|Boiling Point No. 01 - January 1982 (ITDG Boiling Point, 1982)|
This is the first of a series of articles introducing the subject of constructing stoves out of fired clay. We will look at where clay comes from and the two main types of clay most commonly found. Each article will have a small glossary of words commonly used in clay technology.
Clay is an abundant and cheap material which does not need major processing to make it useful. There are few places in the world where it cannot be found and it has been used as a building material for centuries. The fund of knowledge built up by local peoples about its properties and uses, plus its cheapness and ready availability make it a most suitable material for the construction of stoves.
The chief constituents of all clays are alumina and silica, the latter always being in excess of the former. All clays may be regarded as consisting of a mixture of one or more hydrous alumino-silicates with free silica and non-plastic minerals or rock granules. The chemical properties of the clay are dependent on the nature and proportion of these accessory ingredients.
Clay falls into two general categories:
1. Primary Clays: those which have been formed on the site of their parent rock by the weathering of feldspathic or volcanic rock. The rocks are broken down by water seeping through the strata, leaving the clay in irregular pockets. The water action is fairly gentle and so particle size remains large and varied. Primary clays are usually non-plastic. They contain little or no contaminating materials and are recognisable by their purity and whiteness. Kaolin is an example of a primary clay.
2. Secondary Clays: These have been transported by water, glaciers or occasionally wind, to a site far from the parent rock. They are usually composed of a complex mix of different clays and minerals picked up on the way. The action of the water breaks down the particle size and in the process of sedimentation the coarse and fine particles are separated. This means that secondary clays are plastic and hardly ever pure. The colour in - the raw clay indicates the presence of either iron oxides, or carbonaceous matter. Iron which is present as haematite will produce the red, limonite gives yellow; ferrous iron gives gray/green and black. Iron in the clay lowers the vitrification point in firing, so all red clays are low firing or earthenware clays.
Clay used for the construction of cooking pots and earthenware vessels in the Third World is almost always the red secondary clay. It moulds easily into large thin shapes and will harden into biscuit ware at low temperatures (600-800 degrees C) in primitive bonfire firings.
Sometimes the clay is not as plastic as is required and potters have various ways of dealing with this. Some dig the clay, let it dry, crush it, remove stones and unwanted debris, then leave the clay to soak again in pits. Others soak the clay for many months, then dry it, crush it and reconstitute it again for use. This soaking or "ageing" of the clay improves its plasticity because bacteria grow in the clay and produce acid residues. These aid the formation of gels which break down the particles and improve plasticity. Potters of China used to use clay prepared for them by their grandparents. They in their turn prepared clay for their grandchildren to use.
Some Voltaic potters add millet husks, charcoal, grog and donkey dung to their clay. - Why? Find out next issue....
Grog Clay which has been fired and then ground into granules.
Plasticity: The property of a material enabling it to be shaped and to hold its shape. Plasticity in clay depends on particle size. Clay particles are plate-shaped and slide against each other on a film of water. The smaller the clay particle size, the more plastic the clay will be.
Vitrify: to fire to the point of glassification.