This background section was written by Professor James E. Funk of Alfred, New York.
Ceramics is sometimes referred to as the second oldest profession known to man. Early man left his written records on tablets of clay. The Hebrew Exodus resulted partly from labor problems at Pharoahs Clay Works. Pre-Columbian potters produced food vessels and utensils. Chinese porcelains graced the tables of European nobility. Until the present century, however, the production of ceramic articles from clay-based compositions was mostly an art rather than a science. During this century the use of clays has been expanded within ceramics and beyond, to coatings for paper as well as fillers for plastic and rubber products. Even today many producers of complicated ceramics still rely on imperfect and often incorrect knowledge of this technology. However, the science has advanced to the point where the properties of clay-water slurries may now be acurately and consistently controlled to assure performance of the materials in the various manufacturing applications.
Clay has the unique ability to impart plastic properties to a ceramic article due to its crystal structure which immobilizes a layer of water adsorbed onto its surfaces.
In ceramic applications of clay-water slurries, there are a multitude of properties which must be synergistically controlled in order to assure the consistent performance of the slurry in any specific application. For the purpose of this text the explanation of the properties' synergism will be limited to perhaps its most difficult application: slip casting. Other applications, such as preparation of bodies for extrusion of electrical porcelain or dinnerware, or for spray drying, require a similar approach but with different parameters.
A casting slip for sanitaryware casting is usually comprised of the following six ingredients.
1. Ball Clay--This provides kaolinite and other minor minerals as the smallest size particles in the body, including the major portion of the colloidal fraction of the assembled particle size distribution. The colloidal matter provides the high surface area necessary to react with the dispersant chemistry and impart plasticity to the green body and high mechanical strength to the dry body. It is the major contributor to the slip rheology, which is a critical measure of process control. Ball clays sometime contain organic matter, which may be converted to alkali humates and thereby provide autodispersion of the clay in water. The rheology of the ball clay, slurry portion almost alone determines the rheology of the final casting slip. Its natural chemistry and mineralogy also provide important firing and fired properties to the ceramic article. PA0 2. Kaolin--This provides a slightly coarser fraction of kaolinite minerals to the body, modifying the particle size distribution and the behavior of the ball clay. It is more uniform in its crystallography and properties than ball clay, but it is otherwise quite similar to it. It usually fires whiter than ball clay. PA0 3. Feldspar--This is an alkali-alumino-silicate mineral which is used to control the vitrification of the ceramic article during the final firing process. PA0 4. Flint or Silica--This mineral is called a filler, and it further modifies the combined contributions of all of the other ingredients. It also limits the mechanical strength of the ceramic. PA0 5. Water--This provides the vehicle in which the particles of the above four powders are suspended. Water quality is important; the water should not contain an excess of multivalent cations as salts, such as calcium or magnesium. PA0 6. Chemical Additives--They modify the rheology of the slurry and the slip. Monovalent alkali salts, at low concentrations, disperse or deflocculate the particles, reducing the viscosity. Multivalent cationic salts flocculate the particles increasing the viscosity. Formerly these salts were always of the inorganic types. More recently polymeric polyelectrolytes of various types have begun to be used for these purposes.
These four powders in combination also provide a particle size distribution and specific surface area of the solid fraction of a slip.
Slip casting is the process whereby a low viscosity, high solids slip is dewatered in a plaster mold. However, this dewatering must be accompanied by a simultaneous gellation of the colloidal fraction of the particles within the body. These two processes, dewatering and gellation, are both necessary and, in some slips, are frequently opposed to each other. They perform separate functions and, only when balanced in exactly the correct relationship, do they together provide optimum casting behavior.