If you look carefully at this picture you will see all the stages in the clay preparation. Up front there are several basins with clay in different stages. Further back are the white porcelain clay bricks - the "Petunste", in most older litterature mistaken for the secret Chinese porcelain material in itself.
Photo: © Jan-Erik Nilsson, 1992

The crushed stone powder then get watered down in rectangular water basins, about four by eight feet and some four feet deep. The finest clay powder settles on top of the cruder, and is floated over to another basin. The process is then repeated until all rocks are turned into fine clay which is then shovelled up and dried. While still plastic it is cut by a string and bow, and shaped into bricks in a simple four-sided boxlike wooden frame without bottom. The clay bricks are the dried in the shadow of the straw roof.

Modern standard clay is mixed in the proportions 1:4 of kaolin and porcelain stone. For higher quality they use a larger proportion of Kaolin. The "handmade" clays from the water mills are especially sought after for sculptures and hand throwing thanks to its higher plasticity. For this I take their word since I had no means to check this up. For future references I took samples of all the clays I saw, which are still in my possession.