Ceramic Raw Materials
In studying the composition, structure, and properties of ceramics, it is necessary to be familiar with the entire manufacturing process starting with the raw materials used. Although there is a tremendous variety of raw materials used in the manufacture of ceramics, the traditional ceramic industry is largely based on various combinations of clay minerals which impart plasticity, feldspar which forms a viscous liquid at the firing temperature, and silica which is able to form glasses but is relatively infusible. In addition there are widely used mineral fluxes, alumina and aluminum silicates, talc and related minerals, refractory raw materials, and abrasives. Most of these materials are mined and subsequently prepared for use. An increasing number of raw materials are subjected to some purification process; many are manufactured as a secondary step from other primary constituents. In the following we consider some of the most common and widely used of the ceramic raw materials.
The clay minerals are fine particle size hydrous alumina silicates which develop plasticity when mixed with water. They are over quite wide limits in chemical, mineralogical, and physical charateristics. A common characteristic is their layer structure. They are all composed of electrically neutral alumina silicate layers which move readily over each other, giving rise to such physical properties as softness, soapy feel, and easy cleavage. All clay minerals are of secondary geologic origin; that is, they were formed as alteration products of alumina silicate rocks in an environment in which water is present. Most clay materials are the products of weathering and sedimentation, but they may also be formed by hydrothermal activity. The mechanism of the clay forming process is still imperfectly understood.
Clays perform two important functions in ceramic bodies. First, their characteristic plasticity is basic to many of the forming processes commonly used; the ability of clay-water composition to be formed and to maintain their shape and strength during drying and firing is basic to many ceramic processes. Secondly, they fuse over a temperature range depending on their composition in such as way as to become dense and strong without losing their shape. Very pure clays are refractory--impure clays fuse at some lower temperature.
The most common clay minerals may be classified on the basis of structure into the kaolinite group, montmorillonite group, and the illite(hydrous mica) group. More recently the chlorite group has been found to occur and an important constituent of many clay sediments. These classifications are based on crystallographic structure and correspond to the ideal chemical formulas shown in the following table. The ideal chemical formulas of the clay minerals as followed: