwhile the chambers in which the firing has been completed are beginning to cool and so the work goes on continuously. The economy of the method is very great, but a considerable output is necessary in order to avoid stoppage. Kilns are, for the most part, burned with coal, both hard and soft coals being used. Those in the gas belt are success fully burned with natural gas and in some places oil is used. The temperatures at which differ ent wares are burned are about as follows: