Clay Types

Clays are produced by the weathering of igneous rocks, typically granite, which is composed mainly of feldspar, an alumino-silicate mineral. Clays produced within the vicinity of the parent rock are known as primary clays. They tend to be purer materials, less plastic and more vulnerable to distortion and cracking on firing. Kaolin (Al2O3.2SiO2.2H2O), which is the purest clay, comes directly from the decomposition of the feldspar in granite. Secondary clays, which have been transported by water, have a higher degree of plasticity, and fire to a buff or brown colour depending upon the nature and content of the incorporated oxides. Generally, secondary clays, laid down by the process of sedimentation, have a narrower size distribution and their particulate structure is more ordered.

The most common clay minerals used in the manufacture of building materials are kaolin, illite (a micaceous clay) and montmorillonite, a more plastic clay of variable composition. Clay crystals are generally hexagonal in form and in pure kaolin the crystals are built up of alternating layers of alumina and silica (Fig. 8.2). However, in illite and montmorillonite clays, the variable composition produced by sedimentation produces more complex crystal structures.

Ball clays are secondary clays containing some organic matter which is burnt off during the firing process; they tend to have a fine grain size which makes them plastic. When fired alone they have a high shrinkage and produce a light grey or buff ceramic, but they are usually blended into other clays such as kaolin to make a workable clay. Terracotta clays contain significant proportions of iron oxide which gives rise to the characteristic red colour on firing. While the major clay materials used in the manufacture of ceramics are kaolin, illite, feldspar and ball clay, other minor constituents as well as chalk and quartz are frequently incorporated to produce required ceramic properties on firing.

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