Plasticity
This term is used in reference to clays (or more often bodies which are blends of clay, feldspar and silica particles) and their ability to assume a new shape without any tendency to return to the old (elasticity). In industry plasticity is gauged by the way a clay behaves in forming machines and by its stickiness. However potters find that simply throwing two samples of clay on the potters wheel is an excellent way to compare their plasticities. Plastic clays are responsive, large thin pieces can be made, a piece can be made faster, wet pieces can be moved without excessive deformation and plastic clays center more easily. Non plastic clays tend to split at edges during wedging and rolling, they generate alot of slip, they are more difficult to center, they are more flabby and unresponsive and require more finicky refining work in the latter stages of the process.
Plasticity is a function of particle size (normally clay of finer ultimate particle size is more plastic), surface chemistry and charge of particles, particle shape and water content. Bentonites are the most plastic common clay and kaolins the least. Clays of different plasticities exhibit vastly different properties. For example, ball clays are very plastic but they shrink so much on drying that cracks cannot be prevented. Bentonites have such a high affinity for water that it can take a week to dry a specimen and it can shrink to half the size. Kaolins can dry in a short time and have little shrinkage, but they can have very little dry strength (some plastic kaolins are available but their plasticity is usually because they contain bentonite or have a mineralogy that is bordering on ball clay). Thus a typical white-burning clay body might employ as much kaolin as possible for whiteness, enough ball clay to achieve the needed plasticity, and possibly a small addition of bentonite if plasticity cannot be achieved any other way. A white stoneware pottery clay might have as much ball clay as possible to achieve lots of plasticity but some kaolin to reduce firing shrinkage and get better drying properties. A white casting porcelain can be made using only kaolin. Out Bound Links
- (Glossary)
Clay
Clays occur when parent clay-making rocks (there a... - (Materials)
Kaolin - Al2O3.2SiO2 or Al2Si2O5(OH)4 - Hydrated alumina silicate, Pure clay mineral
China Clay
- (Materials)
Ball Clay - Highly Plastic Fine Particle Clay
- (Materials)
Bentonite
Montmorillonite
- (Glossary)
Ultimate Particles
Physical particles of materials are those we can m... - (Glossary)
Splitting
Refers to a phenomenon where a plastic clay develo...
In Bound Links
- (Articles)
Simple Physical Testing to Compare Clay Materials Some of the key tests needed to really understand what a clay is and what it can be used for can be ... - (Glossary)
Bone China
True bone china is a special type of porcelain tha... - (Glossary)
Porcelain
A comparatively white burning clay body (unless st... - (Project)
Ceramic Minerals Overview
The materials we use are powders and we assess the... - (Project)
Ceramic Properties
A property in this context is a created physical p...
Pictures
Albany Slip DFAC dried disk showing the soluble salts and characteristic cracking pattern and cut edge of a low plasticity clay.
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