Clay as a material has wonderful characteristics: Whether it is thrown on the wheel, constructed using sheets, mixed with paper fibres or cast in slip form, it is a plastic and malleable substance which will take on forms and respond to manipulation like no other. With these properties in mind I (like many others), want my finished work to demonstrate these material qualities in the final fired state. Attempting to loosen the static rigidity, which even such a soft material can take on when transformed into fired ceramic. By firing with wood to high temperatures my work again exhibits some of these soft plastic attributes.
I embarked on an extensive exploration of ceramic materials and their
responsiveness when incorporated into a ceramic body and subjected to
wood firing. Firings would often contain work made in ten or fifteen different
bodies, it was not only their abilities to trace the fire and respond
to the heat and atmosphere which was important, but also their workability.
I developed several bodies which gave fantastic responses to the firing
but were horrendous to work with on the wheel.
I now limit myself to three main bodies, (although the process of testing
is still ongoing). These are: a white stoneware, porcelain and a grogged
stoneware. They are all fine particle bodies containing ball clays and
china clay (as their clay content). The flame movement throughout a firing
and the action of flame borne alkalis, as well as atmospheric conditions
within the chamber tends to draw colour best from these fine particle
clays.
The other feature of the clays which I use, is the amount flux contained
within the body. Roughly 20% of both the stoneware and the porcelain bodies
comprise of flux in the form of nepheline syenite. This soda based flux
in theory reduces the maturation temperature of a body and indeed porcelain
pieces at the front of the kiln can be seen to have a sheen at comparatively
low temperatures (1100C). On seeing a sheen at this temperature one would
predict that at the top temperature of 1400C they would be puddles, but
they hold their form pretty well (partially due I believe to the relatively
high china clay content which is imparting pretty high quantities of refractory
alumina), and despite this as I have expressed earlier I like my work
to show signs of Pyroplastic action.
The big ware body which as its name suggests I use for larger pieces including
large platters and bowls is also a fine clay particle body, however it
has a large percentage of Molochite grog added to give it more strength.
It also has a slightly lower flux content. Because I fire these pieces
on their sides stacked along the side walls of the kiln they need the
extra strength imparted by the Molochite in order to keep the Pyroplastic
movement to some degree in check. Both the stoneware body and the porcelain
bodies if used for these large pieces will not have enough strength to
withstand the pressures of side firing and in the past I have been confronted
on opening the kiln with bowls and platters which resemble calzone pizzas.
The inclusion of Molochite does not seem to affect the body's responsiveness
to the colours generated by the fire nor does it mark the surface with
iron bleeds, which (with such heavy degrees of reduction through the firing),
are characteristic of standard firebrick grogs.
In order to develop colours in the bodies there needs to be some iron
present. The iron in all the bodies is contained within the ball clays
used. The tiny amount of iron present in the porcelain body is enough
in some parts of the kiln to generate salmon pinks and rich orange reds
as well as bright whites. It may give all of these elements on one piece
(and frequently does). The stoneware body has more iron, although I would
still class it as a porcelaineous stoneware, however the extra iron introduced
through the addition of a small amount of AT ball clay (a higher iron
ball clay than the Hyplas 71), is enough for the body to generate rich
oranges and purples as well as brighter tones.
The high flux content of the bodies reacts with ash and alkalis to "self
glaze", the bodies on the surfaces which are in direct line of the
flame and ash, and also on other surfaces where the flame has been forced
into turbulence by pots behind. The additional body flux also brightens
the fly ash glaze and encourages it to run and pool.
Although I now generally use only the above three clays, this is as a
result of extensive research into many varying combinations of clays and
fluxes. Now in each firing I will tend to try one new body, placing a
sample both at the front and at the back of the kiln. The stoneware, porcelain
and big ware bodies will all respond differently depending upon their
positioning within the kiln. This is why I have restricted myself. These
three bodies all have the potential to give very varied results and the
more I use them the more I learn.
The responses that the bodies have will also be affected by which kiln
they are fired in. As well as firing several other kilns myself, I am
fortunate enough to have friends who wood fire include small test pieces
in their firings. This has helped to build a picture of the fired qualities
of the clays. For example, the porcelain body when fired in a traditional
Bizen style kiln with no grate for five days will give a much whiter body
that tends to be very glassy, with the fly ash glaze appearing a very
pale green. The same body fired in a square chambered kiln with a large
firebox added to the front with a grate and fired for fifteen days will
give a mottled carbon trapped surface with greens, greys, blacks and whites.
When fired in my kiln it will give surfaces which range from brilliant
white, pinks through to rich reds.
Undoubtedly the types of wood used for the firings also affect the reaction
of these clays. I predominantly use Scots pine and Norway spruce for the
soft wood, Walnut and Ash for the hard. Work fired in other kilns in the
UK and in America have been fired using various Oaks and other native
woods. The fuel has changed the characteristics of the surfaces through
the different types and quantities of alkalis contained within these woods,
given off when the fuel is burned. Also the atmospheres generated by different
kiln configurations can greatly influence the tones of colour that the
clays produce.
Finally the glaze, which I predominantly use, should be mentioned, as
it has also been formulated to respond to this type of firing and give
glaze qualities that work with my forms. The glaze is an ash glaze with
nepheline syenite, Cornish stone and silica. The non-existence of any
stabilising clay makes this glaze pretty fluid and it responds to the
firing in a very similar way to accumulated fly ash glaze. The slightly
unusual aspect is that it contains a ten percent inclusion of titanium
dioxide. This material is generally thought of in ceramic terms as an
opacifier, giving creamy white colouration and in this amount would usually
be considered to make a glaze matt.
In this type of firing however the glaze gives a huge spectrum of colours
from the more traditional cream colours (in cooler [1300C], more oxidised
areas), bright pinks, deep blues, greens and purples in hotter areas with
a more fluctuated atmosphere. These surfaces usually develop macro crystals
increasing in density where the glaze is thicker. Probably seeded by flame
borne elements, the fluid nature of the glaze allows the crystals to form
in the glaze matrix catalysed by slow reduction cooling. The glaze can
also trap carbon, which can introduce interesting elements of black specking,
which add to the depth of the final surface.
Ben Brierley.
