Before putting wares into a kiln, they need to be dry. The best way to dry clay is to leave it in a well-ventilated room. Avoid strong sunlight, as that can cause uneven drying. It’s tempting to put it into a kiln and let it do the work, but this will wear out the elements faster and use more energy to heat the kiln.
Hopefully, you know what kind of clay you’ve used. If you’re unsure, contact your supplier. Your clay will have a name or code, but what you really need to know is whether it’s earthenware (low firing) or stoneware (high firing). Each can require a different firing, and to make things more complicated, your wares will have an influence.
There are usually two stages to fire. The first is biscuit firing—this removes all moisture, hardens the clay, causes shrinkage, and turns it into a porous ceramic. At this point, it’s ready for decoration and glazing. The second stage is the glaze firing, which is usually done at a higher temperature.
*Note: “Biscuit” and “Bisque” are terms you’ll often hear. Today, they generally refer to the same process, though historically “bisque” meant a higher first firing and a lower glaze firing.
During a biscuit/bisque firing, the first part is heated at a slow rate. This allows water molecules to escape without stressing the ware. By 600°C, all moisture is gone, and from then on, chemical/physical changes occur. Traditionally, earthenware bodies can be fired to higher bisque temperatures (1000°C–1140°C) followed by a lower glaze firing (1020°C–1060°C). Stoneware can be fired at a lower bisque (800°C–1000°C) and then a higher glaze firing (1180°C–1280°C).
Some refractory bodies, such as school clays, may be labelled as stoneware but can be treated like earthenware and fired accordingly.
The example temperatures above give you an idea of the temperature window, but the next step is learning how to judge the speed of the climb.
Your Wares
Different wares require different rates of temperature increase. Firing an intricate figurine is very different from firing a hand-built bowl. Consider size, shape, thickness, whether the clay contains grog, how full the kiln is, and even what your kiln can realistically do before creating a firing program.
How to Judge the Speed of Your Firing
A slow bisque fire is best for dense, smooth clays like white earthenware throwing bodies or porcelain, as they release water more slowly. Delicate wares, such as sculptures with varying thicknesses—like thin fingers compared to a thicker torso—are prone to stress. Large, flat shapes like fruit bowls or big tiles can become very hot near the elements while remaining cooler in the centre. Fully loaded kilns with many shelves will also slow moisture removal.
A faster bisque fire works for grogged crank bodies, which have a more open structure and allow water to escape quickly. Simple pots with even thicknesses can be fired faster, and slip-cast wares, when properly dried, can also be fired quickly because they are thin and uniform.
For ventilation, if your kiln has venting holes, leave them open until you pass 600°C, then close the bottom intake and top or side exit*.
*Please note: Nabertherm kilns do not have bungs on the air outlet.
For glaze firings, there is some moisture in the glaze, but with both earthenware and stoneware bodies, the climb can be quicker to 600°C and steeper afterwards to the target temperature. Again, consider the type of wares and other factors mentioned above. “Soak” or “hold” times—periods when the temperature is maintained—help mature the glaze.
*Once again, close the vent at or above 600°C.
Sample Firing Programs
Here are some firing programs we use when testing glazes and producing photographic props:
Universal Program for Beginners (Slow Biscuit): A slow, steady climb suitable for both earthenware and stoneware bodies. This minimises stress and is ideal for those new to kiln firing.
Cone Explanation: Cones measure ‘heat-work,’ which combines temperature and time. The last 100°C before the top temperature is critical. Cones use 15°C/hour (slow), 60°C/hour (medium), and 150°C/hour (fast) as guides. You can program these rates into your controller to hit an exact cone value. Read more about cones here.
Kiln Limitations: Not all kilns can achieve the programmed speeds. Factors like size, power, element condition, and load will affect performance. Most kilns slow near 1200°C, which can lead to more heat-work than expected. To understand your kiln, try a test firing: load medium-weighted wares, ideally things that don’t matter, set to full power climb, (step) on Nabertherm and fire to 1280°C. Record temperatures hourly (every 15 minutes above 1150°C) and record on an x and y graph. This will give guidance as to what you can realistically program into the kiln.
Drop and Hold Technique: Sometimes used to cure pinholes and blisters in glazes. This involves dropping the temperature slightly and holding it for a period. Learn more here: Drop and Soak Firing.
If you wish to fire to higher temperatures or cone values, simply adjust the final segment accordingly.
To learn more about firing and your kiln, join our online class here.
We hope this basic guide gives you confidence to move beyond default programs and gain greater control over your kiln.
For questions, contact us at info@hot-clay.co.uk or call 01934 863040 (option 3).
