The increasingly rapid advancement of technology that we are all
becoming subject to permeates every niche of our existence.
Whether this is necessarily good or bad is up to the individual to decide,
but when it eats its way into a craft which is essentially primitive in nature, it can
only be described as detrimental. Generally, potters in
the West have always directed and controlled the
processes of their craft rather than worked with their
materials in the directions to which they naturally tend.
Now we are reaching a very dangerous stage whereby
craft pottery can either plunge into a sterile
technological environment or return, probably and
sadly too, out of economic necessity rather than the
aesthetic, to its roots. The advent of electricity, gas and
to a certain extent oil, has turned the most creative part
of the potters processes into a mere finishing tool, a
method of mediocrity whereby pots are made
permanent in much the same way as plaster sets in a
mould. Indeed, one can now obtain automatic firing
devices, it has to be wrong to make pots by hand and
then to fire them using computers. Bernard Leach in 'A
Potters Book' wrote, 'There are people who imagine
they can play at potting, condescending in their opinion
from fine art to a craft which they can expect to learn in a
few weeks or months, buying everything ready made
and avoiding drudgery by mechanical devices. Pottery is
still being made and decorated under this mis-
conception, it would be well for both potter and public
to realise that the results of such performances look
precisely what they are, unhappy exhibitions of
dilettantism in a mechanical age'.
The potter has essentially three elements upon which to bestow his creative powers, those of clay, glaze and fire, but at the same time he must strive to be true to these three. For example, if he makes his pots from clay, they should be seen to be made from clay, it is no good making a pot as though it were turned from metal. As potters we have this amazing potential to deviate from the perfectly symmetrical, even on the wheel. There is far too much emphasis put on what goes into a glaze, than what a glaze can do, and likewise with kilns and firing them. A live flame, especially if it originates from burning wood, can be used in such a way as to make formal decoration superfluous if not unnecessary. I have heard of wood firing and its dramatic effects being dismissed as an easy alternative, a way of decorating without decorating, however, if it is used in the same way as a brush and pigment it is incredibly demanding physically, mentally and in terms of accumulating knowledge. One can see what is happening with a brush and pigment, it is controlled. One cannot see what is happening inside the kiln, but it is possible to have a pretty good idea. The following account consists of knowledge gained largely from hard won experience, very little is gleaned from published descriptions of others. While at Farnham, like most other students in an art college, I had the unique opportunity to be totally self indulgent-digging holes in the hillside and firing them was my main pre-occupation!
To commence we must go back in time, to when wood was the only fuel for firing pots, to Japan during the period from the eleventh to the fourteenth centuries. The only active ceramic producing localities during this period became known as the Six Ancient Kilns, three of these being Bizen, Tamba and Shigaraki. The early wares from these kilns were fired in a kiln called the Anagama, or cave kiln. This was simply a tunnel in the hillside. The location of the kiln was determined by a favourable land contour and suitable soil conditions. A sandy soil with a percentage of clay and no rock was necessary. Limestone or shale in the kiln wall would upset the stability of the kiln during firing, and naturally a compact soil was necessary in order for successful digging to occur. This simple anagama, usually six to ten feet long, fulfilled all the essential elements of any kiln. A firemouth for wood burning, a chamber in which to place the wares and to accumulate heat, and a flue for the exit of exhaust gases. In this upward kiln the gases moved by convection, and at the same time produced enough draught for the rapid combustion of wood.
