The chemistry of clay is a complex subject, which I do understand many of you may have no interest in at all – I can hear you yawning as I write! However, even if all you want to do is make pots, it will help to have some understanding of the origins and composition of clays so you can make informed choices for your chosen projects.
The following overview breaks the subject down in simple format so that you aren’t boggled by science and then suggests questions you should ask yourself before deciding on the best clay for what you want to achieve.
What is clay?
In geological terms, clay is a fine-grained earthy material that can be found almost anywhere in the world. The two essential components of clay are silica and alumina which originate from igneous and metamorphic rocks. Naturally occurring clay is formed from feldspathic or granite rocks that decomposed through the action of glaciers and weathering over millions of years.
Clays can be divided into two types:
Primary or residual clays are so called because they are found where they were originally formed and are fairly rare. The most important primary clay is Kaolin – more commonly known as china clay, it is very pure and white but because of its large particle size it tends to be too ‘short’ (non-plastic) for use on its own.
Bentonite is another primary clay but in contrast is extremely fine and plastic, so it’s added to shorter clays to increase their plasticity.
Because of their whiteness, primary clays form the main constituent of clay bodies such as porcelain.
Sedimentary or secondary clays have been further eroded and weathered, and moved from their original source by the action of water, wind or glacier. As a result of this, sedimentary clays have a much finer particle size and are more plastic – this group includes the ball clays.
In the travelling process, secondary clays pick up certain minerals and impurities which among other things affect colour and firing temperature – often making the clay only suitable for low firing. Terracotta clays for example
