The Kiln Firing Process
What happens in a firing in a kiln?
A pottery firing changes the plastic clay body irreversibly into a hard ceramic body. Bisque fires to about 950-1000° centigrade. It still has some porosity and so will absorb water. Over 1000°C the body will start to become vitrified, glassy, and impervious to water. Usually, for ease of handling the pottery, there are two firings, first the bisque, also called biscuit, and then after the pots have been glazed, the glost or glaze firing. To change the plastic clay pots, or greenware, into a bisque, heat is applied by electricity or by burning a fuel such as gas. The rate of temperature rise is modified by the ability of the clay to withstand the increase of temperature; also by the need for the clay to be properly developed at the molecular level to provide structural strength, resistance to wear, and hygienic use. To achieve this, you need to be aware of the progress of the firing, taking in to account the following;
– How much pottery, volume of material, is in the kiln. That is, the density of the packing.
– What is the condition of the kiln and its capacity to reach a high temperature in good time.
– The amount of available time.
– Your deadlines.
The firing procedure is straight forward. Increase the fuel via the burner or turn up the electric controllers. The overall purpose is to give the clay and glaze a strong, beautiful finish. A fast rate of increase, a quick firing, is possible, but this leads to many problems, such as cracking of the ware, glaze imperfections, and poor body strength which shortens the life of the ceramic. We need to consider the reasons for adjusting the various controls and why a controlled temperature rise is necessary.
Think of a lump of clay. The heat has to penetrate deep within the lump evenly through to the centre. During this time the changes to the clay have been taking place gradually from the outside in. The release of water and other combustion gases, the burning out of carbonaceous matter, is not complete unless the heat is given time to penetrate the clay. At the same time the heat is making molecular changes to the clay. Too much heat too fast and surface molecular changes will take place before any expulsions have had time to get out, becoming trapped inside. Such an uneven and impure body is structurally weak. The major points of transition are;
100°C. Water boils and turns into steam.
200°C. Vegetable matter, such as grass and paper, combust into smoke.
300-500°C. Chemically combined water is released.
573°C. Quartz inversion in both heating and cooling.
700-800°C. Carbon and sulphur are burnt off and a strong smell is evident.
800°C. For reduction glost firing, begin light to medium reduction.
900°C. Clay begins to vitrify.
980°C. Bisque firing end point.
1000-1250°C. Variable reduction, light/medium.
1250-1300°C. Reduction soak in the heating cycle to improve stoneware glazes.+
1300-1200°C. Oxidising soak in the cooling cycle to improve stoneware glazes.* +
1100-1000°C. Oxidising soak in the cooling cycle to improve glazes. (Reduction firing.)*
250-200°C. Crystabolite inversion on cooling. Do not open kiln door suddenly/too quick.
* a matter of preference.
+ for any other level of maturity it is a similar 50-100°C range.
KILN FIRING FAQs.
Q.When can I fire my pots?
A. A pot that is too wet is liable to crack, even explode. Let them dry to the touch before you put them in the kiln. Consider whether a pot that is very thick, or is not finished well, really needs to be fired. Perhaps it is better to recycle the clay and make another pot.
Q. What temperature do I fire to?
A. The first firing, a bisque (biscuit), is between 900-1000°C. The second firing, a glost or glaze firing, depends upon what temperature the clay and glaze mature. Earthenware is about 1100°C, Midfire about 1150°C, and Stoneware and Porcelain, 1300°C. To make plain, unglazed, earthenware cooking pots, fire to full maturity of the clay, e.g., 1100-1200°C, and this can be done in one firing as long as all the protocols for a bisque are followed.
Q. What is a bisque?
A. This is a first firing of greenware in the kiln to produce ceramic that is hard, but not fully fused, or vitrified. While the body retains some porosity, the clay has become ceramic and is not reversible. This firing makes the pots easy to handle and easier to apply glaze and other decoration. Bisque with no further treatment, can be kept as an ornament, painted with acrylic, or used plain in the garden. But it is too soft and porous for use with food and fluids.
Q. What is a Glost or Glaze firing?
A. This is the second firing following application of glaze material to a bisqued body. The higher temperature will melt the glaze material and harden the ceramic body. A glaze becomes fluid and can run down a pot and stick to the kiln shelf. Remember to thoroughly wipe the glaze off of the bottom, or the foot, of the pot. Wipe a little from the edge corner of the base as well. A fired pot that shows glaze flaws can often be re-glazed, or touched up, and re-fired.