Once fired, the kiln was sintered to a hard surface and became self supporting. The more firings it had the stronger it became. Long firings were necessary largely due to the dampness of the earth, the kilns took several days to reach 1300, fired on cypress and pinewood which was locally abundant. It took many hours of firing to drive off the water in the walls of the kiln chamber. Until the kiln was quite dry, no build up of heat could take place. Temperature gauging was carried out by eye which resulted in variations between firings. As a result of the firing process the pots had rich surfaces, colours and texture, resulting entirely from the firing, rather than from the design. The passage of flames through the setting of the kiln gave random colouring and qualities to the wares. The most important feature of these pots was natural ash glazing, the deposit of ash being the inevitable result of the placement of the pots in the kiln and the firing cycle. The ash, the by-product of the burning wood flies through the kiln following the flame path, thus settling on the pots, and melting to form a glaze when sufficient temperature has been attained. The success of ashing was dependent on the fusibility of the ash and on the length of the firing. Conifers, such as cypress and pine are especially fusible, and only an extended period of firing would generate sufficient ash. The position, extent and the flow of ash was dependent on the placement of the pots in the kiln, the duration of the firing, and the temperature achieved. Together with 'flashing' from the flame, and ashing, the pots were also enhanced by kiln particles, stacking scars (the pots were simply stacked in the kiln) and packing deformation. No other tradition has managed to capture so much beauty from the firing process and at the same time done it with such innocence. It was not the intention to use the firing as a conscious means of decoration, it was quite simply a means whereby the pots became able to perform their preconceived functions. The main point is that from the simplest of firing methods some extremely beautiful pots were created. I cannot help feeling however that somewhere amidst the battlefield of exhausted men and piles of wood were potters who, like myself, really got quite excited by what they were doing.
There are several well proven principles and designs available for burning wood and firing pots efficiently, however efficiency does not necessarily contribute. towards promising pots, indeed quite the opposite. It is perfectly possible to fire pots with wood using an extremely efficient kiln and to end up with pots having no evidences of wood firing upon them. The emphasis in this case would be one of pure economics, wood as a cheap fuel forgetting its decorative potential, although it must not be forgotten that wood is also labour intensive and requires considerable space and management. The concern here is the use of wood in a dynamic manner, not necessarily as efficiently as possible.
The cross draught principle is logically the optimum means to extract the most from the elements of wood firing that are available. Flame and ash pass between the pots throughout the length of the chamber; in a downdraught kiln for example the flame passes past the pots between the shelves, usually the only pots to capture the excitement are the ones at the top of the kiln. I have come across one kiln superior to the cross draught in this respect and that is one belonging to Takeshi Yasuda which has a flue exit consisting of a hole in the centre of the kiln floor covered by a raised kiln shelf. This appears to be an excellent method of retaining the bulk of ash and dancing flames within the kiln chamber.
The kiln I built at Farnham turned out to be nothing more than a sophisticated anagama. Through a shortage of brick it was built into the ground on a slight slope, the chamber floor being eighteen inches below ground level and the ashpit floor four feet below. Having the ashpit that much lower than the kiln chamber produced a positive pressure principle, the opposite to the negative pressure experienced in downdraught kilns. The kiln chamber had the shape of an elongated egg; from the outset I realized that a live flame naturally following its shortest route to the exit from the kiln would not take time to travel into corners. Thus the kiln chamber, tapering towards the exit flue, was constructed with the organic shape of the anagama in mind. The end result was a kiln utilising both updraught and cross draught principles, very similar to the draught of the anagama. The walls of the firebox and ashpit together with the chamber arch and door were built of firebrick, the floors made serviceable with a 90% grog 10% clay mix. The chamber arch rose eighteen inches above ground level at the front of the kiln tapering downwards towards the flue. By far the best type of firebox for the combustion of wood is the Bourry, a system whereby the bulk of wood is burnt suspended across the ashpit at right angles to the kiln with the ends of the logs resting on two ledges called the hobs. Thus the wood burns in mid air and maximum combustion can take place. The ashes and embers fall into the ashpit and provide a ready source of preheated secondary air. With this type of firebox and an ashpit of some six feet in length I was able to use the effects of flame and ash to their maximum potential.
