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All in the Ash


There are very few firing environments that are capable of developing the types of atmospheres necessary for the generation of such striking clay body colour response, than those generated in a single chamber woodfired cross draft kiln. In the following, I would like to analyse how the combination of fuel and atmospheres develop varying colour and surface responses, ultimately culminating in a characteristic palette of surfaces specifically ascribable to, and unique to, this type of protracted wood firing.


 I fire two anagama kilns, roughly the same size (approximately 100cu/ft). Both have grates in the floor of the firebox, have no side stoke holes in the chamber and are fired to between Orton cone 13 and 15 from the front firebox alone, over roughly 55 hrs. The main difference between the two is that one of the kilns has a traditional anagama style stepped packing space and the other has a sunken firebox and flat packing space. These different configurations of packing space alter the way that the chimneys affect the pull through the back of the kiln. The stepped kiln having a narrower back to that of the flat stacking kiln and therefore a more concentrated pull. The same fuel is used to fire both kilns. Scott’s pine and Norway spruce slab off cuts predominantly, combined with walnut or Oak. Pine generically has relatively high levels of both Calcium and potassium. The hard wood, contributes higher calcium and lower potassium. The hard wood also offers another firing control by burning slower and maintaining the ember bed.   


It is widely acknowledged that many elements are carried through the kiln chamber and therefore the work, during a firing, and that these come into play within varying temperature ranges. Firstly, and the most visible of these, is the ash that is produced. It may be argued that there are 2 periods within a firing that produce different qualities of ash. In the early stages the fire on the grate is small and the kiln chamber is still comparatively cool, therefore the draw from the chimney is less. This creates larger ash particles, which will tend to drift through the kiln in a gentle fashion being deposited on the horizontal areas of work. Mixed with this slow ash is carbon which takes the form of smoke produced in the early stages.


 The build up of large particulate ash and carbon can be viewed through the fire mouths. Visible carbon is burned off the surface of work between 600 -700C, however the larger ash appears to persist up until around 1100C. Once the temperature climbs above this at the front of the kiln the pull from the chimney starts to increase and the flame path through the work becomes more horizontal. The ash that is liberated by the fire appears to become finer due to more turbulent movement from combustion which may break up the larger particles of ash and also through changes in the chemical composition which will be illustrated later. From this temperature and above although there is to a degree a continuation of ash which travels up and is dropped further back in the kiln there is also a significant movement of finer ash on to the surfaces of work orientated towards the fire.


 As well as visible ash, elements that are carried in the flame there are also fine alkalies of potassium and soda and small amounts of other minerals as chlorides and sulphates. These alkaline materials are present as free elements, and are also disassociated from the courser ash at higher temperatures (Hence the visibly finer ash, its mass reduced by this disassociation of volatile alkalies). Anyone who has put their hand into a bucket of unwashed ash will attest to its alkalinity, evident by the burning sensation felt and also by the soapy feel of the water. The elements that are reactive with regards to flame carried materials as opposed to ash bourn, are those materials that become volatile at lower temperatures.


In a study published in Biomass and Bio energy Vol. 4 No.2 in 1993 the authors studied the decomposition of wood ash (Including oak and pine), in the context of bio fuel boilers. In this context the build up of alkalies on the walls of bio fuel boilers is a problem, as it reduces the heat transference potential. However, this information framed by extended wood firing practice provides a start to understanding the way that the fuel that we use accumulates and reacts with our ceramic surfaces.


The authors postulate that ash formed at lower temperatures (600C) is high in calcium and potassium and that as the temperature is increased up to 1300C the mass of the ash decreases due mainly to the dissociation of potassium salts and the decomposition of both potassium and calcium carbonates.


This would suggest that one of the most crucial periods in a firing for the utilization of free alkalies is between roughly 600C and 1000C. This is the temperature range at which the kiln would normally start to edge into its first cycle of reduction, consequently promoting the fluxing and reaction of fine iron in the clay bodies. It is also clearly an important period for the production of flame born potassium that will also begin to react with the ceramic surfaces. Pots which receive high levels of ‘early firing’ ash which has been produced at a lower temperature will acquire a fly ash patina which is higher in calcium (in ceramic terms used as a secondary flux), this calcium rich fly ash will at higher temperatures also begin to loose its remaining combined potassium. It would therefore follow that fuel that is introduced into the fire box of an anagama at temperatures exceeding 1300C would form a much finer ash than that created earlier on in the firing due to this rapid disassociation of potassium and calcium elements from the ash bulk.


If this hypothesis is followed through, one would anticipate a visual and textural difference between the ash build up and body patina of pots at the front of the kiln and those further down the chamber. Front pots may receive proportionally more of the ash produced at higher temperatures and therefore be more calcium rich. With calcium rich glaze one would anticipate that it would have a drier, more crystalline quality, and the more thorough the dissociation of primary fluxing potassium, then the more crystalline these calcium rich surfaces would become. Ash deposits on pots further back in the chamber which have taken longer to heat up will have lost proportionally less potassium and also gained from the free potassium dissociated from the ash further forward, so arguably will possess a glassier fly ash deposit.


Other elements that become very important in the quality of fly ash glaze are the metal oxides, which are also delivered by the deposits of ash. The predominant metal being iron, although small amounts of manganese, zinc, copper and magnesium are also present. It is the reduced iron oxide which will give the fly ash its’ characteristic green colour and when mixed with small amounts of phosphorus also present in ash (more so in the hardwood ash), can give the ash glaze a ‘chun’ quality of light refracted blue. It must be remembered that in high temperature reduction atmospheres, iron in particular is a vigorous flux and will therefore aid in the movement of naturally accumulated fly ash glaze over the work.


It can be deduced that, as kiln design has a significant indirect effect on the colour generation from clays through variances in micro climates generated within the chamber. Also, the time scale of climb in temperature and the wood burnt will have huge influence. Due to the design of an anagama type kiln the back will always take significantly longer than the front to climb in temperature and therefore create quite diverse surfaces and complex actions and reactions in these different zones.


Having discussed some of the basic chemistry behind the actualisation of clay surfaces, the question arises, how does one create these circumstances within the kiln chamber over a finite period of time? Many people fire for far longer than 3 days, but when firing this type of kiln the activity has to be fitted into daily life and when also included in the time frame are 2 days of fuel preparation and 2 days of packing it is hard for me to find periods of longer than 3 days in which firing can take place. So, therefore a set of parameters is set. Investigation then has to be carried out to tease the results out of this time frame, through subtle coercion of the kiln using the controls available.


An intimate knowledge of the kilns and how they respond in any given situation can only be achieved over many firings. However, a theoretical knowledge of the aims can form the basis of an intuitive form of firing where attention is paid to detailed aspects of the kiln. The rate of fuel being burnt, the degree of smoke and back pressure and the way that the flame is seen to move through the pack. All of these will give clues to what is needed next. If an understanding of what is possible is achieved, then there is no reason why the work coming out at the end shouldn’t exceed expectations, creating new spheres of understanding and appreciation both of the forms and of the process that they have undergone. This information then forms the parameters for the next firing.




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