Open-Firing: Principles and Practice
Adapted from an article first published by Studio Potter Journal, March 2021
Like all the best adventures, my journey with open-firing did not follow a straight or narrow path. It started when, as a ceramics student, I joined an experimental firing group – a wonderful mix of archaeologists and potters. We researched and built many styles of experimental kiln, from Romano-British updrafts to medieval dome-tops, hoping to learn more about the ceramic practices of the past. Of all the technologies we explored, it was the simple open-firing that remained the most puzzling. We found that firing without any heat-retaining structure was undoubtedly exciting and full of uncertainties.
My fascination with historical craft practices led me to research the link between ecosystems and material-based actions, a detour that took me away from my journey with open-firing. It wasn’t until I started to manage twelve acres of woodland for field research that, quite literally, the flame was reignited. I found myself having to deal with so much ‘brash’ from tree management – affectionately called ‘loppings and toppings’ – that I realised I had a virtually unlimited supply of sustainable fuel. So, for the last nine years, I have been back on the trail, hoping to unravel the mysteries of open-firing.
Usually categorized as an unsophisticated process, open-firing is anything but a simple or straightforward practice; it demands a high degree of skill, knowledge, and application. It is perhaps the most intensive and nuanced form of ceramic firing.
Although it is widely accepted that open-firing would have been the earliest form of firing technology, there is little detailed evidence of its use in the archaeological record. By its nature, open-firing is a transitory process, leaving little lasting trace. Fortunately, open-firing is a tradition that has continued unbroken in some parts of the world to the present day. Although now declining, there is a significant body of documented research that can help us understand its salient features and technological principles. Combined with work in experimental archaeology and pottery-fabric analysis, we have a sound foundation on which to build.
Open-firing is an integrated system with little margin for error. Clays need to be carefully sourced and blended. Vessels must be appropriately constructed. Fuel must be selected and processed, and the firing itself demands sustained intervention. Open-firing is essentially a single-firing process and from the outset we should be mindful of two major considerations, as they tend to characterise the whole procedure: efficient heatwork and clay-body failure.
Heatwork is an important concept in ceramics. It is particularly significant in open-firings which are usually rapid. The heat must be transferred to the pot as efficiently as the open setting allows. As far as temperature is concerned, a long-probe pyrometer will tell us that top temperatures will range from 700 to 1100 degrees Celsius [1292 to 2012 degrees Fahrenheit], but a pyrometer will only measure the heat of the hot gasses in the fire’s interior. It is not necessarily a good indication of the heatwork transferred to the pots.
The second consideration – connected to the first – is that, due to the rapidity of the firing and the short firing cycle, any water contained in the pot’s body will have little time to evaporate. At boiling point the water would convert to steam. Even so-called dry pots contain water at a level corresponding to the relative humidity level of the surrounding air. When one considers that the phase change from water to steam represents a volume increase of 1600 times, it is no surprise that pots blow apart; this is ‘spalling and it can be problematic. Pre-heating vessels is often necessary to avoid spalling and can be done separately from the main fire. Critically, the pots need to be slowly taken to well above 100 degrees Celsius [212 degrees Fahrenheit] and then fired as soon as possible. Although it is common practice to edge pots nearer and nearer to an established fire, this can be problematic.
The Architecture of Open-Firing
When my journey with open-firing did resume, it was with a modified bonfire. Quickly it became clear that heaping fuel over dry pots was not the way to go. Significant heat needed to get underneath the pots. If pots lay on the ground or sink early in the firing into a bed of ash, they would not be subject to adequate heatwork. In practice, this means the pots should be set on top of fuel and then additional fuel built around the setting, before finally lighting the fire. The configuration makes pre-heating difficult and totally precludes the ‘edging in’ technique. I have found that by using correctly tempered clay bodies, coupled with very dry pots (warmed but still comfortable to handle), I was able to fire with a high degree of reliability.
Traditional open-firing cultures often fire their pots one at a time and this practice makes a lot of sense. Open-firing is extremely adaptable to the individual vessel and surprisingly fuel efficient. One could also envisage that the potters of pre-history, who provided vessels for family groups or small communities, would fire single pots on an ad-hoc basis: they would have little need to fire pots in significant numbers.
I designed a wood-stake system, which provided an integrated structure to burn renewable coppice material or cleaved wood, supplemented by reed or other fine stem materials. By staking the fuel lightly into the ground, in an evenly-spaced arrangement, an effective air supply is achieved. The vertical alignment of the fuel produces a pronounced ‘chimney effect.’ Once the fire is established, with flames actively feeding on about three-quarters of the fuel, the dry pot is placed carefully on top; the central stakes can be made slightly shorter to help with the pot’s stability. Once underway, the burning fuel can easily be manipulated. Top temperatures are achieved in less than thirty minutes. For experimental work, particularly with trial clay-body formulations, a single-pot firing-system is useful.