The firing of a wood kiln has to be an entirely separate exercise from the rest of the process, the concern is no longer with the individual pots, but with a device which affects all of them together. The demands in terms of concentration, usually some form of physical stamina and an almost total reliance on intuition make it inevitable that pressures of making pots and packing them in the kiln are forgotten. The packing of the kiln is the most crucial part of the whole process. It is here that one decides, to a large extent, the fate of any one particular pot. What actually happens inside the kiln is not so much in control as the firing itself. One cannot really predict the outcome, but it is possible to set things up so that the so called 'accidentalism' aspect of wood firing is to a certain extent, calculated. Packing the kiln at Farnham, though only 25 cu. ft., could take as long as a whole day. The same psychological problems are experienced when packing as when decorating pots in a formal manner, however there is a difference. While packing a wood kiln one anticipates potential decoration, it is not possible to see the end result until the process is completed. Hours can be spent deliberating where pots should go, rearranging them and possibly throwing some away. It would take much good fortune for me to be happy with the pack. It may have been made too open or too tight or pots that I really wanted in the best places would not fit.
There are several decorative processes to be considered while packing. The path of burning gases through the maze of pots is dynamically creative. This has to be taken into account, for it carries with it a natural glaze, that of flyash. It is important to decide which part of the pot is to receive all this potential activity, if at all, for expectations can easily be wrong.. The majority of flame will find its easiest route to the exit, it will favour a large gap in preference to a smaller one. It is possible, by way of the pack, and particularly with the cross draught principle, to virtuallly tie knots in the flame path. This is important because where the flame, which is far hotter than the kiln temperature, impinges on a pot, the extreme heat of that flame fluxes the surface, or in more linguistic terms, causes flashing. An alternating cycle of oxidation and reduction occurs naturally in a wood kiln causing the fluxed, exposed surface of the pot to become reddish, in contrast to the greyness of the reduced body. Pots that are unglazed on. the outside can be stacked on the top of, inside or next to each other. Where they touch the surface cannot be reoxidised and thus remain grey, or lighter in contrast to the redness of the oxidised body. This aspect of decoration is of major consideration while packing, along with the positioning and size of wads, if used. The function of wads is either to aid stacking (the use of kiln shelves is minimal) or to prevent pots from sticking to each other or to the kiln furniture. This is usually necessary if high temperatures are attained, or if a lot of ash is generated. 'Sticking scars' are quite acceptable but it gets a bit much if pots have to be broken to get them apart. Wads can be made of clay which leave grey marks on the pot, or alumina mixed with china clay and a little bentonite for plasticity. The alumina wads leave conspicuous white marks, one alternative is to use seashells which cause scarring on the pot wherever they are placed. The calcium carbonate of the seashell forms a eutectic with the silica and alumina of the clay body and fuses, leaving a scar. The remainder, having been reduced to quicklime (calcium oxide) in the firing is easily removable. Additionally, the salt in the seashells volatilises leaving pleasing marks on the surface of the pot.
Major consideration must also be given to flame pattern. In anticipating the path of the flame through the kiln it is important to remember that the flame will always follow its easiest route to the exit. Obviously, I can only speak in detail regarding the kiln described here, however the criteria should be easily applicable to any other. Bearing in mind the shape of the chamber and the cross draught element, if a bagwall is built, be it only three bricks high, the flame is immediately forced to the roof of the kiln. Unless there is a large gap in the middle of the chamber, the flame will then run the length of the chamber at roof height and drop down to the flue at the back. The pots in this path tend to be high fired, heavily reduced and with the bulk of the ash falling on the pots at the back. The remainder of the kiln, the middle and bottom front is usually cooler and comparatively pale in colour. A small bagwall is necessary however to prevent the flame from travelling along the kiln floor to the flue. I have experimented with the positioning of large pots, when packed in the kiln with the pots of more conventional size. It is not really practical to have one large pot at the back, for the part of it nearest the flue tends to be lower fired than the rest of it. Nor is it practical to place one pot off centre in the middle for this upsets the flame currents and causes unevenness not to mention the phsyical problems when packing the space next to it. Two large pots right at the front will restrict the flow of flame and ash to such an extent as to make the firing one big headache. By far the best position on all counts is to have one large pot right at the front in the middle. This automatically splits the flame creating two, major flame paths within the kiln. This in turn creates greater kiln potential. Firstly and inevitably the ashing on this large pot can be very interesting, Secondly, the two flame paths eventually have to converge at the flue, if not before, and under these conditions, fire flashing and ashing are at a maximum. It is important to consider the effect that pots, or stacks of pots in certain positions, will have on this flame path. With a little cunning, a maze of eddies can be conjured up, the more corners the flame has to navigate within the pots, the greater the flashing. If more ash is desired on some pots than others these should be packed more loosely than the rest and in such a position as to be in the anticipated flame path.
