For about 95% of ceramic history, nature did the mixing and potters simply mined the clay. If potters did anything, it was to screen out the trash and stones. “Designer” clay bodies are a very recent development, as is the knowledge of how to mix them properly.
Defining the Terms
Slake: To soak dry clay in water until the clay is fully wetted.
Electrostatic attraction: The relatively weak force between particles with opposite electrical charges that pulls the particles towards each other and, if they make contact, can hold them together.
Temper: An addition to clay bodies, such as sand or grog or natural fiber, which improves workability. These additions may affect the fired result but they are added essentially to assist in forming and drying.
Grog: Ground, fired clay body added to clay bodies, in either the wet or dry stage, to provide texture (both tactile and visual) along with tooth or bite for better control in forming. Grog opens a body up to aid in uniform drying and, because the grog is already fired, it proportionally cuts down on overall shrinkage and the tendency to crack or warp.
Consistency is the Key
Any clay body is fundamentally a mixture of clay, flux, and glass formers. Various forms of temper may be added to influence forming and firing properties, but these are mostly inert materials that essentially go along for the ride. In the case of porcelains, the clay is usually kaolin or ball clay. The glass former is typically ground quartz (silica). The flux is usually supplied by some type of feldspar, which is a naturally occurring mineral composed of alumina, silica, and fluxes.
Mixing these ingredients together would be simple except that feldspar particles tend to stick to each other like socks with static cling from the dryer. Tiny bits of feldspar attach to each other and resist being mixed into the clay body as individual particles. The force holding the feldspar particles together is weak electrostatic attraction but the force is strong enough to form feldspar clumps that, if mixed into the clay body, will melt into pockets of glass and contribute to bloating and slumping when the clay body is fired.
To prove that feldspar clumping causes these problems, a scientist at Rio Tinto Borax (formerly U.S. Borax) tested bars of a clay body made from exactly the same recipe, but mixed by different methods. The best mixed test bars fired perfectly straight while the poorly mixed samples slumped to varying degrees in identical firings in a industrial computer-controlled kiln (see graph at right).
The slumping occurred at about 180°F below peak firing temperature, and slumping corresponded with the conversion of clay to mullite and the melting of the feldspar bits into glass. Slumping is exacerbated when feldspar particles are clumped together rather than dispersed by proper mixing.
The traditional method of mixing clay in large quantities is to use a mixer that runs at very low speed. These devices do an excellent job of incorporating coarse materials into an already mixed clay body so they are useful for changing the amount of grog in a body. However, they do not prevent the issue of feldspar clumping. This can only be addressed by slurry mixing that coats the feldspar particles with clay, thus preventing feldspar clumps.
Slake Mixing 101
It is necessary to use a significant excess of water while mixing the batch to achieve optimum blending of clay body materials, according to Dr. William Carty of the New York State College of Ceramics at Alfred University. Mixing should be accomplished with a high shear mixer—one with a top speed in the range of 3000 feet per minute. The precise amount of water is not critical but it should be on the order of three times as much as will remain in the clay when the body is dewatered to a workable consistency.
Any plasticizer such as bentonite or Veegum should be added to the water first and slaked and mixed thoroughly. This may take up to 24 hours. Then approximately 20% of the clay in the recipe should be added. After it is slaked and mixed well, a process that takes a few minutes, the feldspar should be mixed in. This process coats the feldspar particles with clay and prevents them from clumping. The other non-plastic materials are then added, and finally the remaining clay is slaked and mixed in.
Since extra water has been used, the result is a slurry that must be dewatered to be usable. Industry typically accomplishes this with a filter press, but artists can pour the slurry into a plaster or bisque mold to pull out the water and achieve the same result. Using this process, Carty found, maximizes plasticity of the body, which is achieved within three days of mixing.
This article was excerpted from the January 2011 issue of Ceramics Monthly, which can be viewed here.
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