Tim Willey

Adaptive Construction - Sculpture - Ceramics

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Open Firing - An Adaptive Technology
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The Firing Stack

A long time ago now, I worked with the Experimental Firing Group based at Leicester University’s Department of Archaeology . It was great fun, trying to recreate the firing technologies of the past and the research and findings of the group drove my interest in adapting historic technologies for contemporary practice.

One of the most important lessons learned during this time was the vital role that clay minerology and tempering (opening materials such as grog and sand) made to the success of often, very rapid firings.

At about this time, some fascinating research was being carried out by Patricia May and Margret Tuckson in Papua New Guinea, where they had an almost unbroken ceramic tradition which could be traced back for thousands of years. The research was comprehensively published in
‘The Traditional Pottery of Papua New Guinea’ and I remember seeing amongst its pages a very small image of an intriguing structure which I now call the firing-stack. A few lines of text also mentioned that the stack was used for a single pot and that the pots were not pre-heated!

Now this, to me, was amazing, as we know, a slow preheat of unfired pottery, is essential: that critical period from ambient temperature to about 200c must be tightly controlled, to allow moisture (present, even in the driest of pots), to escape without blowing its way out! The stack was also a paradigm of an adaptive technology where, no doubt, over the millennia, the structure was developed alongside the ‘emergent properties’ of clay-body formulations, which demonstrated a high tolerance to rapid temperature rise.

Well, the potters of Papua New Guinea new a thing or two about clays and tempering clays, as indeed did all potters from ‘open-firing’ traditions, but it took me a long-long time to even come close to a clay that could stand the super-rapid temperature rise of the firing-stack without the need to pre-heat or pre-fire.

It’s not surprising that so many contemporary techniques that fire relatively quickly: pit firing, drum kilns, raku firing, paper kilns etc. all tend to use pre-fired (biscuited) pots. If you’re lucky and very careful, some ‘off the shelf’ clays might work with rapid open-firing, but I must admit, I’ve had very little success.

So, whilst I made plenty of firing-stacks (my first was in the early 80s) I had very few pots that remained intact and I pretty much kept the firing-stack experiments to myself whilst working - off and on - towards a clay that matched its performance.

There was never a eureka moment (although I thought there might be) instead, it was just a case of repeated testing until, over the years, a systematic approach started to develop.

I’ve included below some details on how the clay formulation evolved, but first here is a short (ish) video which shows the firing-stack in action:

P.S. Look out for the deer, top left corner 15 sec in.

Clay Bodies for Rapid Open Firing

Clays, traditionally used for rapid open firings, were empirically developed, that is to say, they were chosen purely on a trial and error basis and the acquired knowledge and understanding passed down through the generations, possibly over hundreds and even thousands of years.

It is still possible to find natural clays which have many, if not all of the attributes needed for rapid open firings. Indeed it may be possible to buy ‘off the shelf’ products that might work, up to a point. However, an understanding of the mechanics, chemistry and mineralogy involved are really useful, at least in understanding why some clays blow apart and others don’t.

For simplicity, I’ve listed what I think are the main criteria for an open-firing body. The list is not exhaustive and by no means scientific (in its strictest sense) but from a makers point of view this is what I think we need to be aware of:

  • a clay body can be viewed as plastic clay-minerals binding together other materials (non-plastics)
  • it is important to have very fine non-plastics in the body, together with much courser fractions
  • fine non-plastic materials become coated with the plastic clay minerals and aid workability
  • course non plastics (opening materials) provide structure and integrity to the body and allow gases (steam) to escape
  • it is beneficial to have a broad size-range of non-plastics, so the spaces between the fine and course grades are bridged
  • many natural clays contain the right blend of clay mineral and non-plastics as a basis for an open-fired body

As a starting point I have found that natural clays are the way to go. Even the really sandy clays can be graded (levigation will remove gravels and very course sands). Lots of testing is required, but it’s great fun.

Usually it’s a case of finding the right clay and adding sand or grog to get the balance right (this is called tempering) and looking at the evidence, this is very much the way traditional open-firing potters went about it, often travelling considerable distances to source just the right materials.
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This is a magnified image of a section of ceramic fabric after firing. It clearly shows the wide size-differential of the opening material.

The finest non-plastics are not so easily identified as they are largely coated in clay (in this case, largely the clay minerals, montmorillonite and illite) and so form the plastic matrix of the body, lending workability.

The ceramic fabric is critically, very open, and inter-connecting fissures can be seen throughout the section (which form when the clay dries and shrinks). These fissures will allow rapid expanding steam to escape without breaking open (spalling) the fabric.

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Glazes and Pigments for Rapid Open Firing

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.
You’ll find some of the results of my work with the Firing Stack in ‘Tales from the Wood’ and also on my Instagram feed.