There are many things then to contemplate while packing. It is at this point that the last say is said, the last direct influence on the pots carried out. From here on, although the kiln is to a large extent controlled, it is the kiln that dictates the final outcome, however this is influenced by many prevailing factors, some of which may cause one firing to be completely different from the next. At this stage the kiln and the firing speak for themselves but always in close relation to these things. Half the enjoyment, half the excitement is in the unpredictability of it all, not knowing whether there will be but one good pot. The final outcome can be entirely transformed by the length of the firing, the type of wood, the amount of moisture in the ground, atmospheric pressure, the weather and a whole host of other factors. Indeed, the only explanation I could offer for a kiln full of orange coloured pots of a tone I never did achieve again was that on the day of the firing it was ridiculously hot and atmospheric pressure was much higher than normal.
It is important after packing and bricking the door to pause and contemplate what is to come. The right mental attitude is essential, for many things can cause the firing to go wrong, half of them directly affecting the firer. The weather and time of day are the most influential elements upon morale, not to mention the effect on the duration of the firing and the atmospheric conditions within the kiln. Twenty four hour firings include the night. Rain during the day is bearable, rain at night, particularly if heavy, can dampen the most enthusiastic of spirits. The dread of a stormy night is a force to be reckoned with. High winds are capable of reversing the kiln, of blowing the chimney down. They can alter the pull of the chimney drastically, and can make chaos of controlled stoking. Rain soaks wood, the ground becomes wet and boggy, the firer becomes wet and cold, then hot and sweaty, then wet and cold again as each stoke is repeated. The most difficult hours to contend with are those preceding dawn. Those first signs of daylight are such a boost to flagging spirits and an exhausted body-but they take such a long time to appear.
A little fire during the evening is a fitting end to the days packing. This will thoroughly warm the kiln and dry out any remaining moisture. It is difficult however to stop and retire home to bed, preferably early. It is essential I think to start before dawn the next morning, there is something unique about those first silent wisps of white smoke mingling with a fresh day. The birds wake, the dew gleams on the trees, and the little fire crackles merrily. Such a small fire and so relaxing, yet warm enough to sort out cold fingers in the winter. Breakfast is next, but it is short, the magic of the first few hours, they soon pass. The fire begins to rage, the flames pour along the firebox floor like a waterfall and work begins in earnest. Wood must be sorted and bundled for each stoke, the firebox must be raked regularly in order to keep all the wood burning, and wood must be stoked into the kiln with relentless regularity. It is difficult however in the early stages to overcome the temptation to force an excess of temperature against time. For myself, I am sure it has something to do with the desire to exert the most violent conditions possible, short of disintegration, upon the pots. It is satisfying to know that they survived, to know that they have been subjected to their limits, to their utmost physical tolerance. To actually see the scars of flames and the distortion of form satisfies me in the same way as being able to see that a pot has been made by taking the clay to its utmost physical capacity. In the same way, the materials used and the method of firing impose limitations on what can be made. Coarse clay will not allow delicate forms, and delicate forms will not survive excessive kiln conditions. Thus, from a given set of limitations a distinctive style will emerge. Similarly, one can take the firing to such a point where the limitations of the materials begin to take effect, this is a distinctly creative part of the process. It is a point of marginal plasticity, when pots with certain forms and inbuilt weaknesses, stacked in a particular manner will almost certainly distort. The glazes used are in such a molten state that they are running down the sides of the pots, flyash is spontaneously fusing to them and if enough has accumulated is behaving in a similar manner. This part of the firing, accounting for a heat tolerance of no more than ten degrees centigrade, is a sensitive period. It can be approached with caution, or with rapidity, it can be frozen by immediate cooling. The critical factor is when to stop. It is handy to have a spyhole with a specimen pot in view, so one can watch and wait for the right moment, to be able to see the extent of distortion and how far the glaze has run.
