Rik Midgley - Ceramics - Cob Ovens - NVC - Qigong
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 generally safer to bisque fire. However, before the modern age of the studio potter, once firing was more the norm, with ceramics being twice fired only for specific reasons. The change came about in industrial times, when a 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.
1. The conservation of fuel – there is no need to repeat the first 900-1000C
of the bisque firing.
2. The time and effort saved to not have to pack, fire and unpack a bisque kiln. Somehow opening a bisque kiln is not quite as exciting as cracking a glost.
3. No shelf space is required for bisque pots waiting for the next glost firing – a real bonus if your workshop is small.
4. I there are a few pots short of a full kiln, they can be made, dried by the fire, and glazed in just a few days.
5. I like the immediacy of the process - making and glazing become a single process potentially leading to more integrated work.
6. Your most recent work may be your best, and it is that which you are glazing and seeing out of the kiln – much more satisfying than having to wait for a bisque kiln to fill, and then finding space in a glost.
7. A high interaction between glaze and body, assuming this is required!
DISSADVANTAGES OF ONCE FIRING.
1. Clay is fragile before firing, and mishandling may lead to damage. I have
had to teach myself good handling techniques, and after an initial period, breakages
2. Fragile delicate work may be damaged if glazed green. With some types of work bisqueing is unavoidable.
3. Glazing errors are harder to rectify – washing off risks damage by over wetting and can remove surface texture. However, minor errors may be sponged off with negligible detrimental effect.
4. The fine coating of glaze particle impedes the escape of gasses from the body during firing, so that the burn off periods for both water and carbon/sulphur are marginally extended.
5. If, as a result of too rapid firing in the early stages, a fat piece of pot breaks off or explodes, then the glazed fragments may damage the kiln or other pots. If a real disaster occurs, such as a shelf collapsing, then the risk is the same as with a glost firing.
Most clay used for bisque firing should be good for once firing. A clay body should be opened with 10-20% grog or sand, of 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 bisque firing – a slightly slower rise in temperature in the first 300C of the firing whilst the water evaporates, along with a longer soak between 900-1000C to burn off the carbon dioxide and sulphur dioxide.
GLAZES FOR BONE DRY GLAZING.
The process of glazing green ware is sometimes referred to as raw glazing, although this is synonymous with the industrial technique of using unrefined or ‘raw’ ingredients in a glaze. I decided to generally bone dry glaze, rather than glaze at the leather hard stage, simply because it is easier 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.
1. A maximum of about 15% clay is generally advised to avoid the glaze flaking
off as it dries. If a glaze with high aluminium content is required a maximum
of 20% china clay may be used, but only if it is brushed or sprayed 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 of the pot.
Glazes with higher clay content are known as slip glazes and are applied at
the leather hard stage of drying.
2. A minimum of 5% clay, to give the glaze itself some strength in the green stage, which minimises chipping from rims. Ball clay should be chosen over china clay if the percentage is this low, since it has the higher shrinkage. It is also lower in aluminium, which is presumably a requirement of a low clay glaze.
3. 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.
4. 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 since 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 in much the same way as with a bisque pot. I glaze my mugs by simply placing them upside-down on a tray, 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, with later touches up. 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 from that with bisque.
I have found over-wetting can be a problem. This is exasperated 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 a jug or a bowl. Experience shows that this choice reduces the chances of cracking, but I’m not sure why. The pots are then left to dry for some hours, or over night if the atmosphere is damp. Next, a second glaze layer may then 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 a fluted pot, 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 to wax it. I collect my old candle butts and melt them down with twice the volume of paraffin in a small tin placed in a saucepan of simmering water. This mix 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 glaze 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, a pot should be left over night to dry before packing into the kiln. Apart from unnecessarily introducing a lot of water 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 once firers who allow their pots to touch. I use aluminium hydroxide to dust my shelves, the powder apparently acts as rollers enabling the pots to contract without cracking,
The firing cycle is essentially the same as normal bisque firing, followed immediately by the end part of the glost firing. Thus, the firing time and fuel for firing to the first ~1000C 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 firing routine for firing my 16 cubic foot propane fired kiln.
I begin the firing with at least an hour at just under 100C, with the door slightly ajar to ensure that the pots are thoroughly dried. The escape 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 firing bisque, you quickly learn when firing is too fast.
An initial temperature rise of 100C/hour should be slow enough to avoid water blowouts. To improve the draw at this stage of the firing, 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 flexi pipe. I find that the heavy rims 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.
The flue clears of steam between 300-400C. The flue 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 200C/hour.
The sulphur and carbon begin to burn off at temperatures over 800C, and so the flue is opened full 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 950-1000C, 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 to near neutral conditions, and soak for 1 hour to avoid
blistering and underfiring in cool spots. The whole process tends to take about
13 hours, and I’ll get 3 firings from two 46kg bottles of propane.