Tim Willey

Glazes and Pigments for Open Firing
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One could easily argue that glazes and fired pigments are totally unnecessary in the context of an open-firing pottery culture and traditionally, open-fired ceramics were decorated (if decorated at all) with pigments, after firing.

It’s no surprise that glaze technology developed alongside kiln technology - open firings are often a very rough-and-tumble affair and any glaze coatings, which are notoriously fragile before fusion, would be prone to physical damage. Also the atmosphere in an open-firing, ranges from oxidisation to deep, smokey, reduction which can have a very unpredictable effect on the glaze.

However, glazes and fired pigments can find a place in a contemporary reinterpretation of open-firing. It is now, pretty much, embedded in ceramic practice that pottery bodies are complimented by similarly durable, fired coatings.
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There are three main problems to overcome in developing fired coatings for open-fired ceramics:

1. avoiding physical damage to the coating in the hostile physical environment of the firing.
2. avoiding the dulling effect on pigments and glazes of a smokey reduction atmosphere.
3. ensuring that the glaze or pigment coatings are fused at the relatively low temperatures of an open firing.

The image on the left, shows how these problems might be overcome:

The glaze has been inlaid into the body, so offering a degree of protection to the glaze, particularly during the early stages of firing, when the glaze is very friable and delicate. This means that, for the present at least, the glaze is used as a decorative medium and not as a protective, waterproof coating (but this does seem aesthetically appropriate to the technology).

You’ll notice that the fabric is heavily reduced but the glaze remains bright and self-coloured. The only way to achieve this is to ensure a good clean oxidised atmosphere at the early stages of firing and then, if needed, one can reduce the body by creating a smokey reduction atmosphere (smothering with wood chippings when the gaze is fused and effectively sealed from carbon impregnation).

This obviously relates to the third problem - of formulating glazes which fuse at low temperatures.

In this respect ‘normal’ low-fired earthenware glazes are just not suitable, as the glaze needs to start melting at 650c - 700c. it’s quite a problem, but
eutectic mixtures do offer a solution.

From our point of view, a eutectic is the proportions of two or more oxides which melt at the lowest possible temperature. You can easily find published ceramic eutectics together with their melting temperatures, and then it’s a matter of converting the oxide proportions into readily available materials; so it effectively becomes a recipe. It’s simpler than it sounds, and only requires a calculator and a few basic calculations.

That these eutectics were investigated and published is a tribute to the work of the ceramic chemist, but for us, they are a wonderful entry into efficient glaze formulation and absolutely essential for open-firings.