Q Why do our liners crack and break during firing?
A If your liners have small cracks or defects before firing, they are likely to break during firing. So,
remember to do the quality check before firing and to remove any stoves with cracks or other defects.
If you have small stones or air bubbles in the clay mixture, these will cause the stoves to crack when
they are fired. Look carefully at the broken surfaces of a few cracked stoves. You may be able to see
small stones or places where an air bubble was trapped. To prevent this, sort the clay well and remove
any stones. Wedge the clay well to remove any air bubbles.
If your stoves are still damp when they go into the kiln or fire, they will crack as the moisture
evaporates and expands. Dry the stoves well before firing. If possible put the stoves in the sun to dry
out for the last few days or into a warm kiln immediately after the last firing and leave them to dry.
If your firing process is too fast or too hot, the stoves will crack. Slow down the firing process by
heating up the kiln slowly. Uneven distribution of heat or cold draughts in your fire or kiln will cause
cracking. If stoves in one area of the fire are broken, this could be the cause. A damp kiln or firing pit
may cause your stoves to crack.
Built-in stresses in the clay can also cause cracks during firing. For example, if a wet stove is
mishandled and then pushed back into shape, it may crack during firing. Certain design features such
as sharp corners also cause stress. Therefore, avoid sharp corners around doors or where the pot-rests
join.
You could also check if the liners have cracks along the joints. Try scratch and slip.
Q My fired liners tend to crumble, especially when they get wet. Why?
A It is likely that your stoves have not been fired enough. They may need to be fired at a higher
temperature for a longer period. Remember that the stoves have much thicker wall sections than most
traditional pottery. If the clay you are using is not used to make other pottery products, it is possible
that it does not have sufficient pure clay in it to fire properly. Try making a thin-walled pot and firing it
to see if it cracks or crumbles.
Q My fired liners are "blue" in colour and have small cracks on the surface.
Why?
A You are over-firing your liners. Either your fire is too hot or you are firing the stoves for too long. Use
less fuel once the kiln or fire is hot. Alternatively, you could reduce the air flow into the fire in order to
control it.
Q Why do my stoves have small cracks around the pot-rests?
A Either the clay you are using for the pot-rests is too wet, or you are using too much water when you
attach the pot-rests.
Q Why do the pot-rests break off from my stoves during firing?
A Either air is being trapped in the joint between the pot-rest and the stove wall, or the joint is not
being made properly. Use the same clay for the pot-rests as that used for the stove body. Make a
strong joint by scratching both surfaces and applying slip. Try to ensure that the clay from the two
surfaces really combines.