Clay
Clay is a mineral with a crystal structure
originated from the geological weathering of the earth' s crust.
Erosion, volcanic action and hot gases resulted in the decomposition of hard rock, mostly feldspar.
There are two types of clays: primary or residual, and secondary or sedimentary.
The first have not moved from where they decomposed from the parent rock,
and erosion was performed by ground water and gases from below.
They are pure and hardly contain other minerals. Kaolin is a primary clay.
The second have been carried by the elements into another location,
incorporating impurities in the form of organic matter, minerals and most commonly iron oxide.
The action of water grinded the clay in increasingly small particles,
leaving all coarse material behind.
This is the most common type of clay. My ceramics are made of clay from the secondary type.
Each type of clay has its own chemical formula, which varies slightly on each batch.
To simplify the work of the ceramic chemist, an ideal formula of one molecule aluminium oxide,
two of silica (powdered quartz) and two of water has been accepted; this is the formula of kaolin.
Clay, when fired, undergoes a chemical change referred to as ceramic change.
The last water not escaped during drying, because trapped in the alumina
and silica molecules, evaporates in the form of steam when the pot reaches around 600°C,
changing the formula to one molecule alumina and two of silica.