"In addition to a clay body, if you have volcanic or feldspatic stone or a type of China clay and ash for glaze, you can make pottery. We must not allow ourselves to complain of the lack of variety or quality of materials... the manner of making natural glazes is generally done by combining a powdered stone that fuses in the kiln whit clay and ash in varying proportions. By varying the proportion of all three materials you will get a large variety of glazes. Then vary the materials a different stone, a different clay and a different ash. There is no end to the glaze you can create when you come to know the nature of the materials to hand."
[da Ash Glazes di Phil Rogers]