Plasticity is a clay body's ability to be deformed under pressure, on the wheel or by hand, without cracking or crumbling, and to hold that new shape once the pressure is removed. It is the property that makes clay usable as a forming material in the first place, distinct from a body's strength, shrinkage, or fired color.
Plasticity comes mainly from particle size and shape: finer, flatter clay particles slide past each other more easily when wet while still holding together through surface tension and electrochemical attraction, which is why fine-particle clays like ball clay are so plastic and coarser, purer clays like kaolin are comparatively short (low in plasticity). A clay body recipe balances plasticity against other goals, since the most workable clays are rarely the whitest or the strongest once fired, which is why most bodies blend several clays rather than using one alone.
Plasticity also changes with water content and with aging: a wet clay that has rested (often called aging or souring) generally becomes more plastic as bacterial activity and full particle hydration develop over time, a traditional reason potters store prepared clay for a period before use.
A quick, informal way to judge a clay's plasticity by hand is the ribbon test: rolling out a coil of clay and bending it around a finger. A highly plastic clay bends into a tight curve without cracking on the outer edge; a short, low-plasticity clay cracks or breaks well before it can form a tight bend. The test is a rough field check, not a precise measurement, but it is quick enough that potters use it routinely to compare bodies or judge whether a batch of clay is workable.
Plasticity resists being stated as a single objective number. The Atterberg limits, a standard measurement from soil science and geotechnical engineering, generally do not apply to ceramic clays, which are far more plastic than the soils those tests were designed to characterize. Ceramic testing instead favors more direct comparisons: alongside the ribbon test, the Pfefferkorn test measures plasticity instrumentally, recording how far a plastic clay specimen deforms under the impact of a falling weight, giving a repeatable number rather than a pass/fail field check.
A body's plasticity can also be adjusted directly rather than just measured. Bentonite, an extremely fine, colloidal clay, is an unusually effective plasticizer: a few percent added to an otherwise short body can measurably improve workability without displacing much of the body's other material. The opposite adjustment, taming a clay that is too sticky or too plastic to handle well, is usually made by adding a non-plastic material such as grog or sand, which dilutes the fine, plate-like particles responsible for plasticity in the first place.