In about 1980 I arranged for about 80 tonnes of clay from Pahiatua in the north Wairarapa, to be dumped on my property close to a small shed I had converted for use as a clay processing area.
Ever since I have been slowly using up this clay by combining it with other New Zealand clays to produce a body that is good for making and for its performance in use. Quite early on I built a shed over the clay so it didn’t wash away.
The seam in Pahiatua is, in parts, 2 metres deep and contains clay varying from quite a dark red/brown to pure white. The red/brown colour is caused by iron oxide. Where the clay is white this iron has been washed out over time.
For stoneware, white clay is preferable since iron lowers the melting temperature of the clay to below the 1300c temperature at which stoneware glazes mature. Red clay is usually used for earthenware or terracotta which is fired to around 1100c.
Pahiatua clay comprises about 60% of the mix in the clay I use. Ingredients are added to increase the firing temperature so that the pots will not deform at even above the firing temperature. (The clay and glaze both actually melt in the firing to form ‘stone’ in the firing but if it is too hot the pot can melt to the point of collapse).
Other clays are added to improve the pots’ ability to handle sudden changes of heat (heat shock), to augment its throwing characteristics, and slightly reduce its shrinkage in drying and firing. These ingredients also affect the colour of the clay when fired.
Some of the raw Pahiatua clay is reduced to a powder by being ground in a hammer mill then stored in bags ready for use. This is a summer job since the clay must be completely dry before grinding.
I also soak dry uncrushed clay in water which is then sieved to make a ‘slip’ which is stored in barrels so that as excess water can be drained off to concentrate the clay content so it becomes thicker. This is another job that needs to be done sometime before making the clay so the slip has time to settle.
The other ingredients are either ground to a fine powder or added wet. I combine the wet and dry ingredients so that the clay is about the right consistency to use when it comes out of the mixer.
I designed and built the clay mixer about 30 years ago and it has produced all the clay I have used ever since. It has two blades and two wires which cut and move the clay slowly until all the ingredients are properly mixed together. It is powered by a single phase motor which is geared down 60 to 1 to do the enormously hard work of mixing around 75 kilos of clay at one time.
When mixed, the rotating blades are lifted out of the bin and I pack the clay by hand into plastic bags for storage. It is best to ‘make’ the clay at least a few months before use because it becomes more plastic (for ease of throwing) and stronger as it matures wet in storage.