All Glossary

Wood Ash Glaze

Common washed wood ash can supply important ceramic oxides when melted, 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 mostly most suitable for potters who are willing to develop and tune recipes for each batch of ash.

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.

The unique nature of ash compared to ceramic materials requires a different approach to glaze making. Generally, the aim is to develop a recipe using the highest practical percentage of a batch of ash that you have accumulated, blended and processed (the larger the batch the better). Ash is most often high in CaO, so it makes little sense to employ calcium carbonate or wollastonite in the recipe. At the chemistry level, a typical ash contains no Al₂O₃ and little SiO₂, the two most important oxides in a glaze. The most common ceramic materials that supply these are kaolin and feldspar so they are the natural companions in a recipe. In the past, it was common to also add Gerstley Borate to help melting and suspension, but it has almost no Al₂O₃ or SiO₂ - it is thus better to use a balanced frit that supplies both (meaning Ferro Frit 3134 is not suitable). 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) - a conservative approach might be to settle for a lower percentage of ash in the recipe (e.g. 35-40%). A higher kaolin percentage should produce a better slurry rheology. Adding CMC gum can also help with adherence and drying issues. Some ashes have significant Na₂O, this will be evident if the glaze crazes - in these cases substitute as much of the feldspar for a frit as possible. The only case where silica should be needed is if a combination of over-melting and crazing happens.

All of the pieces shown here were made by Tony Hansen, they demonstrate what is possible in formulating ash glaze recipes. 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.

Related Information

A starter recipe for a brown wood ash glaze for cone 6

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 is important because every batch of ash is different). This recipe contains ~2% iron oxide. Wood ash generally contains very little Al₂O₃, a critical oxide needed for stable glass development. I thus added 20% EP kaolin (any kaolin should work), it supplies the Al₂O₃ and suspends the slurry. The ash usually contains lots of fluxing oxides, but it still needs help to melt a glaze at cone 6 (so add 30% feldspar, it also supplies more of the critical Al₂O₃). For better melting consider substituting some of the feldspar with an Al₂O₃-containing borax frit (like Ferro frit 3195, even better, frit 3249 since it will prevent crazing). To ease development it is often easier to add more powdered materials to an existing slurry (e.g. it is likely that more frit will be needed), this will push down the total percentage of ash possible in the recipe - thus I often end of with 40% ash.

Cone 6 bowl demonstrates a starter wood ash glaze recipe

A wood ash glaze is not generally something you mix from a recipe. It is something you develop from a batch of ash (see the article below for more information). This ash glaze contains no iron but does have a little cobalt (about 0.25%). The ash is about 50%, with 20% EP kaolin, 20% Custer feldspar and a little rutile (2-4%). If your attempted glaze does not melt well enough consider substituting some of the feldspar for balanced boron frit. If it melts too much and crazes add some silica at the expense of the feldspar.

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.

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 melt fluidity and 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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