Flashing
colors are typical of woodfiring, usually where a reduced atmosphere
has been use in the higher temperature ranges, followed by an oxidised
cooling. The flashing colors are very dependent on the nature of the
clays used, and generally low iron clay bodies fluxed by felspars produce
an orange coloured flash.
I
have for many years been working at improving color response in woodfiring,
both in glazes and on the clay itself. Rather than dull browns and tans
I want reds, pinks, oranges and apricot colors. Experimenting for many
years with clay body composition led to RSF, a woodfire/salt body sold
commercially in Australia by Clayworks. RSF is one of the few specially
formulated commercially available woodfire clay bodies in the world
(the others being in Japan, e.g. Shigaraki clay). It has proved over
the years to be very durable in the most testing woodfire conditions
such as anagama firing, and even refiring multiple times, a practice
that destroys most stoneware bodies. It has been successfully fired
up to Cone 12-13 in the anagama but is best around Cone 10 for color
development. Under the right firing conditions, RSF could produce strong
pink/apricot/orange colours underneath wads, or where the clay was sheltered
from direct flame.
Recently,
because one of the materials in RSF became unavailable, the opportunity
arose to experiment with further improvements in RSF. The new composition
allows an even richer flashing quality.
In
common with the earlier version of RSF, the new clay is also suitable
for saltglazing. It has good throwing characteristics and is especially
suitable for large pots, which I make by the coil and throw method.
It is also used as a handbuilding body especially for large-scale work.
I use it sometimes as paperclay, mixing shredded newspaper and some
paper pulp into the commercial body in a clay mixer. This does not alter
its firing characteristics in any way but allows making difficult shapes,
and larger forms are much lighter than when made from the standard RSF.
Along
with clay body experiments I have for many years investigated details
of the firing process, and it is clear to me that the best clays in
the world will not respond with good color if fired inappropriately.
An oxidising fire, for example, produces no flashing (except an occasional
slight coloration where works are touching during firing). My early
experiments showed that RSF develops color when oxidation and reduction
are alternated during firing, as can happen quite naturally during woodfiring
if stoking is delayed until only brightly glowing ember is left. This
helps temperature rise.
Better
results later came from consistently reducing up to (and maintaining)
top temperature, followed by reduction during cooling down to around
1100 Centigrade (1832 Fahrenheit). This procedure gives orange/apricot
flashing colours. The strongest colors have developed since I started
introducing water into the kiln during firing and cooling. Water in
the kiln during cooling seems to give the strongest colors, which move
towards pinks and reds. There is no final word on these experiments,
which continue with each firing. And just in case anyone thinks that
color development is only possible in long duration firings, I have
seen strong flashing on RSF fired over six hours in a fast-fire kiln,
followed by appropriate cooling techniques.
The
next stage in experimentation will be to develop a higher-iron woodfire
clay. The aim here will be to develop rich red colors as seen in some
Bizen ware in Japan, notably developed under wads in work by Kei Fujiwara.
We are planning that the new clay body will be available later this
year.
