All 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.

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Details

A glaze that employs organic ash (e.g. paper, wood) as a supplier of basic oxides (especially CaO, K₂O, Na₂O). 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. Without large batches of consistent ash to draw on repeatability is poor, so this type of glaze is only suitable for potters.

Many pottery books and websites deal with the preparation (washing) of wood ash. It can be difficult to separate the ash from the unburned material so it may be necessary to calcine or even ball mill it. Most practitioners consider ash glazes decorative and do not use them on functional ware, they thus are prepared to tolerate poor working properties and inconsistent results. However, our approach is to use enough ash to get an earthy appearance but not so much that a glaze is no longer functional or easily usable.

Many recipes online call for "wood ash". But there is a problem with this. The chemistry of different organic ash types varies dramatically. But also for different batches of the same wood. And, more often than not, the type of wood is not even known. Or, it is a mix. If a recipe only calls for a small percentage of wood ash (e.g. less than 10%) this might not be a problem. But for larger percentages, the chances of your recipe firing the same as the pictures shown is very low.

There is a completely different approach: Develop your own recipe using the highest practical percentage of ash. To do this, accumulate a large batch (e.g. 20 lbs), mix it well and then do blending experiments with kaolin, feldspar and silica. The higher the ash percentage the thicker the slurry will be and the more water it will require (and the more the problem with drying shrinkage and cracking). It is thus best to employ plenty of kaolin to help with slurry rheology. Adding CMC gum can also help with adherence and drying issues.

All of the pieces shown here were made by Tony Hansen, they demonstrate what is possible in formulating your own ash glaze recipe. This can be a very rewarding experience, especially when you learn to recognize what material(s) need to be added or substituted to improve the appearance or performance.

Many potters employ fake ash glazes, these are formulated to emulate the appearance of an ash glaze without the hassles of actually using 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).

Related Information

How to create a wood ash glaze

It makes sense to maximize the percentage of wood ash. 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 brown it. 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 more needed Al₂O₃). For better melting add some borax frit (like Ferro frit 3195). If crazing occurs use frit 3249 instead. Use whatever feldspar and kaolin you have.

Plainsman M332 vase with a wood ash glaze fired at cone 6.

The pattern was painted using wax resist and the glaze was applied by pouring. This recipe was the product of multiple previous versions and experience in learning how to process the ash to a finer particle size. The ash was just from the burning of paper and cardboard in our office many years ago. This glaze melted to such a good surface that it was suitable for functional ware. These glazes were among my earliest successes in formulating glazes and hundreds of pieces grew from this. M332 is a coarse-particled clay body and most glazes pinhole on it. Yet these ash glazes produced defect-free surfaces (albeit with plenty of variegation from the impurities intrinsic to the material).

Cone 6 oxidation wood ash glazed vase

I always put the emphasis on ash glazes with functional surfaces, well melted, uncrazed and resistant to staining or cutlery marking. This version gets its color only from rutile. The glaze creates a more decorative surface when applied thinner (like the inside of this small vase). When thicker the surface is functional (like the outside surface). Ash glazes can be very stable (resistant to running during firing) if they contain enough Al₂O₃ (mainly from the kaolin) but also from the feldspar.

Wood ash glazed cone 6 bowl

This ash glaze contains no iron but does have a little cobalt (e.g. 0.25%). The ash is about 50%, with 20% EP kaolin and 25% Custer feldspar and a little rutile (2-4%). If your attempted glaze does not melt well enough consider substituting some of the feldspar for Gerstley Borate or a low melting low expansion frit. If it melts too much add a little silica at the expense of the feldspar.

Cone 8 oxidation wood ash glazed planter

H463 with a wood ash glaze. This ash glaze version was just the cone 6 version (referred to elsewhere on this page) fired to cone 8. The higher temperature produced much more variegation in this rutile and cobalt version of the recipe. A thicker application on the outside caused cracking during drying, that produced crawling that added to the aesthetic. By Tony Hansen.

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