by Rik Midgley
In ceramics schools today, we are taught to bisque our work before the final glost firing, often without actually questioning the necessity of the process. The main reason for this appears to be that work made by night class potters and students is very variable in quality, some of it being rather fragile. This work is then handled by a second person, and the glazer may be inexperienced or the work simply too delicate to be wetted. Hence it is considered safer to bisque fire. However, before the modern age of the studio potter, once firing was the norm, with ceramics being twice fired only for specific reasons. The change came about in industrial times, when high bisque was used prior to a glost technique that required a relatively low temperature. This also had the effect of making the body an inert passive background, which at the time was considered beneficial.
When I set up my studio in 2000, I was determined that all my ceramics would be once fired, although I was completely ignorant of the process. In this article I discuss the peculiarities of once firing, and how I have overcome some of the problems.
ADVANTAGES OF ONCE FIRING
· The conservation
· Time and effort saved in not having to pack, fire and unpack a bisque kiln
· Saving on shelf space, no bisque pots waiting for the next glost firing – a bonus if you workshop is small
· I like the immediacy of the process – making and glazing becomes a single process potentially leading to more integrated work
· Your most recent work may be your best, and it is that which you are glazing and seeing out of the kiln.
· A high interaction between glaze and body, assuming this is required.
· Clay is fragile
before firing and mishandling may lead to damage
· Fragile delicate work may be damaged if glazed green
· Glazing errors are harder to rectify, washing off can damage surface texture
· The fine coating of glaze particles impedes the escape of gasses during firing, so that burn off periods for both water and carbon/sulphur are marginally extended
· Too rapid firing in the early stages can cause breaks and explosions that can damage the kiln or other pots.
Most clays used for bisque firing should be good for once firing. A clay body should be opened with 10-20% grog or sand, or mixed size. This helps the clay stand the shock of being wetted. It will also aid the passage of water vapour and impurities as they escape during firing. It is often recommended to use clays low in ball clay. Ball clay’s small particle size can impede the passage of water vapour through the body, and it also often has a high carbon and sulphur content. The firing problems here are no different than in a bisque firing – a slightly lower rise in temperature in the first 300ºc of the firing whilst the water evaporates, along with a longer soak between 900 - 1000ºc to burn off the carbon dioxide and sulphur dioxide.
GLAZES FOR BONE DRY GLAZING
The process of glazing green ware is some times referred to as raw glazing, although this is synonymous with the industrial technique of using unrefined or ‘raw’ ingredients in glaze. I decided to generally bone dry glaze, rather than glaze at the leather hard stage, simply because it is easier to let all the pots dry and then to glaze them in a batch. Pots are somewhat stronger at the leather hard stage, but the glazes must be designed differently, and generally include at least 20/30% clay so that the glaze shrinkage matches that of the pot. This restriction was unacceptable to me, although I sometimes ‘leather hard’ glaze if I require a high clay glaze.
· A maximum of
about 15% clay is generally required to avoid the glaze flaking off as it
dries. If a glaze with high alumina content is required, a maximum of 20%
china clay may be used, but only if it is brushed or prayed on. A glaze with
such a high clay content will also hold a large volume of water if the work
is thin walled this may lead to over-wetting and a collapse. Glazes with higher
clay content are known as slip glazes and are applied at the leather hard
· A minimum of 5% clay, to give the glaze itself some strength in the green stage, which minimises chipping from the rims. Ball clay should be chosen over china clay if the percentage is this low, since it has the higher shrinkage.
· Additions of around 3% bentonite help the glaze fit onto the green ware. Since bentonite is chemically similar to feldspar, this addition usually makes little difference to the glaze.
· When glazing with a second layer of glaze, it is better that the upper glaze has a lower clay content than the under glaze, to minimise flaking at the rims.
A little more care must be taken when applying glaze to green ware, as it does not have the strength of bisque ware. However once you have broken a rim or two or have been left with the foot of a bowl in your hand, you quickly learn the limitations. Some potters actually alter the forms they make in order to make them easier to handle, such as adding a foot ring to a bowl or a foot to a jug. However this is generally unnecessary since different glazing techniques may be employed to get around these problems.