Constructing a slower, progressive open-firing is another approach to use when testing new clay bodies. Unlike wood-stake firing, a progressive open-firing can be taken through the early stages of firing as leisurely as one likes.
I have designed and fabricated a firing grate, welded together from mild steel round-bar. Not a unique configuration – I have seen a similar structure used by Pueblo potters. Here, rather than preheating separately or edging pots closer to a fire, I bring the fire to the pots. By having an initial fire slightly to the side of the grate I can shovel glowing embers underneath the setting. This cautious approach to open-firing allows the fire to be built at a rate appropriate for testing unknown clays or clays with modest amounts of tempering. The firing grate has proved effective for well-regulated pre-heating.
Fuel for Open-Firing
Fuel needs to be completely dry. This perhaps, should go without saying, but plant-based fuels are hygroscopic and so, if the relative humidity of the air is high, even so-called dry fuel will absorb moisture. Open-firings therefore, are weather-dependant and can often be a seasonal activity.
Fuel also needs to be of the correct dimensions. Finely cleaved logs or branch-wood will burn quickly, transferring heat to the pots mostly by radiation and convection. Larger sections will create glowing embers, transferring heat through radiation and conduction. All these heat-transfer mechanisms are important, and I have found that a mix of dimensions and fuel types create the best firing conditions. Finer, silica-rich materials like grasses and reeds I have found to be crucial in mitigating the sometimes harsh and fluctuating heat transfer from wood – the high ash residue, which tends to retain its original stem structure, has the effect of conducting and then radiating the heat from the wood to the pots in a tempered and more controlled manner.
Firing atmosphere in open-firing, as with traditional kiln firing, will influence the ceramic body. It is common for open-firing to progress from a strongly reducing atmosphere through to an oxidising atmosphere. The colour of the clay body will therefore react accordingly; from carbon impregnated blacks through to brighter oxidised colours. If I need a dependable carbon black, I will immerse – or partially immerse – the red-hot pots in sawdust or pine shavings at the end of the firing, much like the carbon smoking process used in raku.
I have characterised open-firing as an integrated system and the selection and composition of clay lies at its heart. We have two choices: clays found in the wild or those that are commercially available. Rarely will raw clay on its own be suitable. Most will need to be modified to create a viable clay body. Personally, I like to use clay in the wild, but this is, perhaps, more to do with my work in experimental archaeology than for any practical or aesthetic consideration. Whatever the choice, it should be stressed that clay bodies capable of being rapidly fired are rare compared to those that can be adapted for use in a progressive open-firing.
Whether formulated for rapid or for progressive, open-firing, all raw clays need tempering (the addition of non-plastic materials to the clay body). Tempering regulates clay’s workability. In open-firing, tempering plays a vital part in controlling the clay’s tendency for spalling and also its tendency to dunt during rapid cooling.
Tempering does this by:
- opening the body and allowing gasses to escape
- interrupting developing cracks
- lowering the relative clay mineral content and therefore reducing mechanically combined water
- reducing shrinkage
There are three principle considerations for tempering: choice of tempering material, particle size, and quantity of tempering. Dealing with particle size first – tempering materials are usually classified by mesh size – a familiar convention to potters. From my experience, the largest grain size for a clay intended for hand-building would be around sixteen mesh. At the finer end of the scale it is not unusual to include tempering down to two hundred mesh. What I have found to be important is the inclusion of a varied and evenly distributed grain-size, from the courser fractions, right down to the finest dust. In practice, if the potter is grinding their own tempering materials, it will naturally contain mixed particle sizes.
The amount of tempering largely depends on two interrelated mechanisms: workability and firing behaviour. At the risk of generalising: increasing tempering will improve fire-tolerance, but a point will be reached when workability drops off steeply. Although, to a certain extent, this can be offset by using a higher proportion of the finest tempering. Judging the correct addition of temper is always a question of balance, requiring a good deal of attention and testing.
A wide range of tempering material is available, but the four main choices are: sand, grog, organics, and crushed shells. All have been used extensively through the years, but I would look to grog (ground ceramic material) as my first choice. A brief look at the other materials will explain why.
