Stacking for Flashed Color and Scars by Chester NealieThis article was first published in The Studio Potter, Vol. 28 No.2 June 2000 pages 27 to 29; it can profitably be read in conjunction with the article on stacking by Owen; they were published together and Chester and I have shared ideas for many years.
My discussion here is based on experience firing a low arched tunnel kiln up to 5 meters in length with a gentle slope leading up to a long flue and a short chimney. I prefer bundle-stacking pots in this wood fired kiln, believing that superior qualities can be achieved in minimal spaces; subtle flashing occurs where the flame bounces between pots, walls and wadding materials.
What is it I'm trying to achieve in loading a tunnel kiln? My foremost thought is to get the best possible quality of wood firing for each pot. I'm looking to place the clay in the kiln so that all making marks are visible to their best effect. I want unpredictable qualities, directed qualities and that same freedom I try to impart to the clay in making, left on the pots for all time. Ultimately, it’s the story of the making and firing that must stand out.
The second thing in setting these kilns is to make them work. The aesthetic must be achieved through the temperature of the kiln and the placement of the pots so that the wood ash can do its job. The pack has to be directed by the shape of the kiln, the kind of stacking (on shelves or free packed) and where the side stoking will occur. All these parts must tie together in a way that allows the kiln to reach temperature. It’s all very well to have a wonderful stack but if you can't get the kiln hot enough it’s all wasted effort. The physics of moving the flame and of burning the wood vapors so the flame heats the pots is as important as the placement of pots.
Getting to temperature is not the only thing - most people can do it. It’s what to do when you get there and what you do on the way that's important. I'm talking about those indefinable qualities that occur under oxidation and reduction. The stack of pots directly affects these qualities. You can see a very oxidised effect from the early Oribe kilns. Later they discovered that putting a plinth up the middle of the kiln not only supported the arch but also split the big vortex of flame in the firebox, enabling a better reduction process to occur. This tumbling of flame, bouncing off various pots and breaking up the pattern is what we have to consider as well. We're not only playing with temperature, we're also playing with the conditions of the flame.
The shape of the kiln is obviously going to affect the way it can be set. If it’s a high kiln, it’s difficult to bundle stack pots to the top and have the bottom ones support such a weight above them. My preference is to have a low arched kiln and do away with shelving as much as possible.
The way pots are stacked to the roof, leaving enough gap between them for the flame to work is critical. Coupled with this is the need to think beforehand about what shapes will work within each other and which ones will pack together. Spherical forms will very easily pack to the top, but it’s nice to interplay these with cylindrical shapes to stabilize the stack. In tunnel kilns there are spaces that have just the fly ash going through and there are spaces where the ash directly settles on the pots, especially from the side stoking. Pots can be made specifically for these areas. Consider their shapes - you can't have protrusions where they're likely to be knocked off or disturbed by side stoke wood landing on them. Handles and lugs have to be tucked into the edges of necks and subtly kept out of the way of descending wood.
The strength of the clay and its shape have to be considered. The pots have to be thrown with a bit more weight to them if they're going to support a meter of other pots balanced on top. Rims must be stronger and cylinders can't be too wide or they'll squash and end up very flattened forms. If pots are going to be lying on each other, the marks of placement are going to be part of the decoration of the pieces. Wads between them have to be very carefully considered. Kaolin and alumina wads produce pots that separate easily, but the character of the wad marks - a sort of white bird dropping - has a nasty synthetic quality. I feel it is too severe so I look for a much softer blush of color.
I like a wadmix made from a fireclay that's fairly refractory and will break down easily. Mixing this with fine or coarse sand or sawdust will have different effects so you can play all sorts of games with that. Owen Rye introduced me to Hallam fireclay. It has a significant amount of iron present in it but is highly refractory. When you mix it with coarse sand (so much sand you can barely hold the wad together) you'll find the wads just dust off after firing. The coarse sand contacts the surface like a bunch of pin pricks and gives the flame a chance to go between the wad and the clay body, producing a dappled color on the pot.
There are many other things you can use for separating pots. I remember, from
To me the scars are very important. The quality of the scar is part of the passage of that pot and a whole aesthetic develops around what kind of scar you're going to leave. We've all seen the use of straw, either soaked in brine or not, or various leaves or grasses or shells placed underneath leaving their marks. I like this use of materials from the immediate environment to create a permanent record on the sides of pots. In the future these grass patterns could be recognized although the grass itself may become extinct. After all, the pots are going to last a million years whereas the vegetation might not.
The placement of the wadding becomes an aesthetic consideration. I like to apply pats of wadding in large, free-form daubs. Used fairly wet, it adheres better on the raw rather than bisqued pots. Although it appears nonchalant, these daubs are considered for both placement and size in addition to the functional aspect of standing one pot on another. Too many small daubs give a spotted look. Wadding when removed gives an insight or window back into the clay body and/or glaze layer.
Shells are my favorite separators for pots. I prefer shells with texture. The shape of the shell is positioned to give maximum surface contact with clay and the pot is set on wadding with the shell in contact with the pot. The shell becomes calcined during firing, leaving a reverse fossil-type replica, otherwise it is unaffected. After the firing the calcium oxide recombines with the air moisture to form a powder. Reluctant shell wads can be loosened by soaking the pot in water, enabling the shells to swell and separate easily from the clay of glaze surface.
