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Wood ash has been used in glazes since primitive times. When mixed with a clay and feldspar it assists melting and produces the classic variegated and often coveted ash glaze surfaces. Part of the attraction of ash glazes is that each type of ash has a very different chemistry and even ash from the same source is not consistent. People practicing this art thus are always seeking a 'vintage batch'. However many people have managed to control their ash by meticulous attention to consist processing and sourcing.
Ash can be processed wet or dry. In wet processing the often black and non-homogeneous ash is put into water and the unburned carbon material floats and is discarded while the ash itself settles to the bottom. When dried to a powder this material can be quite light in color. However ash is caustic (corrosive to skin and lung tissue). The water used to process it is potentially very caustic. Ash is therefore a hazardous material to work with, so care is needed when handling it. In dry processing the raw ash is simply coaxed through a sieve that catches most of the unburned material and impurities. This form of processing leaves the soluble caustic K2O and Na2O components in the ash and therefore it will be a better flux (of course using any soluble materials in a glaze can lead to problems with slurry flocculation or deflocculation). Dry processing is best if you are making your own ash and therefore have control to make sure the burning process is complete.
Many books for potters deal with the subject of preparing and formulating ash glazes. Generally the key to success is to process a large batch of ash and use a minimum of it developing a recipe tuned to it. When the chemistry of the ash is unknown triaxial blending techniques with feldspar and clay are usually best to zero-in on a good mix (some people also mix ash and a low fire red clay). If the chemistry is known then you can compare the ash to target formulas for the temperature you work at and add materials to supply oxides that are absent or deficient (this in turn dilutes the ones that are in excess).
Since it takes a lot of wood to make a little ash it can be challenging to find a consistent source. When an ongoing supply of ash is available you can get an idea of its consistency by asking about their wood supply and the process that creates the ash. Also, it is important that the wood not be too dirty since the dirt does not decompose, its small proportion in relation to the wood can be a large proportion related to the ash.
The chemistry shown for this is an average of 6 different wood ash types (which are, as noted above, quite diverse in makeup).
Out Bound Links
The hazards of using plant and wood ash in ceramic glazes
In Bound Links
How to have a volcanic ash analysed and them use ceramic chemistry to create a glaze that contains the maximum possible amount of the ash for the desired effect
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...
Hard wood ash
Cherry Wood Ash, Cherry Ash Washed