Available to me is a cross section of earth about 6-10 feet in depth of the hill here. I have concluded that the clay undergoes a transformation due to organic growth - as minerals are drawn to the surface by plants. (More recently I have learned that the grey iron is due to FeO and the red iron Fe2O3 - so the surface action is oxidizing the iron) The red near the surface and grey under it are basically the same clay. They are cross bedded, so the stratigraphy does not reflect this coloration change; only proximity to the current surface. Ball clays in England and Kentucky are a product of this same phenomenon, they are bedded under coal and the plant growth has sapped all the minerals out of it. Thus it fires to around cone 10 rather than cone 6.
However, at 6 to 10 feet of depth, the bleaching of the clay stops, and a uniform brown stoneware clay is obtainable. It is much more compressed however and is beginning the process towards becoming a shale. It chips somewhat as I dig. I am mixing all the lighter-burning clays as though I were mining it. I have identified 8 different clays in this dig. This clay won't be available on a long term basis but the exercise is valuable educationally.
One of them is the light grey and red with marbling as they don't mix readily. Another is the light grey and gold marbled, which is what I have in my yard. I don't know why the clay dig comes up sometimes with gold clay, and sometimes with red, my guess is that it has to do with the original deposit from which the clay comes. The strata is cut diagonally, so from site to site there is variation.
The stoneware has a bit of sand as well as sand sized feldspar, so it has a "short" feel to it but works well. I have made several bowls from the more plastic varieties where the grey clay comes near to the present surface.
