Porcelain Clay Bodies
Unlike stoneware, porcelain can be made only from a body compounded of several ingredients and cannot be made from any single natural material. Unlike stoneware, only a few materials can go into the composition. However, the dividing line between stoneware and porcelain may be very subtle. Many stonewares are very hard, dense, and vitreous. They may be very light in color as well, gray or gray-white. Porcelain is distinguished from such stonewares only by its whiteness, and by the fact that where it is thin it is translucent. Historically, porcelain evolved as a refinement of stoneware, and it is exactly that.
Although the basic proportioning of ingredients in porcelain is quite simple, the exact adjustment of materials and the proper firing requires considerable control. Whereas in stoneware a small change of proportion in the body mixture may make little appreciable difference, in porcelain a change of a few per cent one way or the other may have a very marked effect on the ware.
Essentially, porcelain is achieved by combining kaolin with feldspar and firing the mixture to the point where the body is on the verge of melting or glassification.
Making porcelain today is made relatively easy by the availability of fine, purified and prepared kaolins. Kaolin is produced in quantity not only for use in ceramics but for the paper industry, where it is used for coating white paper. Kaolin is discussed in the previous chapter in connection with stoneware bodies. Here its properties as an ingredient of porcelain concern us.
All kaolins are highly refractory. English China clay has a melting point of about 1800°C, and it softens to the point of deformation at about cone 35. If pottery were made from kaolin alone, it would have to be fired to at least cone 14 to be even as dense and as serviceable as a soft earthenware. The reason for the refractoriness of kaolin is its relative purity and freedom from any oxides other than alumina and silica, both of which have very high melting points. From the analyses already given of various kaolins, it will be seen that the percentages of sodium, potassium, lime, and iron are very small. From a geological point of view, kaolins might almost be considered freaks of nature, because the vast majority of all clays contain considerable percentages of these elements, and as a result, have much lower melting points.
For porcelain making, two properties of kaolin are of greatest importance: (1) the purity of the material, which determines the whiteness with which it will burn; (2) its plasticity, or lack of it.
Although all true kaolins are white- burning clays, there are subtle differences between the various clays which give slightly differing fired colors. As we have seen, Georgia kaolin fires to a slightly darker color than English kaolin because