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Wood Ash

Oxide Analysis Formula
CaO 37.00% 0.56
MgO 11.00% 0.23
K2O 16.00% 0.14
Na2O 1.00% 0.01
P2O5 9.00% 0.05
SiO2 7.00% 0.10
Fe2O3 1.00% 0.01
MnO 2.00% 0.02
LOI16.00%
Oxide Weight 71.07
Formula Weight 84.61

Notes

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).

Related Information

Links

Articles Formulating Ash and Native-Material Glazes
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
Hazards Plant Ash Toxicity
Typecodes Flux Source
Materials that source Na2O, K2O, Li2O, CaO, MgO and other fluxes but are not feldspars or frits. Remember that materials can be flux sources but also perform many other roles. For example, talc is a flux in high temperature glazes, but a matting agent in low temperatures ones. It can also be a flux, a filler and an expansion increaser in bodies.
Glossary Wood Ash Glaze
Common washed wood ash has a chemistry akin to a ceramic glaze, so it can comprise significant percentages in a recipe. Plus it can produce unique visual effects.
Materials Pine Wood Ash
Materials Oakwood Ash
Materials Rice Husk Ash
Materials Rice Straw Ash
Materials Applewood Ash
Materials Cherry Ash
Materials Hardwood Ash
Materials Soft Wood Ash
Materials Eucalyptus Ash
URLs www.axner.com
A good place to buy pottery books

By Tony Hansen


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