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Ash Glaze

A glaze that employs ash from organic (e.g. paper, wood) or volcanic sources as a supplier of oxides (e.g. silica, alumina, soda, calcia). Many books deal with the preparation (washing) of organic ash batches (remember these materials are caustic) and provide example recipes. It can be difficult to separate the ash from the unburned material so it may be necessary to calcine it. The chemistry of organic ash types and batches varies a lot so the best approach is to accumulate a large batch, mix it well and then do blending experiments with kaolin, feldspar and silica to develop a good glaze. The higher the ash percentage the thicker the slurry will be and the more water it will require. It is thus best to have the most possible kaolin present to impart better rheology.

For volcanic ashes, the best formulation method is to get the chemical analysis (or have one done) and use glaze chemistry calculations to compare the chemistry of the ash with a target formula for the intended temperature. Then add the oxides the ash is lacking to make it into a glaze. In this way the maximum amount of ash can be employed in the recipe (and often the most variegation will be achieved). There is an example video at Digitalfire.com on doing this.

Many potters employ fake ash glazes, these are formulated to emulate the appearance of an ash glaze without the hassles of actually using the it. These gazes typically are active melters and form a fine crystal mesh (which can require a long cooling period in the kiln, up to 24 hours).

Woodash glaze cone 6 vase

A wood ash glaze at cone 6

A highly variegated cone 6 cobalt rutile ash glaze

Example of a variegated wood ash glaze at cone 6 oxidation. It contains a small amount of cobalt as well as some rutile.

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

This Wood ash glazed cup has been fired at cone 6. It was the product of a development project, a series test recipes having the objective of maximizing the percentage of ash. Obviously, it 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%) to suspend the slurry (it supplies Al2O3 also). Ashes contain lots of fluxing oxides, but they still may need a little help to melt a glaze at cone 6, I added feldspar (it also supplies needed Al2O3). If that is not enough, I add a little gerstley borate or a borax frit like Ferro 3124. If crazing occurs silica is needed. In the end I got a recipe with about 50% ash.

Making a wood ash glaze

Iron stained wood ash glaze fired at cone 6. These glazes need to be reformulated with each new batch of ash you get (since the ash chemistry changes). This one was formulated by quadraxial blending the ash with feldspar, silica and kaolin to get a sweet spot, then fine tuning and finally adding about 4% rutile to variegate it.

Plainsman M332 with a wood ash glaze fired at cone 6

The pattern was painted using wax resist and the glaze applied by pouring.

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By Tony Hansen

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