Salt Kilns (Soda Kilns)
Salt (soda) makes a very identifiable type of glaze when introduced into a kiln that is heated into the stoneware range. Many know this glaze as orange-peel glaze, due to its bumpy surface which resembles the bumpy outer surface of an orange.
Salt kilns are fuel-fired downdraft kilns that are specifically used for salt firings. When the salt is thrown into the intense heat of the kiln, it goes through a chemical reaction. This leaves a residue glaze on both the ware and the interior of the kiln. The residue shortens the lifespan of the kiln.
Salt kilns are not often used for any other type of firing, since the residue on the interior of the kiln can vaporize and attach itself to pots in unexpected and unwanted ways.
Raku Kilns
Raku is a technique where pottery is pulled from the kiln while it is at its highest temperature, somewhere between cones 08 and 04. The pottery is then usually smoked to produce interesting surface effects.
Raku kilns are smallish and designed to allow easy access to the hot ware so that the pots can be removed quickly. They may be fuel-fired or electric, but they must be placed either outdoors or directly by a door leading to the outside. This is because the glowing hot pottery must be placed within their containers of combustible material within seconds of removal from the kiln.
Because of their small size, temperature limitations, and outdoor placement, kilns especially built for raku are not often used for other types of firings.
Test Kilns
Many studio potters like to develop their own clay body and glaze recipes. In order to do this, tests must be run on and adjustments made to the forumla. This is where test kilns come in.
Test kilns are tiny, usually with less than one cubic foot of interior space. They are specifically designed to fire small test tiles or test pots so that a glaze or clay body can be evaluated.
These kilns may be fuel-fired or electric, but care must be taken that the test pieces are fired in the same manner as the finished pottery will be. The potter must be mindful to follow the firing schedule that will later be used. Test kilns, with their tiny size, can heat much more rapidly than their larger cousins if they are not monitored and controlled.