Ball clay in the context of "Rheological"

Soft-paste porcelain (sometimes simply "soft paste", or "artificial porcelain") is a type of ceramic material in pottery, usually accepted as a type of porcelain. It is weaker than "true" hard-paste porcelain, and does not require either its high firing temperatures or special mineral ingredients. There are many types, using a range of materials. The material originated in the attempts by many European potters to replicate hard-paste Chinese export porcelain, especially in the 18th century, and the best versions match hard-paste in whiteness and translucency, but not in strength. But the look and feel of the material can be highly attractive, and it can take painted decoration very well.

The ingredients varied considerably, but always included clay, often ball clay, and often ground glass, bone ash, soapstone (steatite), flint, and quartz. They rarely included the key ingredients necessary for hard-paste, china clay including kaolin, or the English china stone, although some manufacturers included one or other of these, but failed to get their kilns up to a hard-paste firing temperature. They were called "soft paste" (after the French "pâte tendre") either because the material is softer in the kiln, and prone to "slump", or their firing temperatures are lower compared with hard-paste porcelain, or, more likely, because the finished products actually are far softer than hard-paste, and early versions were much easier to scratch or break, as well as being prone to shatter when hot liquid was suddenly poured into them.

Tonstein (from the German "Ton", meaning clay, plus "Stein", meaning stone) is a hard, compact sedimentary rock that is composed mainly of kaolinite or, less commonly, other clay minerals such as montmorillonite and illite. The clays often are cemented by iron oxide minerals, carbonaceous matter, or chlorite. Tonsteins form from volcanic ash deposited in swamps.Tonsteins occur as distinctive, thin, and laterally extensive layers in coal seams throughout the world. They are often used as key beds to correlate the strata in which they are found. The regional persistence of tonsteins and relict phenocrysts indicate that they formed as the result of the diagenetic alteration of volcanic ash falls in an acidic (low pH) and low-salinity environment, consistent with a freshwater swamp. In contrast, the alteration of a volcanic ashfall deposit in a marine environment typically produces a bentonite layer.

The induration of tonsteins is in contrast to kaolin claystones that can be mined for kaolin clay, such as the ball clays found at Bovey Tracey which formed by the erosion of a nearby kaolinised granite. These deposits are generally softer, white, and plastic.