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Geologists (and mineralogists) characterize shales primarily by their physical properties, and there are many types that vary greatly (e.g. in hardness). Shales contain many minerals (i.e. limestone, quartz, feldspars, iron compounds, Illite, etc.) and the identity of these minerals often determines the suitability of a shale for a ceramic purpose. Shale particles normally do not fuse until at least cone 03+ (1080c) depending on the iron oxide content.
Shales can make up a very high proportion of the body for structural products. High shale mixes for vitrified pipe and roofing can be formed by extrusion with little shrinkage. Since shales can contain significant iron they can fire to varying shades of red. Shale bodies can be fired quite dense (less than 3% percent porosity) and retain reasonable stability in the kiln. When ground fine enough some shales can exhibit a clay-like plasticity and even need to be treated with care to avoid a laminations and drying cracks.
The analysis shown here is an average we found in the Handbook of Chemistry and Physics, however there are many shales that are also low in iron.
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By Tony Hansen