This value indicates the degree to which the material will add plasticity to a body. In pottery this number indicates the amount of water a clay body needs to pug it to comfortable plasticity and workability. The number is subjective because different people and processes require differing stiffness. Typical pottery clays and plastic porcelains used by potters (for throwing) have a water of plasticity of around 21%. Softer bodies would be 22-23% (e.g. for jiggering). Extremes in recipes (e.g. very low or high plasticity, the presence of extremely fine grained materials (like bentonite) or larger ones (like grogs) will affect this number, but not as much as you might expect (1-2% higher or lower).
In industry, this is a physical data value often quoted on data sheets for plastic materials used in ceramics (especially ball clays). Because every particle is plastic (as opposed to bodies where filler non-plastic particles are often the majority) the amount of water needed is higher. It is the amount of water required to totally wet all the surfaces of the particles. This is thus a higher number than that needed to simply make the material into a workable plasticity. At this consistency any low plasticity characteristics can be attributed to the plastic deficiencies of the clay rather than the lack of water. Special lab equipment is required to measure this. Although the topic of plasticity measurement can be complex, generally users simply compare these numbers (usually within a specific manufacturers product lineup) to get an idea of the relative plasticities of the different clays.
There are other measured properties that relate to plasticity. An example is "plastic limit", it is the gravimetric water content at which 3 mm diameter threads of the clay crumble when rolled out by hand.
Enter percent of water
|Tests||Clay Water Content - Powder, Plastic|
Test conducted primarily on materials use to make bodies or glazes.
Methods of Measuring the Plasticity of Clays