Wood varies in calorific value and heat worth from species to species. Softwoods, especially of the coniferous type have a greater calorific value then hardwoods. They also contain a greater resin content, resin being highly flammable. Another factor of softwoods lies in their lower specific gravity and more open texture, which exposes more surface per unit of weight thus speeding combustion. All this is important to remember, during firing it usually results in equal amounts of thin softwood and thick hardwood being used. Thin wood will provide the immediate release of a lot of energy, resulting in rapid temperature rise, whereas thick hardwood will provide mainstays in the firebox for long periods of time and also promote a slow but even release of heat. The type of wood used during a firing can have a marked effect on the atmosphere within the kiln, and is also influential in the colouring of ashing. If a lot of pine is burnt, ashing tends to be very fluid and light green, hardwood will promote drier ashing and tends to be yellowish. Pine can produce the most violent of reducing conditions, also great quantities of black smoke, this sight however is not necessarily indicative of good reduction. Whatever the type of wood used, it must be thoroughly dry before it can realise its maximum heat potential. To fire a kiln with damp wood is a futile waste of effort, for dampness will consume at least half the available heat energy in being driven off. Likewise freshly felled wood will require two or three years to dry out, and will then still maintain a water content of some twenty per cent. Even this will use up a third of the energy available.
Briefly, wood burns in two stages. The first stage of burning is the combustion of volatile gases driven out of the wood during the charring process. If one watches the wood as it is placed in the firebox, it is literally roasted. The gases pour visibly out of the wood and these ignite spontaneously even before the wood itself starts to burn The resin content of the wood plays an instrumental part here, the greater the resin content the greater the heat release during this initial stage of combustion. As gases are being burnt, longer flames are produced. Above eleven hundred, this part of the burning process will produce immense flames. As the wood continues to burn these will gradually recede as the flame length diminishes and the wood takes on a more steady rate of heat release. Droplets of resin are also driven out of the wood, these do not ignite until they have found sufficient oxygen to do so, usually in the kiln chamber rather than in the firebox. The second stage of combustion is from the non volatile part of the wood, charcoal, in the form of embers. This burns with a shorter flame, so that heat release is more localised. The charcoal requires oxygen to burn, and as the bed of embers in the ashpit builds up the oxygen requirement for efficient combustion increases in proportion. After a point, the embers may use up so much oxygen that the volatile gases cannot burn and pass through the kiln without releasing energy. In this case, as I have experienced many a time, usually through a desire for entertainment rather than substandard firing technique, an enormous roaring fire of combustion will light up the whole kiln site, something comparable to near daylight. The ideal, naturally, is to have the gases buring inside the chamber, usually achieved when a soft flickering tongue of flame is issuing gently out of the chimney. Under these circumstances, unless heavily reducing, the kiln should be gaining temperature.
