Wood Ash Glaze
A glaze that employs organic ash (e.g. paper, wood) as a supplier of oxides (e.g. silica, alumina, soda, calcia). Ashes are generally flaky and fibrous and cannot be screened well, this consistency contributes to variegation in the color and texture of the fired surface. Many pottery books deal with the preparation (washing) of organic ash batches (these materials are caustic). It can be difficult to separate the ash from the unburned material so it may be necessary to calcine it. Consistency in firing is poor, so this type of glaze is only suitable for potters.
Wood ash glazed cone 6 bowl
It contains no iron but does have a little cobalt. The ash is about 50%, with 20% kaolin and 25% feldspar and a little rutile. However your ash will be different so you will have to do your own development program.
How to create a wood ash glaze
It makes sense to maximize the percentage of wood ash. Since different batches and types of wood ash have drastically different chemistries how can you have a glaze have a high percentage? This glaze was the product of preparing a large ash batch and a project to develop a glaze specifically from it. This one contains a little iron to stain it brown, this brings out the variegation better. Ash generally contains low percentages of Al2O3, a critical oxide needed for stable glass development. I added kaolin (about 20%), it suspends the slurry and supplies Al2O3. Ashes contain lots of fluxing oxides, but they still may need a little help to melt a glaze at cone 6, so I added feldspar (it also supplies needed Al2O3 also). If that is not enough flux, I add a little gerstley borate or a borax frit. If crazing occurs silica is needed. In the end I got a recipe with about 50% ash.
Plainsman M332 with a wood ash glaze fired at cone 6. By Tony Hansen.
The pattern was painted using wax resist and the glaze applied by pouring.
Cone 8 oxidation planter by Tony Hansen
H463 with a wood ash glaze.
Cone 6 oxidation wood ash glazed vase
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