Clay and pottery clay bodies (I)
THERE are few countries in the world where there is not some form of clay available. It is often very near the surface and can be seen when roads are cut through hillsides, in the banks of rivers or streams and in out crops on beaches and cliffs. It appears as seams which are smooth and compact compared to the layers of sand, gravel, earth and rocks that often surround it. Most clays began as feldspathic and granite rocks which were decomposed millions of years ago by hydrothermal action and weathering agents.
Clay is highly malleable substance and its most important quality, plasticity, enables it to retain a given shape when moulded, leaving the surface smooth and unbroken. The bulk of most clay bodies is made up of the minerals alumina and silica together with small percentages of the other minerals acting as fluxes, which assists the fusing and then melting of the clay body during the firing process. This is a process in which the clay is baked in a kiln at a given temperature, causing it to change chemically and become hard and tough when cool.
The two main groups recognized by geologists are primary or residual clays and secondary or sedimentary clays. Primary clays are those which have remained in their forming grounds, but these are comparatively few throughout the world. The most important to the potter is China clay or kaolin, which is mined using high pressure water techniques to wash the fine clay into suspension, separating it from the gravely rock and fine sand mixture. It is then treated and settled in vast tanks for drying out and preparing for its many uses.
China clay is very pure, but due to the large particle structure, it is non-plastic. China is thought to be the only country with a plastic China clay. Because of its purity, primary clay is not suitable for working in its natural state and can only be used as an ingredient with other clay bodies. It is used in a prepared body for its strength and whiteness and is an essential ingredient of porcelain ware, Bentonite, a clay mineral similar in composition to primary clay, is often added to a clay body to aid plasticity.
Secondary clays are those which have been eroded and carried away by water and earth movements to be deposited in sedimentary layers. In this process of weathering the clay particles have become extremely fine, laminatory in structure and some, such as ball clays, very plastic. Impurities which are picked up as the clay moves will affect the colour of the clay and its degree of shrinkage and drying out when fired. Ball clays are blue to black in their natural state due to the presence of decomposed vegetable matter, but when fired they become white to buff-coloured. When it was first mined and transported to stoke-on-Trent in the early days of pottery industry, it was convenient to shape the clay into, balls for carrying by packhorse – hence the name ‘ball clay’.
Ball clay is pure, very plastic and verifiable and is commonly used in stoneware and earthenware bodies for those qualities. Many other secondary clays have combined with other minerals and organic matter during their formation process. Red clays, for example, contain a high proportion of the mineral iron oxide which is the most common colouring agent and gives us all the red clays found throughout the world.
Red clays are fusible and high in plasticity which makes them excellent for throwing on the potter’s wheel. They have a low melting point and are used for building bricks, flower pots and other terracotta ware. Another secondary clay is fire clay, so called because it is found close to coal seams, is very refectory (high firing), often coarse in texture and is used in high-fired prepared bodies, such as stoneware. When it is calcined and reground, fine clay is a common source of grog. This is added to day bodys to give them added texture and to lessen shrinkage to be used to create colour.
Within these broad classifications, there is obviously on extremely rich variety of clays, each with its own individual texture, colour and working quality. The possibilities are almost endless, which is all part of the appeal of pottery as an art form. Never be afraid of experiment, provided that the basic rules are followed, such experiment will be successful move often than not and you can learn even from mistakes.
Natural and prepared clay
Natural plastic clays are those primary or secondary clays which can be used with the minimum of cleaning or treatment after being dug and weathered. The weathering process is important as it allows the elements, particularly rain water, to penetrate between the fine particles of the clay and hence and its plasticity. The most common of the natural clays are red clays, ball clays, fire clays and some stoneware clays.
Few natural clays are used on their own as, for most purses in pottery, the mixing with other bodies or ceramic compounds is necessary to give the desirable qualities for working the clay. It is important to remember when using a natural clay that its degree of shrinkage from plastic to fired state should not exceed 10 per cent. If it has a higher shrinkage than this and is very plastic, a non shrinkage ceramic mineral or a less plastic clay can be added to bring it to the right plasticity.
Apart from any suitable natural clays the potter can use, the built of available clays for potting are specially prepared by blending different materials. One clay can be added to another to combine their working qualities and ceramic compounds can be added to change the colour, texture and fusibility of the clay body. Ready made bodies can be purchased from a supplier, but many studio potters prefer to make up their own.
The basis of most prepared clays is the highly plastic ball clay, with the addition of ceramic minerals such as feldspar, alumina and silica or non-plastic materials like China clay, China stone or whiting. A coarse body can be obtained by adding some form of sand or grog to a fairly plastic clay. The choice of ingredients for any clay body is dependent on what use it is being put to, what type of ware is being produced and what method of decoration is applied to the pottery. Each clay body has a different mechanical strength, colour, texture and resistance to heat and the essential differences in pottery ware results from the individual qualities of the clay bodies and the heat to which they are subjected in the kiln. Only with experience and experimentation can the potter choose the right ingredients in the right proportions.