Q. Do I need to bisque?
A. A bisqued pot is not affected by wetting and so applying glaze to bisque is easier and safer. The glaze recipes found in books, and the mixed glazes in buckets, refer to glazes to be dipped or sprayed onto bisque, unless specifically stated as once-fired or green-ware glazes. It is possible to apply suitably prepared glaze to green, unfired wares, skipping the bisque stage, and firing only once to a high temperature. Since the green clay softens when wet these are a little harder to handle and special care is necessary when glazing, loading the kiln, and firing. Another method used commercially is to fire a high bisque, say 1250°C, and then low fire with a low temperature glaze, say 900°C.
Q. What glaze can I use?
A. A glaze is usually tested to mature at a certain temperature/cone number, to fit a particular clay body, and to fire under certain conditions. This information is always shown clearly on the label or by reference to technical information. This should not restrict trying any prepared glaze in a different way, straight from a bucket, as long as the range of the glaze properties are not severely over stretched. An earthenware body, apart from an earthenware glaze, could be fired to the midfire, or low stoneware range. A stoneware body could be fired, apart from a stoneware glaze, at the lower range of an earthenware and midfire. The main point is not to fire earthenware to high stoneware because of the danger of the glaze running onto the shelf, if not the earthenware body slumping, nor to underfire a stoneware glaze and have no glass formed. An earthenware glaze can have a range from say 1100-1200°C and a stoneware glaze from 1150-1300°C.
Q. What cone do I need?
A. An electric kiln often has an automatic thermostatic cut-out and/or a cone sitting in an automatic cut-out, as well as there being a timer. For a bisque use cone #08(955°C) or #07(984°C). Otherwise set a stand of cones in line with the spy hole as follows. For any glaze, enamel, lustre, firing, the glaze has been formulated to mature at a set temperature range, and hence certain cones are needed. It is wise to set three cones, one on either side of the maturing cone, e.g., as in the case of stoneware, #7, #8, #9. Make a small raft of clay and stand them up, low to high, left to right, with the cone markings facing the front. They should lean slightly to the left. Place the cone setting in line with the spy hole. For a more thorough record of the heat work in other parts of the kiln, cone settings can be placed anywhere on shelves. The cones bend and show maturity when fully bending with the tip arching over to touch base. The cones being inside the kiln are affected by the heat, and so show the heat work on the ceramic material. Sometimes there is a discrepancy with the temperature probe and meter which measures only the local ambient temperature, not heat work. If there is a Kiln-Sitter or other automatic timer, pyrometric cut-out, these will also need to be set. Where you have set cones to be visible from the spy hole, the kiln-sitter cone can be omitted.
Q. What kiln can I use?
A. An electric kiln is best for bisque and for oxidation firings. The electric elements are, however, likely to be working at a reduced capacity with age, and so high temperatures become difficult to achieve. For that reason, stoneware oxidation firings, other than bisque, are best done in a gas kiln. For a reduction firing a gas (or, oil, wood) firing kiln is best. Since there is no flame in an electric kiln there is no immediate way to create reducing conditions. It is possible to artificially cause reduction in an electric kiln but this is detrimental to the electric elements or to the soft bricks where there is this kind of insulation. For all kilns, once you have begun firing, place a sign indicating the kiln is operating, warning of the danger, and avoiding interference.
Q. On an electric kiln what temperature controller settings do I use?
A. Each kiln varies according to size, type of construction, age of elements. Work carefully and systematically. Try the lower settings and observe the change in temperature over time. Record the temperature on a time chart. Make adjustments according to the rate of rise. Keep in mind the desired end temperature and the time it will take. There are two main types of kiln, 1. Electric and 2. Gas.
