The article below was written in 2000 and was published in Ceramic Review issue 196. It came about as a means of documenting my explorations into clay bodies for use in extended woodfirings.
The Perfect Body
For the last three years, myself, Paul McAllister, Stephen Parry, Jim Gladwin and David Scott have collectively been firing the 68cu ft anagama kiln built in 1998 by Izumihara Masanobu. The kiln is sited in the grounds of Wysing Arts near Cambridge. This article will hopefully convey some of the research that I have carried out into clays, which utilize the atmospheres, generated though a protracted wood firing. This investigation was embarked upon as a result of being unable to find a pre formulated commercial clay, which would satisfactorily achieve the surfaces we were looking for.
In an anagama wood firing with a reasonably long duration, there are many variables: the path of the flame, the length of the flame, the length of slow burn and the length of fast burn, the structure of the pack, the duration of the overall melt and the types of wood used (We use a mixture of soft wood: Great Fir and Scots pine with approx. 10% hard wood in our case, walnut for the coaling properties; all are slab wood off cuts.).
Some of the early firings of the anagama kiln were, with hindsight, carried out with a type of gung-ho naivety. Using predominantly white St. Thomas from Potclays, The Crank/Raku body from Les Bainbridge Ceramics and a standard white stoneware used in the Ceramics Department at Loughborough University School of Art and Design. All these clays were great for making and they worked well in the salt kiln and the down draft wood fired kiln, as well as in standard oxidised and reduced firings. However, when they were subjected to the anagama firing they yielded very disappointing results.
The St. Thomas took the temperature well (about 1400C at the front and over 1300C at the back), but tended towards a very flat grey, although it took on a little more colour at the front of the kiln just beyond the reach of the coals. This clay did not utilise the amounts of ash that it had been subjected to, especially in areas that achieved the most heat and movement by the flame. It was also very prone to what can only be described as a skinning on the surface. A very dry puckering of the clay surface which, if chipped with a fingernail, would crumble off.
The crank developed some seductive colours in areas of the kiln, which in the earlier firings reached around cone 10, but were very prone to producing a flat shiny dark brown. It was not a good recorder of the flame's path. Generally the ash, which landed on pots, sat and moved only a little although provided some lovely dry olive greens. The white St. Thomas produced runs right at the front of the kiln but further down the kiln gave only, in our eyes, a rather insipid dry pumice grey. Glazes, which had a relatively high alumina content and low flux content tended to crystallise massively in the cooling of the kiln and shiver off.
It was all very perplexing. It became apparent to us all that when undertaking firings of this nature, much of the acquired ceramic knowledge was wildly inadequate preparation for these extended firings. I started to experiment using a basic white stoneware recipe. I had to test new clay bodies in our downdraught twin fireboxed wood kiln. The kiln is fired to cone 10 in approx 18hours. Although not a close match for the anagama in duration, it was at least fired with wood and as I used no bag walls, tests placed close to the flame inlet could pick up a fair amount of natural ash.
The white stoneware recipe is:
Hyplas 71 ball clay - 65
China clay - 22
Quartz - 13
To this base recipe I added 20% nepheline syenite, which I considered to be quite a large addition. To some of the test bodies I added percentages of AT ball clay and to others Red earthenware powder or Potclays keuper red in to introduce varying amounts of iron.
The resulting tests showed promise. The pots directly in the flame path showed increased gloss due to the increased flux in the clay, far more than the white St. Thomas (the clay being fired in the bulk of the kiln). I had dusted some of the tests with fine sieved ash in an attempt to replicate the ash build up in an anagama firing, these surfaces produced some good ash runs, the movement of the ash and colour in the body was again promoted by the increased body flux. The base white body on its own showed little flashing and remained dry where as the clays with the small additions of either AT ball clay or red clay and flux, indicated more of a propensity to flash from the flame impingement. A high soda flux appears to generate the brightest colours.
For the next set of tests I tried expanding on combinations of high and low iron bearing ball clays and flux contents, using Nepheline Syenite, Potash feldspar and Cornish stone, keeping the quantities of china clay and quartz static. Some of these tests again showed good results in the down draught kiln.