Residual salt from seashells tends to leave an orange halo around the perimeter of the shell. This is particularly effective on white and flashing bodies and seems to happen under both reducing and oxidizing conditions. An extreme case is to set the pots buried in shells. Any shell particles remaining and subsequently refired combine with the ash and flux to form significant glaze runs, occasionally corroding the surface of the ware.
I find placing the pots evenly like rows of soldiers gives a very boring flame effect. I like to zigzag the pots across the kiln with some in front and others staggered back. I place a large pot in the middle so the flame hits that and spills to the ones behind. I'm playing a rebound game with the flame. The spaces are as important as the actual pots so, as I load, I always consider what the flame is going to do and where I will leave room for it to go. It’s easy to pack the bottom of the kiln so tightly that the flame can't get through. It’s harder to leave room for the flame to pass through to affect the pots behind.
I try to make a variation of pot forms so they'll stagger through the kiln. That way the flame bounces back-wards and forwards off the walls, off the sides, in between and around the pots. I'll also stack the pots against the wall of the kiln. Without the restriction of shelves I can use the entire space for my construction. I'm making a sculpture with a purpose - a huge installation in itself.
One of the joys I find with tunnel kilns is that there is a reasonable variation
of temperature throughout. Consequently, I make pots that will benefit from
the different temperatures. Terracottas, slips, all sorts of coverings, high
temperature clays, highly refractory clays are all placed in suitable positions
to give flashing. For instance, terracotta which can take temperatures of 1200-1250°C
(2192- 2282°F) can give magnificent colors in the cooler areas. I'm not
trying to get a porous body but am always wanting a fluxed clay for that soft
wet look of the melted ash.
Coarse white feldspar clays can be placed throughout the kiln. At the front
and sides of the firebox, such pots can achieve a purple colored ash glaze,
and yellow to orange flashes from alternated oxidization and carbon inclusion.
These strange purple blues occur in slightly lower temperature areas where pots
are partially covered by ash and where there is a certain amount of free oxygen.
The tunnel kiln tends to oxidize around the sides of the firebox and behind
horizontal side-stoke holes. I stack pots in these places knowing the probability
of these effects.
Bowls and platters are very difficult to place without sitting them on shelves. However, they can be accommodated, especially bowls, if you remember that the flame is always wanting to move upwards. If they're stacked upside down they'll give some beautiful cornet-like effects as the flame slides in between the wads in its struggle to go up through the pots.
In the rest of the kiln, I am looking for strong reduction from 800°C (1472°F) onward because from that temperature a reaction has started between the iron and the fluxes in the materials. The amount and quality of reduction is not necessarily related to the black carbon coming from the flue. As the kiln is built directly on the ground, the hydrogen from the ground water will act as a reducing agent from 1000°C (1832°F). Carbon monoxide from the fuel combustion, however, is the main reducing agent.
The reduction is kept on a tight edge continuously, balancing the fuel with the amount of oxygen coming in. The speed and color of the flame is more important than masses of black smoke. It is what is happening inside the kiln that is important. The natural wood firing processes of regular stoking gives variable oxidation/reduction.
After 1300°C (2372°F) has been reached, (and because the pots are raw, this takes 24 hours), the kiln is kept at this temperature for 70 to 80 hours. During the firing I use sidestoking to build up a heavy load of unmelted ash. I then bring the kiln temperature up again to melt this ash down. This process is repeated over several days, building layers of melted ash on the pots. I sometimes use different woods to vary the colors in these layers. This cycle should finish with a hot melt throughout before the final stage. The kiln temperature should be well in excess of 1300'C at the front, while the back, where the feldspathic glazes and dense iron bodies are placed, should have reached 1260-1280°C (2300-2336°F).
Following this, the final reduced cooling begins. I stoke heavily over the next four hours until a large bed of embers shows as high as possible in all the stoke holes. At the end of this period, I completely fill the front of the kiln with large logs interlaced with thinner wood. This heavy carbon reduction gives a mattness to pot surfaces. I leave the firebox lightly sealed so that the embers will continue burning for at least 24 hours in a slow lazy atmosphere which leaves the flame patterns on the pots.
When the pot comes out of the kiln there's yet another process to consider. How much ash surface to rub off and how much to leave? After firing, most of the wadding and ash is removed by rubbing with a hand stone or wire brush, or gently chipping with a cold chisel rather than grinding with a tool. I use a coarse to medium hard whet stone with water to rub down the pots or I can sandblast or chip off the wads to leave those wonderful scars. I think of them as birthmarks, because to me they tell the story of the pot's creation and are an important part of the aesthetic of the whole. The essence is to leave the naturalness of the process so that the story of making and firing is revealed without obvious contrivance.
But the pots aren't always finished after their first firing - they could be multiple fired. Owen describes once-fired pots as novices just beginning their process. A shiny ash surface may be covered with a dry slip or a dry ash surface covered with a glaze of shino and again refired. The surface subtleties continue with each firing. Each time new surfaces are presented that may be outside your previous experience and may take time to understand. When is more too much? Its left to you to work out.
Stacking these kilns takes time and I find it the hardest of all the processes. The loading is more than placing pieces on shelves - it’s to do with using the palette of color from the flame and ash to paint the pieces, not unlike brushing on canvas.
But when you think about it, all the problems are just part of the learning curve. The destroyed kneecaps are the hair shirt of the anagama potter and the effects, when you get them, make it thoroughly worthwhile. To place a simple little pot in the kiln so that the flame and ash leave their magical mark is worth all the agony of the hours of loading.