Flashing on Australian commerial clay bodies

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Red/orange flashing and reduced cooling                            by Carol and Arthur Rosser
Flashing is just one of the infinite number of effects which can result from long wood firings but clay bodies often do not cooperate and desirable colours can be elusive. It is common practice to apply slips and shino-type glazes to obtain similar colours but that is one more thing to do, and anyway there is something very satisfying and direct about firing the unadorned clay. For a long time we have been encouraging flashing on clay bodies based on mixtures of a local iron bearing clay and white firing stoneware bodies. To ensure the development of red/orange flashing we use a particular cooling procedure and if we deviate from this the results are unsatisfactory. The white firing bodies were either commercially available bodies or bodies we have mixed ourselves from commercial milled ingredients
 pots with woodfire flashing

Flashing on pots made from bodies not available commercially

The Skardon River clay body used for the lidded jar in the photo has this composition:
Skardon River clay body
Skardon River kaolin   33
Crediton clay              27 
Soda felspar               23
Silica                         13
The Crediton clay is dug about 15km from home and has about 2.4% iron oxide. Skardon River kaolin deposit is located near Weipa, Queensland
Reason for testing commercial clay bodies
 In order to ennable helpers to make pots of their own to bring to firings we have felt the need to find clay bodies which do not involve Crediton clay and which will produce warm flashing colours under the same firing regime.  So we have run a series of tests of commercially available clay bodies, and mixtures of these bodies, and some of the results are shown below.

Types  of wood  used
The main wood used in all our firings has been Blackwood (Acacia melanoxylon), with a variety of eucalyptus woods used in the early stages only. Flashing does not happen when the natural ash glaze is thick, so this is a major concern when packing the pots. It is possible to get huge quantities of natural ash glaze with Blackwood so the duration of stoking with it must be considered carefully.  Obviously the quality of the flashing will depend on the wood used and in this respect it may be that Acacias are more useful than Eucalypts as sources of wood.  We would really like to know what happens using wood from other species but Blackwood is what is readily available in our area and we have become familiar with the variety of ash effects it produces so that is what we use. Blackwood's prodigious production of ash glaze seems to cause crawling on shino glazes if used too early in the firings so we use eucalyptus wood until the temperature at  the front of the kiln has reached 1,000 degrees C.
The kiln The tests were fired in our Oztrain kiln.
Firing Our firings are usually 40 hour reduction firings to cone 10 or higher,  and we use the Rosser reduced cooling procedure in the early stages of cooling. This involves some cooling under reduction followed by a period of oxidation, and  is dealt  with at length here.  A short description of the cooling of the firing of the test mixtures is given  here. We use an oxygen probe to make sure we are maintaining the right atmsopheric conditions during the critical periods during cooling. As usual the main wood used was Blackwood, with Flooded Gum (Eucalyptus grandis) used in the early stages.  Although we usually use green ginger leaves as a reducing agent during cooling this was not necessary on this occasion because the ember bed was so big that reduction occurred naturally in the 1,000 to 900 deg range.
The clay bodies
A few tests involved a clay body which has this composition:
Rosser body
Crediton clay       80
potash felspar       8
silica                   12
The iron oxide content  of this clay body is 1.9%
With the exception of Clayworks TMK and Kagero the commercial clay bodies were as available in early 2014. The Kagero used was a few years older and the TMK sample was much older, possibly a decade older.  There is no guarantee that the clays of the same names being sold now have the same composition as the ones used in the tests.  Tests of TMK bought in 2016 were much less promising, indicating that the TMK body has indeed changed over the years. We once had an unfortunate anagama load involving a mixture of Rosser clay and a new batch of Feeney's white stoneware clay. To our disgust this brew did not produce orange/red flashing at all.   Public flogging should be reinstated as a punishment for clay manufacturers who change the composition of their clay bodies without informing customers.

Test results
The photographs were taken in full sunlight and the saturation setting on the Olympus OMD EM5 camera was  "Natural". The colour rendition on your screen will depend on your particular device. The clays were mixed dry then mixed with water and dried out to plastic state and formed into test tiles. When mixing larger quantities of clay bodies in plastic state it is sufficient to knead the mixtures thoroughly or put them through a pugmill.

Interpret the numbers like this:

The tile at near right consists of 2 parts Clayworks TMK mixed with 1  part Rosser clay

 2 Clayworks TMK + 1 Rosser
2 Clayworks SWS + 1 Rosser

Keane Flash
2 TMK + 1 Keane Flash

 SWS Clayworks SW
1 SWS + 1  SW

1  SWS + 1 Keane 5B
2 SWS + 1 Clayworks Kagero

Clayworks RGH 2 TMK + 1  RGH

Keane 5B 2 TMK + 1 Keane 5B

Clayworks LGH    Clayworks CWR

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