Clays and Ash Glazes
Clays and ash glazes
I mix up my clay body from powder in an old Crypto-Peerless dough-mixer. I use two parts Hyplas 71 to one part AT ball clay, by weight, to which I add 10-15% sand / grog. I sometimes add gathered grits to give tooth and character. I make the clay soft/sloppy and dry back to the required stiffness (still soft), after a minimum of two months storage. This mix gives a good conker colour in reduction firings if the exposed body is given a wash of strong ash-liquor or a light wash of shino.
A glaze is basically a skin of glass which has melted onto or into the surface of the clay body during firing. Glasses may be formed from silica and alumina with small amounts of other metal-oxides which act as fluxes to lower the melting point of the mix. The cocktail of compounds will determine the properties and appearance of the glaze, as will the way in which the glaze is fired. For instance, higher amounts of alumina, or the presence of magnesium oxide can lend mattness and opacity to a glaze but a glaze which appears 'glassy' after one firing might appear softer and more opaque after firing to the same temperature if it is allowed to cool more slowly giving crystals more time to form. Also, at stoneware temperatures (eg 1280C), the glaze and the underlying clay-body react together and fuse so the makeup of the clay affects the appearence of the glaze.
I mostly make my glazes from a mix of hardwood ash; granite/feldspar dust; clay and small amounts of other materials such as flint and dolomite. Woodash contains the minerals taken up from the soil by the tree throught its lifetime - mainly silica, alumina, calcium oxide; potassium oxide and magnesium oxide with smaller amounts of oxides of other elements, including iron. On its own, woodash will produce a glaze of sorts but at stoneware temperatures this will tend to be too runny/glassy. Mixing it with only one or two other materials such as granite and clay can achieve the right mix of compounds to provide beautiful and practical glazes. The colours ash glazes produce are mainly influenced by the amount of iron present and whether oxygen was limited (reduction) or present in excess (oxidation) during the firing and also the composition of the underlying clay body.
A traditional mix that I use a lot as a starting point for tests is:
40% feldspar (a granite)
I have tried substituting the feldspar in the above recipe with washed, sieved and dried sludge from a local granite worktop sawmill and after much testing found two lovely glazes.
The advantage of using this approach is the variation, interest and subtle beauty of the resulting glazes that is hard to achieve when buying heavily processed minerals from large suppliers. A lot of experimentation is required to determine the exact recipes and this must be repeated with each new batch of ash.
My wood ash comes from the studio or home/friends wood-burners, allowing me to keep ashes from particular trees seperate. Many hours of work are required to make ash glazes; the ashes are repeatedly washed and settled until the liquor is non-soapy and tasteless and then sieved (80 mesh), and dried in porous biscuit-fired dishes and stored for weighing into tests or batches at a later date.