Reduced Cooling 

Article by Owen Rye

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I see two main approaches to reduction cooling. I think of them as Bizen style cooling and Shigaraki/Shino style cooling although both may have nothing in common with what happens in reality in those places/styles. And each is different with each different person who does it.

The ‘Bizen’ approach is used with high iron clays. John Neely and Dan Murphy are some of the best users of this technique. They teach at Utah State University where I recently did a residency and used their firing techniques. They have experimented with a wide range of high iron materials. Using laterite as a slip seemed to give the strongest reds – a rich red being the major aim of this technique – although some parts of the kiln are oxidised to give a rather nicer chocolate colour than straight oxidised cooling; and the neutral atmosphere iron purples happen sometimes in out of the way parts of the kiln.

In this technique they have a very ‘tight’ kiln (John Neely’s train kiln design) and near the end of the heating phase all open joints, cracks etc are sealed over with fireclay. The damper is almost shut and the kiln stoked with small pieces of wood (like two pieces an inch square and a foot long) through small openings in the base of the firebox. This is enough to keep the kiln in reduction. The stoking goes on for a long time (12 – 18 hours would not be unusual) until the temperature has dropped to around 800⁰C. The rate of cooling is the natural cooling rate of the kiln.

A variation on this, which has been used by various Australian woodfirers, still using high iron clays, is to stoke charcoal in among the pots when the kiln has cooled to around 800⁰C. This gives swirly flame markings on the clay surface. Phil Cornelius used this technique with porcelain although I think at a higher temperature, around 1100⁰C.

The ‘other’ common reduced cooling approach is used with low-iron clay bodies in order to produce red/pink/orange/apricot flashing colours on exposed areas of clay body; and my experience suggests that similar cooling cycles are also relevant to producing the pink/red colours of Shino and related glazes. The common factor is composition: soda felspar (potash felspar tends to produces oranges rather than pinks and reds), a high alumina content and low silica, and a low iron content – as a guess half a percent or less. Increasing silica kills the colour. The alumina should be combined in other materials (kaolin and soda felspar are the usual suspects). It is not effective if added as pure alumina or as the hydrate. I am interested in experimenting with other sources of alumina but since that’s in the future I won’t give any hints here. So don’t ask.

In this style of cooling the reduction is again started at peak temperature – or as soon as cooling commences is a better way of saying it. The reduction is continued until the temperature has dropped to – and here everyone has a different idea, which may in reality reflect use of different materials, but in the range 1150 to 1000⁰C. For me the temperature is around 1050- 1080. The kiln is then oxidised strongly. This is the kind of reduced cooling that Arthur is doing with his ginger (ginger????- why not something hallucinogenic – or at least incense-like?).

Over the years I have tried all kinds of approaches to the cooling phase – including all kinds of wood, oil, various ways of introducing water etc. Sorry to say, no ginger. I am now happy to do it all as simply as possible. So the cooling starts by sealing the kiln, shutting down the draft so the work does not get a coating of spiky unmelted ash, and stoking with green wood. The best results come when I (or my nominee) cut the wood straight from the tree and stoke it straight in, like some kind of primitive sacrifice. I tend to use pine because that’s what I have but in workshop firings hardwoods have worked just as well. That to me says that the kind of wood does not matter as long as it has a good water and possibly resin content ie green wood. I fill the firebox. If the chimney is smoky, throwing small amounts of water in on the wood gets rid of the smoke (yes, the water gas reaction. If you do not know what the water gas reaction is then you have no business firing wood kilns. Study it).

I then watch the oxyprobe and as soon as the reduction begins to drop I either introduce water or stoke more green wood. The cooling in my kiln takes around 4 hours to reach the changeover temperature. I then leave the kiln alone until it starts to oxidise. I worked out years ago that is you just go off and start drinking booze and think the firing is finished, the kiln will climb all the way back to its original temperatures as the fuels burns away in oxidation. Meaning, an oxidised cooling. So – I watch the pyrometer (or preferably get someone else who can be trusted to do this while I start drinking). Every time the temperature starts to rise I throw in water, to keep the kiln cooling in oxidation and avoid any temperature rise. If you are now confused about the role of water – its increases reduction where carbon is present, and heavy reduction; it increases oxidation where carbon is absent and the atmosphere is oxidising – above, of course, the temperature where water dissociates. There is a very good discussion of this in Kwi-Rak Choung’s master thesis. Again, study the water gas reaction.

What happens then depends on your kiln but usually if the temperature gets down around 900⁰C you can seal it up (leaving about a two square inch air opening in the front and go and start your partying.

The most extreme example I have ever seen of the effect of heavy reduction all the way down, with the kiln totally sealed was in 1992 where Tony Nankervis cooled that way and then opened the anagama at the Lismore woodfire conference. Everything was black. Right through, not just on the surface. Tony was embarrassed but it was a very useful experiment – said what not to do.

The effect of reduction cooling on glazes is an area that would reward experiment- it has for me. I am mightily puzzled about why people who fire gas kilns don’t do some of this experimenting. I saw an exhibition in Washington DC in the early 1970s of earthenware temperature glazed pots by Otto and Gertrude Natzler that had been oxidation heated (electric kiln) and reduction cooled by stoking a solid fuel of some kind into the kiln. They had some of the best glaze effects I have seen. Alan Peascod did a lot of work on cooling in all kinds of atmospheres – he told me once he ordered a cylinder of every gas CIG sold at the time, apart from any they thought might explode, and injected these into cooling kilns. There is a whole unexplored world out there for anyone willing to put in the time and trouble – no doubt many of the results will be bad but you never know until you try it.

My work with glazes in the anagama, reduction cooled as described above, has led to some very pleasantly matted surface qualities in some glazes. High iron glazes should be avoided, they tend to crystallise with sharp crystals sticking out everywhere to lacerate the unwary. Other glaze additives like tin oxide do similar things. The reduction cooling seems to enhance blues such as the Chun/Jun type.

Just for fun I’ve thought about doing a fully oxidised firing followed by a reduced coolingbut have never done it.