Sand is a common tempering material owing to its relative abundance and its naturally graded particle profile. It is reliable when used in modest amounts, but in relation to open-firing, we are sometimes dealing with over fifty percent tempering material. Many sands are predominantly composed of silica, and as such, are subject to the quartz inversion. At 573 degrees Celsius [1063 Farenheit] quartz will suddenly increase in size by about .45% and on cooling it will contract. Sand often has rounded grains which have little grip, when combined with the mechanical stresses of the quartz inversion (particularly on the cooling cycle) the grains become detached from the surrounding material. The result is a weak body.
Organic temper (which effectively burns out after firing) has been commonly used with open-fired bodies. In large amounts, however, it creates many post-firing voids and the ceramic fabric may become frail and unusable.
Shell temper (or shell-sand temper) is in many ways an ideal choice for tempering open-fired bodies. It is easy to source and prepare. It exhibits a plate-like particle section that provides workability and strength. Its expansion/ contraction rate is closely matched to the predominant clay minerology. However, being composed almost entirely of calcium carbonate, it can cause problems with ‘lime popping,’ where lime – converted from the calcium carbonate during firing – hydrates and expands. There are strategies for overcoming this, a fact endorsed by traditional practice; but it is a risk!
Grog, made from crushed and graded ceramic material is many ways an ideal choice. It is easy to make and grade. It has a near identical rate of expansion and contraction to the surrounding fabric and so imposes little additional stress. Its sharp, angular structure sits tightly in the surrounding ceramic matrix.
Historically, pigments have been used for open-fired vessels and in many cases pigment was applied post-firing, which ensured bright colours, unmuted by the firing atmosphere.
Pigments applied prior to open-firing are best sourced from natural earth deposits, these tend to not only contain chromophores, responsible for the primary colour, but also accessory minerals such as clays and carbonates. Clays minerals, in particular, help the application and adhesion of pigment and also contribute to the sintering of the pigment layer during firing. Furthermore, pure colouring oxides, such as those obtained commercially, are often inappropriate for burnishing – a technique often employed for open-fired pots – as they remain powdery and subject to smearing. Also, they do not take on the burnished lustre of clay-bearing pigments.
For the contemporary potter, a vast array of commercial stains and colours are available and it is perfectly feasible to formulate synthetic pigments which are colour-stable and adequately sintered at low temperatures. However, for the reasons already outlined, they would not be suitable for burnishing. As a starting point I use this rule of thumb recipe, in parts by weight:
Stain 38 : Low-temperature Frit 48 : China Clay 14
This base recipe has a high percentage of stain, at a level that will give strong saturated colours. The frit regulates sintering and may, with some stains, edge the pigment towards a more glaze-like surface. The china clay is useful in binding the pigment and helps brush application.
One could argue that glazes are totally unnecessary in the context of open-fired pottery. Traditionally, open-fired ceramics were decorated (if decorated at all) with pigments and often after firing. It’s no surprise that glaze technology developed alongside the advancement of kiln technology. Open-firings are often a rough-and-tumble affair and any glaze coatings, which are notoriously fragile before fusion, would be prone to physical damage. Also, the atmosphere in an open-firing, ranges from oxidisation to deep, smoky, reduction, which can have an unpredictable effect on the glaze.
However, glazes can find a place in the contemporary reinterpretation of open-firing, but there are three main problems to overcome:
- Avoiding physical damage to the coating in the hostile physical environment of the firing.
- Avoiding the dulling effect on pigments and glazes of a smoky reduction atmosphere.
- Ensuring that the glaze coatings are fused at the relatively low temperatures of an open-firing.
Glazes can be inlaid into the body, offering a degree of protection, particularly during the early stages of firing when the glaze is friable and delicate. Similarly, the glaze can be used in discreet areas as a decorative medium rather than a protective, waterproof coating.
To formulate glazes which fuse at low temperatures, it is normal to use a low-temperature frit as its basis – similar to those used in raku.
The Future of Open-Firing
Open-firing, despite being regarded by some as an archaic technique, has much potential for interpretation by the contemporary potter. It gives us new creative possibilities through its directness and its ability to integrate with modern ceramic techniques and technologies. In terms of sustainability, open-firing relies on renewable-energy sources and, despite its obvious lack of insulation, energy is not wasted as might be in the prolonged heating of a substantial kiln structure. Open-firing is, by definition, a revealing process which, as an educational tool, has the ability to engage the student with real-time ceramic events – events that might otherwise be hidden in the secret interior of the kiln. It is a ceramic technology that has lasted, unbroken, for thousands of years and will, I am certain, endure as an exciting and stimulating process that uniquely connects us to the ceramic cycle: from clay – through fire – to pot.
Note: Open-firing is potentially dangerous, so I always wear appropriate personal protective equipment and take all the precautions necessary for this type of work.
I’ve included two videos on the video resources page that might be helpful.