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The reason I asked is that around 500-600 there is a big change in the expansion rates of different materials and they're all different. In addition clay undergoes the quartz inversion change. Although we usually sneak in under this temperature range, when we fire the centre of the fire will be hotter. I once killed a whole kiln full of pots in a wood fired kiln by taking this temperature range too fast. The pots were full of little cracks. Wood firing is hard to control and I think that is probably what killed your sand/clay sample. Castable refractory manufacturers state that the stuff is unstable in this range and recommend a rise of only 50c/hr from memory.it is possible to fire pottery by gently heating it and pushing it gradually closer to the fire and finally pushing it right it, but expect more than 50% failure rate even when done really carefully. Hope this helps.
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