Postscript about the clamp firing:
A volume of incompletely burned wood and charcoal was left in the clamp, though it must have achieved quite a heat: the pots are definitely fired, as the intact ones ring. The ‘waste’ goes into the next firing, making this a more energy-efficient process than it looks, and a larger clamp would be even more efficient.
Fire marks the ground, creating the reddened earth and scatter of charcoal archaeologists look for. The site of each clamp remains visible for about two years’ worth of gardening, and after that dissipates into the plot.