Plasticity is influenced by the past history of the clay, type of clay, amount of working the mixture has received, the amount of souring, temperature and moisture content.

More precisely the plasticity is effected by the physiochemical relationships between colloidal clay particles and water, so is effected by mineral composition, particle size and particle size distribution, cation-exchange capacity, cations and pH and the surface tension of the water.

Plasticity of natural clays depends on the nature of the very fine fraction, montmorillonites may be found in the fine fraction, but not in the bulk of material.

Relative plasticity of clay minerals as given by E. C. Bloor, Plasticity: A critical survey. Transactions of the British Ceramic Society.

There are many plasticity tests available; however, our test method is based on the Atterberg plastic limit test used in soil technology.

Clay is crushed and ground to the usual size distribution as is used in typical production plants, usually less than 3 mm. A sample is sieved through a 500 micron test sieve to collect the fines, coarse section is discarded.

A small amount of this fine clay is wetted to form a plastic ball, this in turn is split into sub-samples and the sub-samples worked between finger and thumb into threads. These threads are rolled on a glass plate down to less than 3 mm in diameter. The moisture content is measured when the sample shears both longitudinally and transversely. The resultant moisture content is the plastic limit.

The liquid limit of the bulk clay is assessed using a ELE cone penetrometer covering a penetration range of 15 to 25 mm, a graph of moisture content vs cone penetration.

 The plasticity index = Liquid Limit – Plastic Limit