Ceramic Technology 1 - Clays & Clay bodies

This week we will begin  to look at the Technology of Ceramics: the relationships between clays and glazes, the different types of ceramics and the way technologies are used for utilitarian and aesthetic outcomes. 

Ceramic Technology - Clays and Clay Bodies

Descriptively, the word clay is likely to raise memories of sticky, earthy material oozing between toes or stuck to the soles of shoes, or perhaps more purposefully, a soft material yielding to our touch in the formation of shapes. It is this characteristic plasticity of clay, arising from extremely fine particles of abundant earth minerals combined with water that give it such excellent malleability. The abundance of plastic clay in many geographical locations has resulted in its widespread adoption as a medium for creating cultural objects, often of a distinctive regional character determined by a  clays particular geological quality. Whilst clays may be found that exhibit ideal working qualities in their prospected state, the desire to develop specific working properties and fired characteristics has led to the deliberate sourcing and mixing of particular clays and other earth minerals for particular purposes, raising a distinction between the terms ‘clay’ and clay body’.In addition, different formulation and processing is used to prepare clay for different typoes of processing, whether in it’s plastic state for hand or wheel forming, pressing or extruding, or as liquid ‘slip’ for casting

Clays are essentially weathered igneous rocks that have their physical mineralogy altered by actions of wind and water. 

The majority of clays are characteristically red, orange of brown burning, indicating they have been weathered from iron bearing rocks or have been contaminated by iron during the process of transportation and weathering. These clays are commonly found in shales, swamps, or other locations where sedimentation has taken place. Other geological forces such as folding may relocate these sedimentary deposits to areas where the clay becomes exposed and commonly visible, such as roadside cuttings. Sometimes, laid clay beds underlying vegetation may have iron leached from the substrate, yielding more refined, pale coloured and even white clays. White burning clays, on the other hand are less commonly found, the most pure being formed at igneous origins, their lower plasticity arising from the ‘in situ’ geothermal weathering of felspar. In circumstances where Kaolinitic clays are subject to transportation weathering with low levels of contamination from iron or other minerals, plastic ball clays form; and in more unusual circumstances, plastic kaolins can form.

The following table characterises four major clay types :

Plasticity imparts a significant working property of a clay body, but it is not the only consideration. Strength, shrinkage and colour are equally important factors at both green and fired stages of the ceramic process. For this reason clay bodies are developed to balance green state working properties with those of the desired fired ceramic state. A clay body may be formulated for a particular forming process, such as hand or wheel forming, pressing, or liquid slip casting. Firing considerations concern colour, surface and application ( eg vitreous china vs. porous gardenware ) hardness and durability. Sometimes a fired property dictates the development of an unorthodox forming process, for example in advanced ceramics where polymer platsicisers are used to enable casting of non plastic alumina.The mineralogy and particle size of ingredients also contribute to the design of the clay body, where particular fired nuances can be achieved through their selective inclusion, for example the use of calcined bone in translucent ‘bone china’, coarse felspar particles in traditional ‘shigaraki’ ware, and powdered soda glass silicate frits in ‘faeince’ paste.

Generally, clay bodies formulated for standard ceramic processes balance the use of plastic and no plastic materials.

Plastic clays exhibit higher green strength and shrinkage rates, the former desirable for workability but the latter contributing to distortion due to closely packed particles that inhibiting drying. Consequently, plastic clays are mixed in proportion with lower plasticity clays and non plastic powdered filler minerals such as felspar and silica to extend particle size distribution and reduce shrinkage. Non Plastics also contribute to the appropriate degree of melt to achieve the required fired characteristics such as durability and porosity/vitrification.The following generalised recipes show two different examples of clay body formulation for different forming processes at different temperatures.

 The following video series provides a good grounding in the identification, selection and preparation of clays for various purposes.

Learn How to make pottery clay. For more Ceramics How-To Videos & Articles, visit WonderHowTo.