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Anagama clays

Clay as a material has wonderful characteristics: Whether it is thrown on the wheel, constructed using sheets, mixed with paper fibres or cast in slip form, it is a plastic and malleable substance which will take on forms and respond to manipulation like no other. With these properties in mind I (like many others), want my finished work to demonstrate these material qualities in the final fired state. Attempting to loosen the static rigidity, which even such a soft material can take on when transformed into fired ceramic. By firing with wood to high temperatures my work again exhibits some of these soft plastic attributes.

I embarked on an extensive exploration of ceramic materials and their responsiveness when incorporated into a ceramic body and subjected to wood firing. Firings would often contain work made in ten or fifteen different bodies, it was not only their abilities to trace the fire and respond to the heat and atmosphere which was important, but also their workability. I developed several bodies which gave fantastic responses to the firing but were horrendous to work with on the wheel.
I now limit myself to three main bodies, (although the process of testing is still ongoing). These are: a white stoneware, porcelain and a grogged stoneware. They are all fine particle bodies containing ball clays and china clay (as their clay content). The flame movement throughout a firing and the action of flame borne alkalis, as well as atmospheric conditions within the chamber tends to draw colour best from these fine particle clays.

The other feature of the clays which I use, is the amount flux contained within the body. Roughly 20% of both the stoneware and the porcelain bodies comprise of flux in the form of nepheline syenite. This soda based flux in theory reduces the maturation temperature of a body and indeed porcelain pieces at the front of the kiln can be seen to have a sheen at comparatively low temperatures (1100C). On seeing a sheen at this temperature one would predict that at the top temperature of 1400C they would be puddles, but they hold their form pretty well (partially due I believe to the relatively high china clay content which is imparting pretty high quantities of refractory alumina), and despite this as I have expressed earlier I like my work to show signs of Pyroplastic action.

The big ware body which as its name suggests I use for larger pieces including large platters and bowls is also a fine clay particle body, however it has a large percentage of Molochite grog added to give it more strength. It also has a slightly lower flux content. Because I fire these pieces on their sides stacked along the side walls of the kiln they need the extra strength imparted by the Molochite in order to keep the Pyroplastic movement to some degree in check. Both the stoneware body and the porcelain bodies if used for these large pieces will not have enough strength to withstand the pressures of side firing and in the past I have been confronted on opening the kiln with bowls and platters which resemble calzone pizzas.

The inclusion of Molochite does not seem to affect the body's responsiveness to the colours generated by the fire nor does it mark the surface with iron bleeds, which (with such heavy degrees of reduction through the firing), are characteristic of standard firebrick grogs.

In order to develop colours in the bodies there needs to be some iron present. The iron in all the bodies is contained within the ball clays used. The tiny amount of iron present in the porcelain body is enough in some parts of the kiln to generate salmon pinks and rich orange reds as well as bright whites. It may give all of these elements on one piece (and frequently does). The stoneware body has more iron, although I would still class it as a porcelaineous stoneware, however the extra iron introduced through the addition of a small amount of AT ball clay (a higher iron ball clay than the Hyplas 71), is enough for the body to generate rich oranges and purples as well as brighter tones.

The high flux content of the bodies reacts with ash and alkalis to "self glaze", the bodies on the surfaces which are in direct line of the flame and ash, and also on other surfaces where the flame has been forced into turbulence by pots behind. The additional body flux also brightens the fly ash glaze and encourages it to run and pool.

Although I now generally use only the above three clays, this is as a result of extensive research into many varying combinations of clays and fluxes. Now in each firing I will tend to try one new body, placing a sample both at the front and at the back of the kiln. The stoneware, porcelain and big ware bodies will all respond differently depending upon their positioning within the kiln. This is why I have restricted myself. These three bodies all have the potential to give very varied results and the more I use them the more I learn.

The responses that the bodies have will also be affected by which kiln they are fired in. As well as firing several other kilns myself, I am fortunate enough to have friends who wood fire include small test pieces in their firings. This has helped to build a picture of the fired qualities of the clays. For example, the porcelain body when fired in a traditional Bizen style kiln with no grate for five days will give a much whiter body that tends to be very glassy, with the fly ash glaze appearing a very pale green. The same body fired in a square chambered kiln with a large firebox added to the front with a grate and fired for fifteen days will give a mottled carbon trapped surface with greens, greys, blacks and whites. When fired in my kiln it will give surfaces which range from brilliant white, pinks through to rich reds.

Undoubtedly the types of wood used for the firings also affect the reaction of these clays. I predominantly use Scots pine and Norway spruce for the soft wood, Walnut and Ash for the hard. Work fired in other kilns in the UK and in America have been fired using various Oaks and other native woods. The fuel has changed the characteristics of the surfaces through the different types and quantities of alkalis contained within these woods, given off when the fuel is burned. Also the atmospheres generated by different kiln configurations can greatly influence the tones of colour that the clays produce.
Finally the glaze, which I predominantly use, should be mentioned, as it has also been formulated to respond to this type of firing and give glaze qualities that work with my forms. The glaze is an ash glaze with nepheline syenite, Cornish stone and silica. The non-existence of any stabilising clay makes this glaze pretty fluid and it responds to the firing in a very similar way to accumulated fly ash glaze. The slightly unusual aspect is that it contains a ten percent inclusion of titanium dioxide. This material is generally thought of in ceramic terms as an opacifier, giving creamy white colouration and in this amount would usually be considered to make a glaze matt.

In this type of firing however the glaze gives a huge spectrum of colours from the more traditional cream colours (in cooler [1300C], more oxidised areas), bright pinks, deep blues, greens and purples in hotter areas with a more fluctuated atmosphere. These surfaces usually develop macro crystals increasing in density where the glaze is thicker. Probably seeded by flame borne elements, the fluid nature of the glaze allows the crystals to form in the glaze matrix catalysed by slow reduction cooling. The glaze can also trap carbon, which can introduce interesting elements of black specking, which add to the depth of the final surface.

Ben Brierley.