Techniques that we use for producing outdoor frost resistant architectural ceramic sculpture or tiles. 

We use clay that is in a squishy/sticky condition during the beginning stages of creating each clay piece. This ensures that the clay ‘knits' (bonds) together as it dries and does not separate where one piece was pushed against another.

The clay is 40 to 50 percent grog in content for warp and crack resistance. The particles of grog range in size up to at least 1mm.

The initial drying period is slow. The clay pieces are initially covered loosely with sheets of paper or polythene, then dried from 'leather hard' over a period of two to three weeks. The clay is dried more quickly in a warm dry atmosphere once the clay surface has a lighter powdery colour. 

During the initial firing we fire up to 150 degrees centigrade at a rate of between 10 to 15 degrees centigrade per hour increase. This encourages any trapped moisture to escape as water vapour.

For work that is between 20mm to 40mm thick we fire at a rate of between 40 degrees C per hour and 30 degrees C per hour respectively up to top temperature. This surprisingly slow increase in temperature allows every part of each clay piece to be at a consistent temperature and reduces conflicts in expansion between one part of the clay or ceramic body and another.

When firing at these relatively slow rates we look at the range of maturing temperatures as recommended by the supplier for whatever clay we are using and choose a top temperature that is in the middle of this range (Clays that have a large proportion of flux e.g. black clay, may require firing at a lower top temperature ). We soak at this temperature for between one and two hours. The vitrification that occurs will enable adequate frost resistance.We test by placing a drop of water on unglazed ware which should then be repelled and not absorbed. Increased vitrification is required for those pieces that will become absolutely saturated with water and are then exposed to heavy frosts.

When loading a kiln we sit clay or ceramic pieces upon soft high firing clay balls that have been dipped in kiln placing powder. They are placed at 100mm intervals. This gives even support for each piece and allows better ventilation and heat transfer within the kiln's atmosphere.

The fired ware is cooled from top temperature at a rate of not greater than 40 degrees centigrade per hour. The kiln door is opened and the bungs are removed once the kiln has cooled down to below 100 degrees centigrade.

This page was last updated June 2004.