SOME NOTES ON CLAYS AND FIRING

All our clay bodies are in plastic form and most sold in 12.5Kg bags, apart from paper clay whicch comes in 5Kg bags and Southern ice which comes in 9.25Kg bags. When stored in a cool but frost free enviroment, free from draughts and direct sunlight, the clays will retain their condition indefinitely.

Gaze suitability/Firing temperatures:

We recomend biscuit firing at 1000-1060 deg.C followed by a glaze firing at a higher temperature which matures both the clay and the glaze at the same time.

Each clay and each glaze is listed with its firing range. When selecting a glaze for use with a particular clay it is important to select one with a firing range above the lower temperature given for the clay. If this is not done the clay will be under fired and the glaze may craze.

Alternatively you could biscuit fire the clay above the lower figure given for the clay then glaze fire to the firing range of the selected glaze, however this method could cause difficulty with glaze pick - up when dipping altough this can be overcome by adding Magnesium  Sulphate to the glaze which assists suspension and application to relatively vitreous biscuit ware.

Firing rate:

The optimum rate depends on the thickness of the pots and clay type e.g thin cast and grogged bodies can be fired quicker than thrown and and ungrogged ones. Ideally,biscuit firings would be slower than 100 deg.C per hour up to200 deg.C and 100-200 deg.C per hour beyond that point and ending in a short soak.

Glaze firings should be at 100 deg.C -130 deg.C per hour but at 100 deg.C per hour maximum from 700 - 1100 deg.C with high carbon ball clay or fireclay based bodies.If clays are fired  above their vitrification point then bloating is likely.

Bloating may also result from insufficient biscuit firing or too fast firing in the later stages of glost firing.Over- long firings may give excess heat work and cause bloating.