1.1. An electric kiln has a regulator knob usually with an indicator dial showing 1-5, Low-High, or 0-100%. The adjustment varies the time the power to the elements is on and off. So there is a click ON- sounds of operation- then OFF- silence (at low this maybe for a minute), then click ON again, and so on. It is a good idea when first turning the kiln on, to stand around to hear the first sounds of operation, because often something needs to be set correctly for this to happen. 2.1. A gas kiln has burners with a non-calibrated adjusting tap. The adjustment increases the amount of gas flow and hence the strength of the flame. However, at first, use only the pilot light, or pilot burner, lit by holding in the press-button. Light the main burner later. When there are a number of burners, these can be lit in stages. This gives more control when a low temperature preheating is required. On the burners, there are adjustable flanges, for regulating the primary air. At start up is best to have them closed to prevent a draught of air extinguishing the flame. Once the initial warm-up has passed, and use of the main burner has begun, after a few minutes open the primary air as much as can be done without a flame-out. Since a closed primary air is also the setting for reduction, this step of re-opening the primary air should not be neglected, even though getting the flame right can be fiddly. Each kiln is different depending on size, age of electric elements, type of construction (brick, fibre), design of kiln. Work carefully and systematically. Try the lower settings and observe the changes in temperature according to the expected rate of rise.
Q. In an electric kiln when do I plug the holes?
A. There are a number of holes in an electric kiln. Usually one or two on top, one or two at the front bottom, one at the front middle, and one at mid side. The plugs that are used to block the holes are called bungs. The hole at the side is where a pyrometer probe can be inserted. The one at mid front is the spy hole (peep hole) to check the cones. One has to set the cones to be visible from here. Leave this closed or the cold air will affect the cones. The two at front bottom are for air flow in the early stages. Close these during the firing, say at 600°C. Later, cooling to about 200°C these can be removed prior to opening the door. The two at top are for the smoke and gases to escape and can be left open at all times unless in the late stages it is preferred to retain heat. After firing, close them so as not to cool down too quickly. Then on cooling to about 200°C these can be removed prior to opening the door.
Q. What is the rate of rise?
A. As the temperature rises from minute to minute, hour to hour, as shown on the pyrometer, the change can be charted and the increase over time can be calculated. For a bisque the recommended rate is 60°C per hour. For a glaze firing 100°C is a good average. During the firing even if a faulty pyrometer is not giving an accurate reading of the actual temperature in the kiln, it still indicates a change of temperature and is very useful for that purpose. There are critical points in the development of the ceramic body and glaze and as long as these are given attention, in between the rate of rise can be much higher, 150°C, 200°C, 300°C per hour. If the kiln accidentally turns off at some point during a firing it doesn’t hurt to do a fast catch-up within limits. With awareness of the critical points (see following) it is possible, with increased attention, to do a relatively fast firing, but perhaps it is still best to work safely and carefully. A slow firing produces a stronger body. In any case, for the first couple of hours from ambient temperature of about 20°C, slowly pre-heat the loaded kiln up until 100°C, when water is released as steam. The rate of rise should be 30°C per hour. Where possible setting the pots in the hot sun before loading into a kiln saves a lot of time and fuel/energy by shortening the pre-heat. Alternatively the kiln is allowed to fire very slowly, steaming overnight.
Q. What are the critical points in the development of the ceramic body?
A. There are points where some care and attention is required to allow changes to take place. Water boils at 100°C and steam is driven off with force. If we arrive at this point suddenly then both the clay and glaze might be damaged. Allow plenty of time for slow drying, say three hours, up to 100°C. Leave the top bungs out so that the gases can escape. With thicker ware, more time may be necessary. You will notice at about 550-600°C there is a lag in the rate of temperature rise. It is tempting to turn up the controller once again as you have been doing. But at this point (573°C) there is a change called quartz inversion taking place within the molecular structure of the ceramic. The heat energy being applied is now being used for this inner change, and so the temperature rise automatically goes on hold. Allow 15-30 minutes for this to take place, then you should see a resumption of temperature increase. Between 550°C to 850°C strong oxidation is necessary to clean out carbon and convert iron compounds. Above 800°C the clay begins to irreversibly vitrify and so a thorough oxidation needs to have taken place to clean out the clay before then. For some reduction glazes it is important to begin reduction at this point for the same reason that vitrification of the clay will begin to close the process off for the clay.
