“Form and Surface”, 1989

This article first published in Pottery In Australia Vol 28 No 3 (August 1989) pages 19 to 21

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My discussion is limited to woodfired effects in the anagama or tube kiln, and specifically to what I see as the different zones in this type of kiln, and the differing effects produced in these zones. This discussion is based on some nine years experience of long-firing kilns and having been involved in about 30 firings I have formed some general notions about this type of firing.

The first is: kiln design does not matter much. My experience suggests that the slope of the tube kiln is not too important provided adequate draught is supplied; the internal shape can be virtually a straight tube or can be the elegant porpoise shape currently fashionable ( I do prefer some rough or projecting brickwork inside to create local turbulence); and that the simpler the overall conception of the kiln and firing the more appropriate. Provided the back of the kiln is constricted enough to produce considerable reduction and that side - stoking is possible at least each metre along the kiln, the full range of effects is available with virtually any kiln design.
The second notion is that packing the kiln is to me the most important part of the whole process. Selecting which clay bodies and which forms go in various parts of the kiln, deciding the relationship between pots in space to give desirable flame markings as well as providing a proper flame path through the ware, and the placement of wads or stacking of pieces touching each other, are the critical factors in determining the final result on each piece. I might he exaggerating slightly in saying that as long as the firing goes for long enough, as long as there is a general tendency to reduce more than oxidise, and as long as the kiln gets hot at some stage near the end, the results will be worthwhile provided the packing was right.

A third notion is that the fashionable Japanese concept of ‘acceptance’, which suggests the primacy of accident and lack of conscious thought as factors in firing these kilns, can too easily be seen in some work as simple inexperience. With increasing experience, gained from careful observation of details, the results obtained from these kilns become relatively predictable. Not totally predictable, because there are so many interrelated variables that complete replication of long firings is impossible, and each firing takes on its own characteristic hue when the work is laid out after opening. Fortunately, the multitude of variables ensures that this style of firing will always contain an element of challenge, and scope for growth in one’s work.

ZONE 1. The area at the rear of the main or front firebox, before the first side stoke-hole. This is an area of very variable ash deposit; partly very heavy, well-melted ash, with runs quite common; partly light ash on black or grey areas which have been kept cool through much of the firing. Not much flashing occurs. This area is best suited for larger, heavy-walled forms rather than small pieces. Often extensive cracking occurs, especially when firing is rapid, so bodies used in this area need to be resistant. I like to add coarse river sand to commercial clays, or use naturally coarse-textured clays to build up layers of texture on these very variable surfaces. Definitely not a suitable area for crockery.

ZONES 2. 3 and 4. These are the hot areas of the kiln. Flame tends to rise from the firebox to the top of the kiln and travel over the top. I prefer to set these kilns with biscuit-fired pots (which admittedly is much easier with access to college kilns for biscuit firing) rather than raw, because a raw setting shrinks more arid encourages more flame to travel up and over the ware rather than through the pack.

Ash deposits tend to he uniform and relatively fine in these areas, and the ash coating becomes thinner towards the back of the kiln. Through much of the firing the flame is relatively fast moving through these zones, and tends to produce ‘comet’ markings around wads. The edges of these markings are usually fairly crisp without much halo effect, although towards the back of the kiln the halo effects become more complex. This may have some relationship to the slower flame speed towards the back. These hot higher areas need very refractory bodies towards the front of the kiln, with good resistance to warping. I use a body based on Hallam fireclay which has a warm fired colour, with alumina added. Very few commercial bodies will stand the treatment they receive in the top front of the kiln in a hot firing, although some survive further towards the hack. These areas of the kiln are well suited to the ‘crockery’ types of ware such as teapots, platters, bowls, and the like. I set many miniatures in these areas.

ZONES 5. 6 and 7. The kiln I currently use has side-stoke holes about nine inches above the floor-level, in contrast to many other kilns which have side-stoke openings more towards the top of the kiln. The lower stoke-holes permit use of longer and larger pieces of wood, giving complete control of temperature along the kiln. I stack pots on their side below the stoke-holes, right across the kiln. These receive a very heavily carbonised ash deposit on their upper sides and the other side exhibits a light ash deposit but strong flashing effects. I like to produce forms with heavy incised lines or texture for these zones. The lines or textures are partly hidden and partly revealed by the variable ash deposits along the sides of the work, but on the upper surface there is no point in elaborating the surface which is covered with a thick ash deposit. These zones of the kiln are most definitely not suited to utilitarian wares. For me, tall slender forms work best. The extreme variations in temperature mean that forms should not be too thick walled or cracking is inevitable. Coarse-textured clays are aesthetically more appropriate on than fine. Higher-iron clays are very suitable for producing deep red flashings on the lower sides. White clays do not produce good results.

ZONES 8, 9 and 10. These areas are the most affected by side-stoking practices. If stoking through most of the firing is mainly from the front of the kiln there wilt be a considerable temperature drop-off from front to back, and these zones will not have a very significant ash deposit. I normally begin side-stoking either at the back, or right along the kiln, as soon as the temperature is high enough for the wood to ignite. These are the most variable areas in the kiln. Ash and charcoal spills both backwards and forwards from the side-stoke areas, usually insulating and cooling portions of vessels and forming greyish areas. Thick, well melted ash deposits form in the area behind the side-stoke and uneven ash deposits in front of the side-stoke. The ash particles are usually quite coarse so the ash deposits are very variable. Often strong flashing occurs because of the turbulent flame and uneven atmosphere. Work placed near the stoke-hole will have spiky, uneven ash deposit, and near the sides of the kiln, where cooler conditions are caused by air entering the stoke hole, these are often poorly melted. Fine detail will be retained on the sheltered parts of pots, so fine bodies are suitable. Many commercial clays tend to be suitable, especially the stonewares rather than porcelains, the lighter-coloured bodies being more susceptible to flashing. I set a lot of miniature pots in these areas.

ZONES 4, 10 and 11. Those zones at the back of the kiln produce the greatest variety of flashing effects. Ash deposits tend to be fine and uniform but wadded or contact areas produce the rich flashing. Some lovely, dry, ash surfaces can be produced it temperatures are allowed to drop at the back of the kiln, and these dry surfaces contrast well with the rich flashing. Shorter firings will produce little ash deposit hut considerable flashing can still develop. These zones of the kiln are well suited to fusible bodies, especially those intended for shorter firings to about Cone 5 or less. The most fusible porcelains work well, as do some fine-grained, high-iron bodies such as the New Zealand-produced Abbots Red which can produce very elaborate flame markings.