The next firing of the anagama contained several of the more promising tests from the down draught kiln. They were packed in different areas of the kiln, so that any variance could be recorded. The pieces, which came out of this firing, were starting to achieve what we were looking for. Plenty of ash runs on some pieces towards the front of the kiln, some of the tests were totally glazed and in some of the fine low iron bearing clays subtle blushes of colour. Carbon entrapment and flame flashing were visible beneath the glassy surfaces. The overall impression was that the pots in these new clays were coming out bright and vibrant rather than the more sombre colours often associated with these types of firing. The glazes which had been applied unaltered to the new "high flux" clays, appeared to be cured of there shivering tendencies and were giving some very rich colours and surfaces.
While on holiday in Cornwall I collected some bright yellow clay from a beach near Falmouth. Its yellow colour indicated that there must be some iron content. I brought it home and dried it out, slaked it down and sieved it through an 80's sieve. The resulting clay was very fine and plastic. I added 5% and 10% additions of the Cornish clay to a high flux porcelain, which I was testing. Even with only 5% addition the working properties of the clay were greatly improved.
By this time some of the students and I had built a small anagama in the kiln yard of the Ceramics Department at LUSAD. In the second firing (after a few tweaks), we fired for 42 hours to cone 12. Some of the new addition porcelains as well as other clays were tested in this firing. Both the 10% and 5% additions worked well. The clays were fluxed and had integrated with the natural ash glaze. The 5% addition mix took on a bright orange glassy surface and showed no signs of pyro-plasticity. The addition of the found clay had also added organic and mineral impurities, which showed themselves as veiled carbon trapping in the glassy body. The stoneware clays also worked very well, with many colours emerging from the ceramic surfaces. All these surfaces were a direct response from the material, to the firing, surfaces unique in their elusive attainability.
Our next firing of the Wysing anagama three months later contained pots made from 10 different clay bodies. Clays were positioned in the kiln with previous knowledge of the atmosphere, and the way that the flame moved throughout the kiln pack, so that the clays would best utilise these factors. On opening this firing we were very pleased with the results. Clays, which had been tested in the Loughborough anagama, had all become richer and had brightened in the longer firing, producing wonderful flashings under the natural ash glaze. The flame born ash had integrated well with the fluxed bodies producing chunned glaze nipples ("dragonfly eye" as Steve calls it), on the under sides of side fired bottles.
There had not been too much movement from the clays in their prolonged pyro-plastic state and running ash had formed spiders webs of rivulets on the pots all around the kiln but most strikingly at the front where I had used a 60 high flux stoneware, 30 white St Thomas mix. The white St.Thomas addition producing along with the oranges and whites of the No. 1 body, rich black/greys through to titanium white with olive green ash runs which chunned where thick. All this from the clay, the fire and the atmosphere.
Some of my glazes that have continued to be problematic are fired in open topped saggars. The saggars protect the matt carbon trapping Shino glaze from the direct flame on a low flux porcelain body, but allow flame born ash to deposit on the top of the pots. The saggars are perforated to allow a little gentle flame impingement. This has worked well with the fluid almost white natural ash glaze contrasting with the matter carbon trapped surfaces. The same glaze when applied to the high flux stoneware body and fired without a saggar, produces a bright orange to brown glaze with a mother of pearl lustre when placed in the front of the kiln, and more dramatic carbon trapped orange and white surfaces towards the back. At last we were getting the surfaces and qualities, which we had hoped for.
The research carried out on these anagama clays has spawned an interest among some of the Loughborough students into developing their own clays for this type of firing, and also for use in other types of firings and applications. All the members of the group firing the Wysing anagama have diverse aesthetic aims as far as the objects they produce, and it must not be forgotten that the most important element in all our minds is that the surface is appropriate for our work. The surfaces that can be achieved with this type of firing are unattainable by any other means, and by the manipulation of our raw material, appropriate surfaces can be achieved for sculptural and domestic work alike. It is clear that there are an infinite number of ball clay and flux blends that are still to be investigated, all of which could utilize the anagama
Ben Brierley 2000