If a pot is easily held at the base, glaze may be applied by dipping or pouring. I glaze my mugs by simply lacing them upside-down on a try, and pour glaze over them, thus avoiding any thumb marks. Big bowls with no foot ring may be placed on a couple of canes and then glaze poured over, touching up later.
However procedures like this must be done fairly rapidly or else there is a risk of the rims starting to dissolve. The processes of spraying and brushing on of glaze differ very little form that with bisque.
I have found that over-wetting
can be a problem. This is exacerbated by the fact that the throwing clay I
currently use is high in ball clay, and consequently has a high expansion
on rewetting. After watching a row of bowls slowly break up, I tend to be
rather cautious, and the following procedure has evolved.
· First the concave surface is glazed i.e. the inside of the jug or bowl. Experience shows that this choice reduces the chance of cracking, but I’m not sure why.
· The pots are left to dry for some hours, or over night if the atmosphere is damp.
· A second glaze layer can be added if required, followed by another drying off period
· Finally the outside of the pot is glazed.
If a pot is badly thrown,
in that it is uneven in thickness, then the thinner parts will expand more
rapidly than the wider, and cracking may follow. If an uneven thickness is
desired, such as in fluted pots, then this is potentially a problem, although
the risk is reduced if the flutes are vertical and thus perpendicular to the
throwing stress. The best way to ensure that an area remains unglazed is wax
it. I collect old candle buts and melt them down with twice the volume of
paraffin in a small tin placed in a saucepan of simmering water. This is cheap,
easy to apply and dries quickly. Any glaze stubbornly remaining on the waxed
area can be sponged off.
There is some scope in raw glazing for creating effects that cannot be achieved with bisque fired work. After an initial glazing, a leather pot may be deformed, turned, or cut, thus integrating texture and glaze.
PACKING AND FIRING
After glazing, pots should be left over night to dry before packing into the kiln; the pots are more fragile when recently wetted. Pots can be tightly packed when once firing since they have much of their shrinking still to do I have heard of some firers who allow their pots to touch. I use aluminium hydroxide to dust my shelves, the powder apparently acts as rollers allowing the pots to contract with out cracking.
The firing cycle is essentially the same s normal bisque firing, followed immediately by the end part of the glost firing. Thus, the firing time and fuel for firing to the first 1000ºc is saved. Without compromising the quality of my work in any way. I like to minimise the firing time so as not to waste both fuel and time. With this in mind I have developed the following routine for my 16 cubic foot propane fired kiln.
I begin firing with a least an hour at just under 100ºc with the door slightly ajar, to ensure that the pots are thoroughly dried. The escaped of gasses from the pots during firing may be retarded at the surface by the fine glaze particles. This is to some degree balanced out by the more open clay bodies used in once firing which facilitate the passage of these gasses. As with bisque firing, you quickly learn when firing is too fast.
An initial temperature rise of 100ºc per hour should be slow enough to avoid water blowouts. To improve the draw I directly warm the flue for half an hour. Also, the height of the flue can be increased by temporarily adding an old chimney pot or a length of pipe. I find that the heavy foot rings of large bowls are prone to cracking at this stage. I avoid this by placing the bowls on shards of clay, broken from a thin rolled out sheet, thus allowing air to circulate under the bowl.
After the flue clears of steam between 300-400ºc it is now partially damped off, so that the kiln runs under near neutral conditions, at which it is most efficient. The temperature rise is now up to 200ºc / hour.
The sulphur and carbon begin to burn off at temperatures over 800ºc and so the flue is opened fully to ease their escape. This burn off must be completed by the time either clay or glaze vitrification begins; else gasses are trapped leading to bloating. Since the rate of burn off increases with temperature, I continue the rapid temperature rise until 950-1000ºc, when I soak for 1 hour. Following this, I proceed as with a normal glost. I reduce hard for 2.5 hours up to cone 8 down. At this stage the glazes are molten and cannot be penetrated by gasses, so that more reduction will have little effect. I reduce the power, and back off the flue near neutral conditions. And soak for 1 hour to avoid blistering and under firing in cool spots. The whole process tends to take about 13 hours, and I’ll get 3 firings from two 46 kg bottles of propane.
has recently published a technical article in Ceramic review
Understanding Stoneware Glazes
March 04 issue 206