A pottery firing changes the plastic clay body irreversibly into a hard ceramic body. Bisque fires to about 950 -1000° C . 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, 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. And 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;

  • What kiln firing is It? Is it Earthenware, Mid-fire, or Stoneware and Porcelain?

  • Are there some glazes which perform better at the bottom, at the top, and so on?

  • How much pottery, volume of material, is in the kiln. That is, the density of the packing. Generally kilns work better with a light packing at the bottom. For reduction firings pack very closely.

  • Are there firing cones set and are they visible through the port?

  • What is the condition of the kiln and its capacity to reach temperature in good time. Older kilns with old electric elements take longer.

  • The amount of available time. There may be set time limits for the use of the kiln, due to a buildings opening hours, community restrictions, or fire regulations.

  • The fuel and resources available. You don't want to run out of gas.

  • Do you know how to operate the various instruments and apparatus, as well as adjusting the burners and flues, to fine tune the firing, for reduction, or not for reduction, and for temperature?

  • Your timetable. A kiln can not be left unattended nor can the responsibility be casually undefined leaving no one in attendance. Any other commitments need to be reorganized.

Roughly we can say the Bisque firing is 60°C per hour up to about 1000°C. The Glost firing is 60°C per hour until past 100°C, and then 100-150°C per hour.

Generally the firing procedure is straight forward. Increase the fuel via the burner, or turn up the electric controllers. A digital kiln controller can automatically fire an electric kiln and is now very common. But the settings in the controller need to reflect these same considerations. 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. So 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 center. 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 right into 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, sealing the outside, before any expulsions have had time to get out, and trapping volatile materials inside. Such an uneven and impure body is structurally weak.

This is all from a technical standpoint. There is much about ceramics which has a beauty, a liveliness, perhaps achieved pragmatically, not knowing or applying anything about all this. But to avoid, cracks, crazing, underdeveloped glazes, glazes running onto shelves, clay over-fired and brittle, there are fundamentals worth knowing. 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.  The molecular changes in the clay will take the heat energy and there will be less of a temperature increase. Between 500-600°C, don't panic and think you need to turn up the kiln. After the quartz inversion, the firing naturally returns to temperature increase. By wrongly turning up the firing, the changes will be interrupted. 

  • 700-800° C. Carbon and sulphur are burnt off and a strong smell is evident.

  • 900° C. Clay begins to vitrify.

  • For Reduction Glost firing, begin reduction. Strong reduction evident by flickering orange/red flame.

  • 1000° C. Bisque firing end point.

  • For Glost firing continue.  The maturity of the clay and glaze is what determines the end point.  For example, Earthenware might be 1100° C, Midfire 1180° C, Stoneware 1260° C, and Porcelain 1300° C

  • For Reduction Glost firing

    1100-1300° C. Variable reduction, light/medium, depending on time table.

  • Soak or Hold; For each level of maturity there  is a similar range of about 50°C to soak .

    # temperature rising:  e.g. Before maturity. 1150 - 1180° C, 1250-1300° C. Slow the rate of increase, and take 1/2 to 1 hour to gradually get to end.

    # temperature drop: e.g.  After high point. 1180-1150° C, 1300-1250° C.  When the end cone is just tipping, turn kiln off, and let drop 30° C to 50° C.  Then hold at that lower temperature for about 1/2 hour.  You can also creep up to towards the mature temperature, but watch the cone, as the heat work can be done even though the temperature reading is below the top temperature.  

    #Note. Do not start a Hold or Soak at the mature temperature or when the cone is down, because the continued heat work will be an overfire.

  • At end of firing switch off or turn off gas supply. Close all kiln exit ports and flues.

  • Further possibilities:

  • down - 1100-1000° C. Oxidising soak in the cooling cycle to improve glazes. 

  • down - 250-200° C. Crystabolite inversion on cooling. Do not open kiln door or vents.

  • Under 200° C , you can open kiln very slightly to hasten cooling.  Don't unload until not hot to touch. 

    The ordinary impulse in a kiln firing is to clamp the kiln, that is to block all the exits and outlets such as the holes in the top of an electric kiln, or the flue in a gas kiln. The idea I suppose is to keep the heat in the kiln. In the early stages of the kiln firing this is a mistake. As the kiln heats up it is giving off steam and is burning off materials, producing smoke and other gases. These gases have to be vented otherwise they will affect the clay and glazes. Perhaps most importantly is when sulphur is burnt off starting at about 700°C it mixes with water vapour to produce sulphuric acid and this attacks the metals. You can see the corrosion around the doors on many older kilns. This is completely unnecessary. Leave a top port open until above 800°C, or more.

    Although 1300° C is often given to be an ideal high temperature firing, the preference for firing to 1300° C is not really a standard, and in a lot of places it is 1250° C. It is said this preference arose from the historical search many centuries ago in Europe looking for the secret of Chinese Porcelain. The required high temperatures were unknown at that time in Europe and it was a quest to develop the skills and kilns to get higher and higher temperatures. Today this quest is pointless, and clay and glazes are easily developed to mature at 1250° C, saving time, effort and energy.  

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