Producing a zero-porosity cone 6 stoneware is not as easy as you might think. People expect stonewares to be plastic and fit glazes well. That means there needs to be lots of ball clay and silica in the recipe. These are refractory materials and they don't leave much room for the material that produces the vitrification: Feldspar. If the body does not need to be white there is another interesting approach: Use a red terra cotta material to supply plasticity and maturity. In this case I have made a 50:50 mix of a red-burning, super plastic, low fire clay (Plainsman BGP) with a refractory white-burning ball clay (Plainsman 3C). The result vitrifies to a zero-porosity, beautiful light tan body that even glistens in the light. One problem: Fired shrinkage. This body shrinks almost 17% from wet to fired.
Maturity A term used in the ceramics industry to signify the degree of vitrification in a fired clay. Mature clays are dense and strong, immature ones porous and weak.
Vitrification The term vitrified refers to the fired state of a piece of porcelain or stoneware. Vitrified ware has been fired high enough to make it very strong, hard and dense.
Stoneware To potters, stonewares are simply high temperature, non-white bodies fired to sufficient density to make functional ware that is strong and durable.