Controlled Reduced Cooling 5

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After convincing ourselves that the cooling regime could improve our results overall, without doing too much harm to the better pots coming out of the anagama, we tried applying it. Fortunately we had a plentiful supply of the decorative ginger  growing in our garden, because we needed a lot. The damper of our kiln did not properly seal the kiln, and even blocking off the top  of the chimney still allowed a lot of leaks, so it was hard to keep the kiln in reduction between 1100 and 900 on the way down. About 30 cubic feet of ginger was stuffed into the firebox, and in the end we took to opening the stoking door and heaving in bundles of the stuff, just to keep the oxy probe reading heavy reduction. 

When we reached 900 on the way down, we stopped stuffing, and waited until  the  probe started to indicate oxidation. Then we tottered off to bed in or usual state of post-anagama exhaustion, confident that cooling would be slow enough to conform to the proposed cooling plan. The next morning I cautiously peeped into the firebox and was appalled to see all the pots covered with black carbon. Two thirds of the pots in the kiln were Carol's, and to return to the house to wake her with the news that all the pots were ruined risked grave domestic repercussions.  A period of desperate thought turned up the possibility that when the oxy probe registered oxidation at 900 there was still a great mass of unburnt ginger in the firebox, not yet burning but willing to do so after a period of drying out. If this finally burnt it would have done so under extreme reduction, possibly at temperatures below  which carbon burns off, thus yielding carbon covered pots. So I was able to wake Carol and nonchalently announce that there had been a slight change of plan and that we were going up to top temperature again in order to repeat the controlled cooling cycle. Although Carol found this difficult to believe she was too tired to resist, and after overcoming the incredulity of our two visiting  stokers we did as proposed. The second time we made sure all the ginger had burnt away before closing up. The results were encouraging enough to cause us to use the controlled cooling in subsequent firings.

Above is a graph of the cooling cycle as applied to our anagama. The kiln has relatively thin walls, fibre lid sections, and no buttressing and so did not hold much heat. While this meant that the period of ginger stuffing was relatively short (or would have been, had we not done it twice) it also meant that the cooling between 900 and 800 took less than the planned 4 hours.  The firing of the kiln prior to starting cooling is not the topic of this article, but the general aim was lots of reduction. We did not use Final Stage Reduction, and have not used it since then. In later firings we took more notice of the period between 900 and 800 and if necessary introduced a gas burner to slow this part of the cooling while maintaining oxidation. Sometimes, depending on state of the coal bed at the start of cooling, the cooling is slow enough not to need the gas burner.

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Use of the gas burner is convenient but but repugnant to dedicated wood firers so we are adopting the more sensible procedure of increasing the thermal mass of the kiln. In the image at left my nephew Andrew Rosser, who regularly helps with the stoking, is making a start by adding more bricks to the base of the sides of the kiln. We have not yet added enough to be certain that the gas burner will not be needed.