Woodfire Tas 2011: the Nubeena firing
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Immediately before the Woodfire Tas 2011 conference Carol and I ran a firing workshop in a kiln almost identical to the Oztrain. One of the objectives for this firing was to test our Rosser Reduced Cooling procedure using clays, sorts of wood and reducing fuel differing from the ones we use at Eungella. Our conclusion was that the procedure was successful in that context. The kiln and participants of the workshop and the conference can be seen on Conrad Calimpong's album of photos (the images will be larger if you click through them one-by-one rather than activating the slideshow)
Kaowool insulation on top of the kiln glowing at top temperature
|Summary of Rosser Reduced Cooling
The procedure is to execute a normal reduced wood firing and then, during the cooling cycle between 1000C and 900C, maintain a reducing atmosphere. At 900C the atmosphere is allowed to resume oxidation. For large, slow cooling kilns that is all there is to it, but for small kilns the cooling between 900C and 800C should be encouraged to take at least four hours. A convenient way to slow the cooling without slipping back into reduction is to introduce a gas burner, and the cooling during reduction can be ennabled by introducing fleshy vegetative material into the firebox.What it achieves
We think the method diminishes the sort of mid-brown colour which is a common result of firing with wood, while encouraging the development of red flashing on lightly ashed clay bodies without discouraging green fluid ash glaze colours. At its best the mid-brown colours are replaced by very dark brown or black. Importantly, red surface colours on shino-type glazes are also encouraged, or at least not suppressed. (Disclaimer: no colour is intrinsically bad - it depends on the context - but there is no doubt that firing with wood, particularly with Australian Eucalypts, does tend to produce a certain unrelieved mid-brown which Carol and I find undesirable. I am not fond of glib pejorative terms but Jack Troy's characterisation of this colour as "boot polish brown" is apt)
Zoe Rosser and Dave Judge harvesting bulrushes for reduction while cooling. The workshop was held at their kiln, located at Saltwater River, near Nubeena, Tasmania
Variations to the procedure
At Eungella we have recently been firing with Flooded Gum (Eucalyptus grandis) for the initial stages of the firing and then using Blackwood (Acacia melanoxylon) for the later stages. The reducing fuel is always a decorative ginger (Hedychium coronarium), chosen because it grows profusely in our garden and is moist and fleshy. At Saltwater River, not wanting to vary the procedure too radically, and using fuel readily available, we used Stringybark (Eucalyptus obliqua) for the inital stages, followed by mixed Acacias for the later stages. The Acacia wood was roughly 60% Blackwood (Acacia melanoxylon), 30% Silver Wattle (Acacia dealbata) and 10% Black Wattle (Acacia mearnsii). For the reducing fuel we used the fleshy lower parts of plants know locally as Bulrushes, harvested from a local farm dam just before use.
Various clay bodies were used. The bodies we mix and use at Eungella were used as controls, and workshop participants brought pots made from clays commercially available in Australia, either mixed or as bought. Conrad Calimpong brought pots made from clays available in California. Most of the pots shown are the ones made from the clays we use at home because these provide direct evidence that this reduced cooling procedure is useful with wood types, and reduction material, differing from the ones we use at Eungella.