Natural Clays
Clay refers to the finest-grain particles in a sediment, soil or rock. It is primarily made up of hydrogen and oxygen, as well as aluminum and silica (which both can be found in disintegrated volcanic rock). Clays also make up a large part of shale, which can be found in pore spaces in sandstones and have also been located in limestones, dolomites(which have a similar texture as limestone) and coals. An important use for clay is pottery making. There are two main types, shell tempered and grit tempered. Originally, mussel shells were ground up and mixed in the clay for the tempering process. The addition of the shells prevents shrinkage and keeps the pots from exploding during the firing process. The other type was made with crushed stones. The coloration of the clay is a result of the firing process as the chemicals present change color. Color is an important factor because the color of the product is influenced by the state of oxidation of iron, the state division of the iron minerals, the firing temperature and degree of vitrification, the proportion of alumina, lime and magnesia in the clay material itself. The very first step to pottery making digging out the clay from the earth. The second is crushing the stone or the shells and adding them to the mixture; the clay is then rolled into a coil shape. Tools can then used to smooth the walls of the coils. The final product is then fired in an open bonfire or a primitive kiln (shown below in Figure 4).
Types of Rocks used for Clay Making
Kaolinite, Al2Si2O5(OH)4, is a type of stone in which clay is commonly found. Kaolinite, hydrous aluminum silicate, is the basic raw material for ceramics, and large quantities are also used in the manufacture of coated paper. It is commonly made by the decomposition of rocks and contains aluminum silicate compounds, such as feldspar. Its largest deposits can be found in formations of clay beds in lakes. It varies in color from yellowish-gray to brown depending on various impurities present. When mixed with water, kaolinite becomes plastic and very easy to mold. It is only soluble in concentrated, hot sulfuric acid. Kaolinite also has the same chemical make up as both dickite and nacrite, which are also clay/kaolin minerals.
Montmorillonite, hydrated sodium calcium aluminum silicate, is a basic constituent in rocks called bentonites. It is usually found in earthy and scaly amorphous-looking masses and comes in either white or gray. The interesting thing about this material is that it has the capacity to expand by absorbing water or other liquids in its presence. Also, it has the potential for ion exchange; the cations (Na+, K+, Ca+) present between the layers in the structure are replaced by other cations present in the solution.
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Figure
5: Kaolinite
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Figure
6: Montmorillonite
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