Q. What is reduction?
A. Normally the atmosphere in a hot kiln is hot air and gases, and is said to be an oxidising atmosphere. Burning fuel requires oxygen, and when this is all consumed in the burning, the lack of oxygen is called a reducing atmosphere. Reduction brings about colour changes to many of the stoneware glazes. It converts the metallic oxides when the chemical compounds break down and lose their oxygen radical, e.g., green copper oxide becomes the metal, red copper. To achieve this, starve the burners of fresh air, by adjusting the primary air control, and also starve the atmosphere in the kiln of air by closing any openings, adjusting the flue, and the burn will go deeper into the clay and the glaze in search of oxygen. You will see a more yellow/orange colour in the flame and in the kiln. The atmosphere will be cloudy. A little sooty smoke should be visible coming out of the flue. A flickering flame searching for oxygen should be visible from any small exit holes. The temperature at which you begin reduction depends on the type of glaze. In the case of Celadon glaze start reduction at 800°C. A light reduction is recommended and only in the last 50°C is heavy reduction needed. Soak as normal and re-oxidise at the end. For Copper Reds, as with Celadon, begin reduction at 800°C, and unlike Celadon reduce heavily throughout. With a Tenmoku Glaze reduction can start later at about 1000°C, and continue with medium to heavy reduction, ending with heavy reduction, a soak, and re-oxidize. As well as there being different techniques for each glaze, the firing regime also depends on the characteristics of the kiln. It is not impossible to put any number of different reduction glazes in the one firing and utilize the differing conditions of the firing atmosphere but without intimate knowledge of the kiln it’s pot luck and some glazes suffer at the expense of others.
Q. Why is it important to be oxidising, not reducing under 1000°C?
A. In an oxidising atmosphere there is sufficient oxygen to combust the carbonaceous matter (paper and plant material) within in the clay, or as used in fabrication. However, in the second glost or glaze firing, having previously bisque fired up to 1000°C in an oxidising atmosphere, this stage has been done once already. For the first stage of a glost firing the main reason is that a reduction flame is very heat inefficient, a waste of fuel, and polluting. Reduction is required only when the glaze is beginning to melt, e.g. over 800°C.
Q. When is a firing complete?
A. For a bisque the kiln sitter/controller may or may not have automatically cut-out. The automatic pyrometric cut-out can also be unreliable, or inaccurate. Where cones have been set, check through the spy hole. The first cone should be flat, the middle cone well bent over and touching base, and the third cone starting to bend. Check the pyrometric temperature reading. For a bisque a temperature around 960-980°C is desirable. Either wait for the cut-outs to operate, or if temperature has been reached turn off the kiln. Do not leave the kiln unattended, relying on the automatic cut-offs. Even when there has been an automatic off, make it a habit to turn all switches to off, or zero, including the wall switch. Make sure the bungs are all in place. For a glost or glaze firing check the cones and the pyrometric temperature reading. Take the reading from the cones more than from the pyrometer. The pyrometer is more useful to give the rate of rise, but in the end the cones give a better indication of what has happened to the glaze. Turn off the electricity/gas, both at the kiln and at switches/valves at the wall. For a gas kiln close the flue/chimney damper, and where possible block any other ports. This will limit the influx of cold air.
Q. What is a soak?
A. In the development of a glaze, better results, smoothing out of imperfections, development of colour, can be done by soaking a glaze. This means holding the temperature for a half hour or so, rather than continuing the normal progress of heating or cooling. This could mean towards the end of the firing, turning down the energy input, and arriving at the maximum temperature very slowly for the last half hour. Or it could mean finding the adjustment to the input where the temperature hovers at one point and holding it there for a half hour. Or, having reached maximum temperature, adjust the input so that the kiln is cooling, but the temperature drops very slowly, say 100°C per hour, for a half hour or more. A soak should be in oxidation, so for a reduction firing the primary air needs to be re-opened. A soak can be extended all the way down to 1000°C, or parts thereof, as this often gives much improved glazes. This varies with each glaze so experiment.
Q. Why did my pot crack?
A. When the clay is too thick it cannot heat, dry, nor shrink evenly and stress develops in the clay. Make pots as thin as possible, remove excess clay from sculptures and fabrications, and allow to dry naturally before the firing. The familiar ‘S’ crack in the bottom is due to throwing technique. While throwing, finish the inside bottom by running the fingers over the base with pressure, from the outside in, toward the centre. Pots often crack during cooling and the most critical point is around 200°C. Do not open the kiln door too quickly.
Q. Why did a piece fall off?
A. Smaller, thinner parts dry more quickly than larger, thicker parts. The stress of heating, drying and shrinking, and gravity, can cause smaller parts to break away. It is sometimes possible to re-join a bisqued piece, depending on the position, with a thin coating of glaze acting as glue in the glaze firing. There are commercial pottery adhesives which can be re-fired. In some cases, it is just as easy to use an ordinary adhesive suitable for ceramic, on glazed pieces, without refiring.