Firing of the flashing tests

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Woodfire flashing on kitchen pots
A collection of pots from our kitchen
One of the advantages of encouraging flashing with the right clay body and cooling cycle combination  is that the undersides of the pots develop flashing readily because  pots fired upright receive less ash on the down sides.  Of course, if flashing is what you value most, the pots can be fired the other way up, or at odd angles, but  if you are pursuing different effects on the exposed sides it is comforting to know that flashing will brighten up the sheltered surfaces without any particular effort. Casual revelation of these colours in the kitchen  is  an enduring source of delight.

Diagram showing the crucial stages of cooling. A period of reduction roughly from 1,000 down to 900 degrees is necessary, followed by at least 4 hours of oxidation between 900 and 800.The temperature readings shown in the graph should not be taken too seriously because they imply a degree of accuracy which simply does not exist.

The tests shown on the Test results page were fired in our Oztrain kiln and subjected to the  Rosser reduced cooling  procedure. The firing took 38.5 hours to the start of cooling and was supervised during cooling for a further 7.5  hours. The wood used was Flooded gum (Eucalyptus grandis) for the first 9 hours, then we changed to Blackwood (Acacia melanoxylon). The up cycle was subjected to heavy reduction, with the usual atmospheric cycle occurring within each stoking period. The ember bed was very deep at the start of cooling and no additional reducing material was needed during the crucial reduction period between 1000 and 900 deg. An LPG gas burner was introduced during the oxidised cooling between 890 and 795 degrees to extend this period of cooling to 4 hours.
An  oxygen probe was employed during cooling to make sure that the kiln atmosphere was as required during the crucial stages. No kiln atmosphere readings were taken during the cooling from top temperature down to 1020 degrees. The kiln atmosphere may have been reducing during this period but in other firings was certainly oxidising and the red/orange flashing results were similar to the ones achieved this time. Quite a lot of embers remained when the kiln reached a reported 795 degrees and supervision of the atmosphere ceased. There would have been air leakage into the kiln for the rest of the cooling as no attempt was made to seal it with slip. Initially at least the atmosphere in the kiln was oxidising as cooling continued below 795 degrees, but it is conceivable that the atmosphere became less oxidising as the ember bed burnt down.

Good for shino glazes and slips
This reduced cooling procedure is not only good for flashing on clay bodies - it is also very kind to shino glazes, probably because the development of red colours on shino glazes and shino-based slips is basically the same process as the development of red/orange flashing on clay bodies.

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