However well dried the pots and the kiln are, it is essential to start slowly. A small campfire for an hour or so followed by a large campfire for another hour or so will ensure a slow temperature rise of up to two hundred. Another two or three hours up to six hundred is recommended. Anything much faster than this, though it may not cause disintergration of pots, some cracking and partial splitting of the clay surface may occur. Over this period the size of the fire in the ashpit is progressively increased in size until around six hundred when wood can be stoked in from the Bourry firebox door, so that it lies at an angle, with one end in the ashpit and the other resting on the hobs. It is important to criss cross the wood as much as possible so that maximum combustion is achieved by having a maximum surface area of wood free burning. The embers from this wood are allowed to accumulate beneath the hobs in order to bring them up to red heat, and thus to a point where stoking across the hobs can commence. The temperature at this point is near to that where reduction commences, around nine hundred.
Reduction is achieved by one of two methods. Inefficiently speaking, one can overstoke, that is feeding an excess of fuel into the firebox, which in turn cannot be burned by the available oxygen. This causes an excess of black smoke from the stack, a factor not indicative of the state of reduction. It is merely a waste of heat worth in the form of unburnt gases. Under these conditions, temperature rise will cease, and in all probability will fall.
The alternative, and by far the more efficient method is to decrease the amount of oxygen per constant of fuel, by closing up the secondary air vent of the ashpit. Using this method, very little heat potential is wasted, and although temperature may not rise, it will not fall. This inevitably calls for an alternating oxidising and reducing cycle, for under oxidising conditions, temperature will rise rapidly. Therefore it is necessary to be constantly aware of proportionate kiln atmospheres and to react accordingly.
It took many firings for me to learn and understand the functioning of the Bourry firebox. Temperature rise after eleven hundred is difficult to achieve, and after twelve hundred nigh on impossible unless a subtle, understanding touch is available. Reduction, especially if fierce, will almost inevitably cause. a drop in temperature. In order to gain temperature it is necessary to have a subdued oxidising flame. This flame, as it does not carry any unburnt, cold fuel, is hotter than the kiln will ever get, it can be seen as a slender white tongue flicking into the kiln and dispersing before it reaches the flues. Under these conditions it is possible to get a temperature rise of up to forty degrees with one stoke, though twenty degrees is a normal figure. Severe reducing conditions with violent, rushing and long, yet very desirable flames can drop the temperature as much as sixty degrees, so one has to engineer the stoking so that a staggered yet gradual rise in temperature is achieved.
The embers in the ashpit, the by-product of wood burning across the hobs, are instrumental in achieving temperature rise. If the embers rise above a certain volume, the supply of oxygen to the hobs is severely reduced; it is being used up in burning away the embers, therefore efficient combustion cannot take place and temperature rise either stagnates or falls. If the bed of embers falls below a certain volume, the air entering the kiln by means of the secondary air port is insufficiently preheated and causes similar effects regarding temperature.
The method by which the volume of embers is controlled is quite logical. If a lot of thin wood is burnt, a lot of embers accumulate. This is because the thin wood does not take long to burn through, and will do so before the bulk of the wood is completely burnt, thus ending up as embers in the ashpit. Larger logs are advantageous for a more controlled firing. They take a long time to burn through, at the same time giving a constant flame to the kiln. Temperature rise is thus slower, but more sustained. The embers that do fall in while the stoke is burning, are sparse but continuous, and are constantly being consumed with ease and a minimum of oxygen by preheated secondary air. Thus when the stoke does get to the point of collapse, there is sufficient combustion space within the ashpit for the subsequent embers. Also, if all has gone well, one will have gained twenty five degrees, five more than the amount that will be lost due to the violent, reducing flame caused by the embers falling in (which causes a temporary, but excessive lack of oxygen). This alternating between a soft oxidising and a raging reducing fire is sufficient to produce well reduced pots if maintained, and is also particularly desirable for its range of colours on the raw clay body.
It is possible to fire for great lengths of time without causing any large collapse of the stoke, at the same time maintaining a constant reducing atmosphere and temperature rise. This is done by periodically stoking using two or three main supports of large logs covered with thin pieces criss crossed on top until the firebox is full. The large logs prevent the smaller wood from falling in once burnt through, thus it will be free burning in mid air throughout its entire combustion The large logs fall in gradually and rarely all at once. Thus in between each complete fresh stoke, it is possible to fit in two or three stokes of thin wood on top. In addition to every fresh stoke, the embers are raked through vigourously, this keeps them turned and continually burning and breaking down. In doing so vast quantities of flyash are generated.
