Hot Clay Firing Advice
Here is a basic guide to firing with some suggested programs.
Before you put your wares in a kiln, you will have dried them to what is called greenware. The best way to dry newly made pottery, is to put it in a dry and well ventilated place. Do not leave them in sunlight. When fully dried and hard to the touch they are ready to go into the kiln, but at a molecular level they still contain a lot of water.
Hopefully you know what kind of clay you have used, but if you are unsure, you should contact your supplier and they should advise. It will have a name or a code, but what you will need to know is, is it earthenware (low firing) or stoneware (high firing)? For each there is a different firing program, and to make it more complicated, your wares will have an effect on the firing you do.
There are usually two processes to firing, the first is bisque - this will remove all moisture, it will harden, shrink and become a porous ceramic. It is then ready to be handled for decoration and glazing. The second is the glaze firing. This is at a higher temperature. The ceramic surface will melt with the glaze and the two surfaces become one.
In a bisque firing you heat up the kiln slowly, this gives time for the water molecules to escape without stressing the clay. By 600°C all the moisture has gone and thereafter it becomes a chemical change. Your wares will shrink because of the moisture leaving the clay, but expand at certain points due to the molecules undergoing dramatic changes. With earthenware bodies you tend to fire to higher bisque temperatures (1000°C - 1120°C) and follow with a lower glaze firing (1020°C - 1060°C). With stoneware you can fire the clay at a low bisque (950°C - 1000°C) and follow with a higher glaze firing (1220°C - 1280°C).
But firing an intricate figurine is different to firing a hand built bowl. The differences between the size, shapes, whether the clay has grog* or how full the kiln is, should be factored in to a firing program too.
A slow bisque fire should be done on clays that are dense and smooth, like white earthenware throwing bodies or porcelain, as they will lose water at slower rate. Delicate wares such as sculptures with areas of varying thickness like delicate fingers on a hand, compared to a thicker torso, will undergo severe stress. With shapes that are large, flat and wide, like fruit bowls or big tiles, these would become very hot close to the elements but have cooler centres in the middle of the kiln. Fully loaded kilns with many wares on many shelves will affect the atmosphere in the kiln. it will take a longer for the moisture to leave the kiln.
A faster bisque fire can be done on grogged crank bodies as they are much more open in structure and water will escape quicker. Simple pots with even thicknesses can be fired with a faster climb in temperature, slipcast ware when properly dried can be fired quicker as they are thin and even.
For Ventilation, if you have venting holes leave them open until passed 600°C then close the bottom intake and top or side exit.**
With Glaze firings there is some moisture inherent in the glaze, but with both earthenware and stoneware bodies the climb can be a little quicker to 600°C, and steeper once more to the target temperature. Again, take into account the kind of wares and other factors as above. 'Soak times', which is the period of time that a temperature is held for, helps mature the glaze.
These firing programs are guidance only and not to be taken as definitive. Modern kilns can differ from older kilns and have a lot more insulation, therefore hold heat for longer and can overwork glazes. You can adjust for this by removing the soak time or taking down the target temperature. Firing is a temperature and time calculation which together make 'heat-work'.
*Grog - A universal term for additives put into a clay body, These can be sand, crushed firebrick or molochite.
**Nabertherm top loaders have no bung in the side. Skutt’s small firebox kilns have no venting.
We hope this provides a very basic beginning that helps you with a new kiln. If you have any more questions, please feel free to contact us either by email on [email protected] or telephone 01934 863040.