Chapter Five
FIRING THE STOVE LINERS
Firing is the process of baking sun-dried clay into pottery. During firing, the clay structure changes and it
becomes a brittle substance which will no longer dissolve in water. Most clays change colour during firing.
Stove liners can be fired in a traditional potter's bonfire or pit. However, it is important to remember that
because the walls of the liner are thicker than most traditional pottery products, they need to be fired for a
longer period than pots. The experience of firing Upesi liners in Kenya has shown that if temperatures of
600-700°C are reached and held for two to three hours, the liners will be adequately fired. The total firing
time is, of course, much longer as it includes the heating and cooling time.
Fig. 5.1 A traditional potter's bonfire pit
While you can never totally prevent pots or liners from cracking during firing, you can minimize these
losses. It is realistic to expect a few liners to crack. If fewer than 10 per cent of the liners crack during
firing, then you are on the right track. Liners will crack if the temperature in the kiln rises too fast as the
firing begins. There are several possible reasons for this. If any remaining water in the clay reaches boiling
point too quickly, it gets trapped, expanding suddenly as it becomes steam, and cracks the liner. The clay
itself also expands as it is heated. If this happens too fast, the liner may crack.
Cracking will also occur if the temperature in the kiln drops too fast during cooling. This is because the
pottery will contract suddenly and crack. Liners may also crack if heated unevenly. This could happen if
the temperature rises too fast on one side of the bonfire, or if the bonfire cools down unevenly due to wind
or draught.
Experienced potters should be familiar with what causes cracks and how to prevent them. Trial and error
will help you to understand the causes of cracking and how to remedy the situation. Chapter 9 gives you
some practical advice on what to do if your liners crack during firing. Bear in mind, however, that every clay
is different, and you will need to experiment with your clay to discover the best way of firing your stoves.
The most important points to remember in order to improve the firing are:
• slow the heating stage of the firing
• slow the cooling stage of the firing
• make sure that the heat during firing is evenly distributed
• maintain temperatures of 600-700°C for at least two to three hours.
Stove liners have thick walls and you should be careful that they are properly fired. Partly fired liners may
look all right, but will crumble when they get wet or after a short period of use.
If you hold a liner with one hand and tap it with the other, or using a short stick, you should hear it ring. An
under-fired liner will sound dull. Traditional potters will be familiar with this process as it is used to check
pots.