Raking through the embers to do this can be quite a conscious exercise, and if this is done often enough and long enough it can promote amounts of glaze on the pots concurrent with several days firing, even though the firing may only last twenty four hours or so. It is important to arrange the pile of embers after raking, just inside the entrance to the ashpit, and just before the hobs. This accounts for maximum preheating of the secondary air, and leaves a clear space underneath the hobs for the passage and easy combustion of the flame and gases.
Thus it is quite feasible to control the rate of temperature rise, and also the ratio of oxidising and reducing flames, this factor alone is capable of causing one firing to be completely different from the next.
One would automatically assume that due to increasing stoking skill and efficient use of wood the length of the firing cycle would diminish. This was not the case however, it either remained around twenty two hours or got even longer, simply out of choice. Up to a point, the longer the firing cycle the better. Speaking purely in terms of efficiency, I could fire the kiln in fifteen hours using a single cord of wood. Why then would I fire for twenty four hours using two cords of wood and lose twice as much sweat in the process? Initially I was obsessed with the effect of ash. I still am, but that is not all. Between 920oC. and 1250oC. one is merely setting things up and getting the clay body into a state where it is technically 'correct'. There is plenty of ash on the pots but it has not fused, and has only settled where it will not fall off. Between 1250oC. and 1300oC. the surface of the clay body becomes sticky and specks of ash will adhere and fuse spontaneously. It is also at this point that the flame is at its most dynamic because the clay surface can more readily respond. Thus it is inevitable that at some point a long soak is desirable. 1 found it best to soak short of maximum temperature rather than soaking for a long period at maximum temperature, I did use some glazes, and this tended to produce effects concurrent with overfiring.
Initially I would reach 1300oC. and then carry on firing, soaking the kiln with filthy kiln atmospheres until such a time as I felt it right to finish, or physical limitations made it a necessity. This inevitably resulted in dull colouring of both clay and glaze, and those glazes high in alkali and iron ran too far, resulting in a distinctly overfired appearance. Hence I decided to soak forty degrees short of top temperature for up to six hours, varying the kiln atmosphere considerably. This is to promote stark variations in body colour on the pots, a method of firing similar to the Japanese technique known as higawari-that of changing fire. I would then fire slowly with a clean atmosphere up to 1300oC. This resulted in a much improved firing cycle. Colours of both glazes and bodies were much brighter, due to the oxidised end to the firing, and because the end was slow high clay, high feldspar glazes were not adversely affected (because of their viscous nature, a rapid rise usually results in excessive crawling).
|Old stoneware clays mixed with 10% china clay
grog and feldspar sand
Nepheline syenite 20
|High lime, high iron
|High clay, high feldspar
Most clay bodies low in iron will flash well. Very
pleasing dry flashing can be obtained with aluminous
bodies, a siliceous body will promote shiny flashing. I
have pots solely from HVAR ballclay, which with the
combination of ash and fluxed surface look as though
they have been salted. A good shino type glaze will have
a temperature tolerance of anything up to one hundred
degrees although it won't look the same at either end of
the scale. Ash glazes high in flux are a lot more sensitive
than this and thus have a rather more critical
Advantages of using a pyrometer are few, cones are a much better idea. A pyrometer is beneficial when learning to fire, it is easy to see whether temperature is rising or falling, and subsequently to discover why. Every kiln is individual, and the only way to determine the right way to fire it is through experience, experimentation and some calculated guesswork. A pyrometer may reduce a lot of this work, for it is easy to see from it the results. What a particular action provokes.
