Controlled Reduced Cooling 7

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Although the controlled reduced cooling procedure is entrenched in our firing method there is no certainty that it will be beneficial to firings with different clays using different wood. We have not changed the woods we use, our usual procedure being to start the firing with Forest Red Gum and change to Blackwood later in the firing. The Forest Red Gum produces much less ash glaze than the Blackwood, and the deposited ash melts at a higher temperature. We have a suspicion, but only a suspicion, that a number of Australian Eucalypts produce ashes which share these characteristics, and it may be that there are other Acacias which behave like Blackwood.

Our clay bodies have changed over successive firings, and are selected to produce results which suit our tastes. Having had a bad experience with a commercial clay, which changed characteristics without warning, so that we lost the red blush, we now mix our own clay bodies.

I am keen to know what happens if this procedure is used in other kilns, with other clays and other woods. For a direct comparison  the procedure would have to be followed fairly closely, using an oxygen probe and digital pyrometer. The reducing agent could be any moist, fleshy weed.  Possibly water could be used, and if so I would certainly like to know about that. If you try this cooling regime, please let me know what happens. Negative results are just as important as positive ones.

To  see what times might be involved in an anagama of more conventional construction, with good insulation and heavy butressing, so that more heat is retained and the cooling is slower, I have constructed the following diagram from information provided by Len Cook.

The jagged line shows the cooling temperatures in Len's anagama as automatically recorded by  a device supplied by one  of Len's friends. Len uses Final Stage Reduction, and careful examination of the graph reveals that the heavy reduction caused by the Final Stage Reduction procedure lasted for just 1.5 hours, while the temperature dropped to something like 1150 C.  Len lets his kiln cool naturally after Final Stage Reduction, so the grey area in the top half of the graph shows the period during which ginger stuffing WOULD occur if our controlled cooling procedure was adopted.  The pink area of the top half of the graph simply indicates that Len's kiln cools slowly enough to satisfy the four hour period of reoxidation between 900 and 800 that we regard as desirable. There is no obvious reason why Final Stage Reduction and our controlled cooling procedure should not both be used in the same firing.

To make controlled reduced cooling practicable an oxy probe of some sort is essential. The composition of the atmosphere in the cooling anagama can change very rapidly, and constant feedback is essential if you want to be confident about what is going on. Oxy probes are expensive, but the ones without the pyrometer are about half the price of the  original probes with the inbuilt pyrometer thermocouple. To avoid exposing ours to days of ash deposits it is inserted at the start of the cooling cycle, very carefully, over a period of ten minutes. Absolute concentration is essential, but as you can see is sadly lacking in my case here.

The other essential requirement is a damper that can be shut completely, and our damper system is shown in the image below.  It is located just at the back of the anagama chamber, and is accessible by removing loose standard bricks so that when completely closed it can be  further sealed with strips of ceramic fibre.  Messing about with hot bricks and hot kiln shelf dampers, while flames are shooting out any gaps, at a stage of the firing when total exhaustion is near, is quite dangerous.

Bad, but satisfyingly spectacular,  things happen if the dampers are completely closed while there is still fuel burning in the firebox, so there is a need for care,  if only to avoid legal action from your stokers. Obviously the stoking door has to be completely sealed for ginger stuffing to be effective. For a well sealed kiln enough ginger can be introduced through one or both of the secondary air ports. The ginger going in can completely occupy the port so no air goes in with it. With care, the amount of ginger needed for reduction cooling in our kiln can be as little as a few barrow loads.

Some dodgy chemistry

What do I think is going on with this cooling cycle? If we assume that one way or another most of the coloration on these anagama pots is due to iron oxide, and make the further heroic assumption that the iron compouinds which form during the firing behave in a way reminiscent of iron oxide itself during cooling, then it makes a sort of sense. The red form of iron oxide is stable under oxidation below 900 C, and starts to convert to the black form above 900C under reduction. The brown form is a mixture of the red and the black. I have removed pot shards from a kiln at temperatures above 1100C and dunked them in water and found no trace of brown or red.  This suggests that keeping the atmosphere in reduction during cooling down to 900C will tend to suppress the formation of brown colors, and that any colors willing to change at 900 should be given enough time to move as far towards red as possible before becoming unreceptive to change below 800. If you think this explanation is sound chemistry then perhaps I can interest you in some shares in the Sydney Harbour Bridge? They will be available at a very reasonable price as soon as the ink is dry.

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