44 pots of varying sizes were packed tightly into the chamber, tile/amphora sherds (actually roughly made curved slabs of fired clay) were then laid over the pack. On top of this a very coarse mix of mud, gravel and organic material was spread and finally I plastered a mix of sand and clay over the surface, leaving three exit flues at the back of the kiln, through which hot gasses could escape.
I'd made the decision to attempt a black-burnished ware / greyware firing which involves starving the kiln of oxygen towards the end of the firing causing an intensely reducing atmosphere within the chamber. This has the combined effect of pushing carbon deep into the pores of the clay and converting iron oxide within the clay body from its red Ferric form to black Ferrous oxide. In the case of this firing this was achieved by closing down some of the exit flues for the last 200 degrees and, once we had reached the target temperature of 900 degrees, completely sealing the top of the kiln then stoking as much fuel as possible into the firebox and then sealing it closed.
The kiln was lit at 8:30am and we began sealing the kiln at 6pm giving a total firing time of nine and a half hours and an average temperature rise of 95 degrees per hour. Once begun, firing is a continuous process, even being distracted for a few moments can result in a temperature drop. As a 21st Century potter I have the advantage of an thermocouple and pyromenter (High temperature thermometer) my Roman predecessors would have had no such technology at their disposal. Their temperature measuring techniques would have relied on their senses: in the early stages of firing a potter needs to take things very slowly, ensuring that trapped moisture in the clay doesn't blow his/her pots apart, a hand placed over the exit flue will give an indication of temperature and whether the gasses are moist or not; once the kiln is over 600 degrees C the colour of the pots in the chamber, seen through the exit flues will give the potter all the information they need.
I think the results speak for themselves, everything was well coloured by the reduction, the variations from pale grey to black being very much in keeping with Roman originals. If you'd like to own one of the pots from this firing watch the Vindolanda website and my Blog for news, or drop me an email and I'll let you know when they come up for sale.
Visit my website at www.pottedhistory.co.uk