The pyrometer is however not only sensitive to overall kiln temperature, but also to flame temperature. The needle is fairly stable up to eleven hundred after which it will rise and fall like a yo-yo. A hot oxidising flame will send the needle rocketing thirty degrees or more, while a relatively cold, yet violent reducing flame will cause it to fall. It is so easy to become glued to the needle to the point of neurosis. With a little experience it is easy enough to see and feel the rise in accumulated heat. There will come a point where the pyrometer can be thrown away and senses relied on entirely, for with a little practice they are far more sensitive than that wretched figment of technology.
I found it necessary to start heavy reduction quite early on, around 920oC. This could then be eased off after 1100oC., decreasing up to 1260oC. To start much later than 920oC. resulted in pale, anaemic looking pots. An hours soak at 800oC. would ensure that all carbon was burnt off, thus preventing any tendency to bloat through carbon trapping later on. The kiln would always stick for two hours or so around ten fifty for no apparent reason. There are two factors which may account for this. At some point the kiln structure itself must reach a certain temperature for any further internal atmospheric temperature to take place. Secondly, the kiln is in the ground, and apart from being a poor insulator, ground holds a large percentage of water. This must be driven back to quite a considerable depth before any further temperature rise is attainable. It is this water content of the kiln that gives the pots qualities that I believe cannot be achieved in any conventional kiln. The simple fact that the kiln is dug out of the ground will leave a distinct mark of individuality upon the wares. This is usually borne out by subtle colour variations of the clay body tending towards pinks. I am also convinced that such stark contrasts of colour in the clay body of any one pot cannot be achieved with such effect in a conventiuonal kiln, without the addition of some artificial means.
As a final note on technique, when I resolved to wood fire, one reason for doing so was a growing awareness of the overpowering nature of technological gadgetry. I wanted to forget knobs, dials, gauges and complicated firing cycles. However, unbeknown I was immediately plunging into a self imposed system of pre planned analysis and calculation, albeit without the added sophistication of industrial elements. Instead of a precision made venturi to control air intake, an old house brick is used to plug up a hole between two more old bricks. The principle is exactly the same, however it is the latter method that is more in keeping with the processes of the craft. If it were not for recording each firing by means of an analytic graphing system, reaching conclusions on how best to fire the kiln would have taken much longer. Whether the method was true to the means I am still undecided upon. Compromises always have to be made, it is important that they are always seen as temporary and eliminated one by one.
As potters we have a moral duty to provide an education, something we have learned, to the undiscerning eye of the public at large, for they are so easily influenced by the wrong things. The question one then might raise is, how do we convince them that wood fired; coarse textured pots with ash all over them and running or crawling glazes are good, and they should buy them? The answer is that we don't. It is not just a case of making money, it is a case of making pots. I make pots because it is my job and I enjoy making them. Pots made, my job is done, it is then up to others to decide upon them by whatever criteria they choose to use. However, as far as the maker is concerned they must be made in a manner true to his own beliefs, and hopefully struggling towards his own astronomically sacrifices and effort, with all sorts of categorical high standards. One can go to all sorts of lengths and attitudes, but at the end of it all one has to judge everything on the pots, that is what it is all about. The look at, rarely do they consider the processes by which pots are the final statement, it is the pots that people they have been made. The craftsman has to provide his contribution, no matter what magnitude he feels it represents, and thus bring a little joy into someone's home, and hopefully at the same time make a little profit from love.
Patrick Sargent kiln design
Anagama kiln shape
Unglazed jar, fired to 1300oC, glazed by flame and ash
18" shino glazed plate
10" dish, shino glazed, ashing from wood
Unglazed jam jar
Unglazed 5" bottle, ashing has run down side, flecking from feldspar particles
18" unglazed jar, flashing from firing caused by touching other pots
15" unglazed bottle, circles caused by bowls placed on pot which was fired on its side
8" shino glazed teapot, fired to 1330oC
6" shino glazed bowl, wax resist, chrome green dots, ashing