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The term 'clay' is used in different ways. Potters often refer to their 'clays', these are typically recipes or mixtures of clay minerals, feldspar and quartz (more correctly these are clay bodies, or, just bodies). A lump of the material that has been mined from a deposit is also referred to as 'a clay'. However, it its strictest sense, the term 'clay' refers to flat microscopic particles from which that lump is composed. Actually, even more precisely, it refers to 'some of the particles' (since the lump will invariably have particles of many other minerals also). Clay particles have a surface chemistry that imparts an affinity for water (the other particles are just dead micro-rocks).
Clays occur when parent making rocks, referred to as 'clay-making minerals', break down physically (by weathering) and hydrate to form new mineral particles with new properties. This hydration involves insertion of complete water molecules into the crystal structure (whereas with most minerals oxides are converted to hydroxides on hydration).
Clays have plasticity. This property is a product of the fact that the surface chemistry attracts water electrolytically. The water thus becomes a glue and a lubricant that gives billions of particles the opportunity to express that collective property of plasticity.
From a mineral point of view, clays are hydrous-layer silicates of aluminum (kaolin is pure clay mineral, its chemistry is Al2O3.2SiO2). Clays have a wide range of particle sizes and shapes and mineralogies and these are related to the identity of parent rocks, mode of conversion and whether they are primary (on site of alteration) or secondary (moved by water or wind). These factors produce different tactile properties, plasticities and drying shrinkages. The different chemistries of clays and amounts of contaminants produce different firing behaviors (e.g. temperature of vitrification, color, strength, efflorescence).
Sometimes the data sheets of clay suppliers can miss the point of what their material really is. They do this by providing only the chemistry. But this does not answer questions like: Is it plastic? Does it have a fine particle size? Does it have a high soluble salts content? What is the porosity and fired shrinkage at various temperatures? What is the wet particle size distribution? What are the drying properties? What does it look like when fired to various temperatures?
Out Bound Links
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In Bound Links
By Tony Hansen