Low-fire bodies are defined by when the temperature at which the clay body matures, generally considered to be between cones 09 and 02 (1700 and 2000 degrees F or 927 and 1093 degrees C). Low-fire clays tend to have good workability and usually will not shrink, warp, or sag excessively. However, they are softer which means that they are less durable and will absorb liquids.
Low-fire clays are divided into two types according to their color after firing. Darker-colored bodies (most commonly red), and the white and buff clay bodies.
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Red or Dark Earthenware Clays
Darker earthenware clay bodies can range from an orange-red to a dark brown, with red being the most common. Their color derives from the iron-bearing clays used their clay bodies. The iron already within the clay body acts as a fluxing (melting) agent, which matures the clay at relatively low temperatures. Earthenware clays melt at such low temperatures that they seldom become fully vitrified. Because of this, the fired ware will continue to absorb liquids. For this reason, functional ware is almost always glazed. Appropriate, non-toxic glazes must be chosen, however, as some glazes are also mildly absorbent at this temperature range.
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White or Buff Earthenware Clays
Because of an increased interest in low-temperature firing, new varieties of low-fire clay bodies have been developed. These clay bodies have also been given the label “earthenware” due to the fact that they mature in the earthenware temperature range.
The idea of low-firing white clay bodies actually began further back to Europe, when pottery factories began trying to duplicate the porcelain ware that had become available from eastern Asia. These clay bodies required large quantities of fluxing agents, in order to lower the melting temperature for the relatively clean mixtures of kaolin and ball clays. The white bodies of today are still composed of about half clay and half added fluxing agent, such as talc.