We have a variety of kilns at Morris and James, from the relatively small to several 'walk-in' ones. Although larger than most found in a Studio Pottery, they are really quite small at 9 cubic meters. Over the years they have been through numerous iterations, with gradual changes being introduced to improve performance and longevity. They are not inexpensive to rebuild, and over time the extreme heat breaks down both the ceramic wool lining, fire bricks and metal structure. They require ongoing maintenance and major work every six or so years depending on usage.
Cars have been built to transport the ceramics in and out of the kilns, and around the pottery, and these too have to be sturdy enough not to distort when heated...getting that right took a number of years, but as you can imagine a double deck kiln car with a load of ceramics buckling during firing would be a disaster. There have been a few!
The mix of gas and oxygen is carefully controlled and is maintained at the same ratio during the whole firing cycle. The Matakana clay, taken from our quarry along side the river, is not particularly old, being formed somewhere between 10,000 and 340,000 years ago. In comparison to deposits elsewhere it is quite young, the forming having been accelerated by the copious amounts of Kauri leaf litter. This organic matter created an acidic environment which speed up the conversion of the Waitemata sandstone into clay. Because of this and the abundant other plant debris that accumulated in and over the deposits, we need to heat the clay with flames that have a slight excess of oxygen to ensure that any organic matter is completely and cleanly burnt away.
The kilns are loaded and then heated in a number of carefully controlled stages, to ensure that thermal shock in heating and cooling doesn't damage the pottery. Depending on the clay condition and the various glaze types, car loads are also left to 'soak' at certain temperatures, ensuring that the different sized pots 'cook right through'.
The kilns are always unloaded with a combination of excitement and trepidation, as you never quite know what you'll get!
- Nick C