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.
