by Ben Brierley
The subject of wadding
for use in extended woodfirings may not be particularly glamorous, however,
when one is attempting to fire ceramics for three to five days to Orton cone
15 in an anagama style kiln, and wanting to get the pots out intact at the
end, it becomes an increasingly interesting and pertinent one. Anyone who
has undertaken a long woodfiring and experienced the highs and lows of the
process, will empathise with that magical moment when the kiln door is un-bricked,
and one gets a first look at the pots. Instinctively a hand reaches in to
retrieve the choice piece of work in view, liberating it from its ordeal,
and it will not move. The only option is the rather brutal hammer and chisel.
If this is successful and the piece comes out without breaking, then it will
undoubtedly have to undergo surgery by angle grinder to remove the remnants
of kiln floor/shelf adhering to it. As if the firing was not enough in itself!
How this problem can be avoided in the first place, then, becomes a big question
as the next firing looms.
I was prompted to write this article after a particularly hot firing, in which
the scallop shells that I used at the front of the kiln to provide an extra
element of release, failed to do so. Rather than staying inert through the
firing ready to be dissolved afterwards, the shells fluxed into a rather unsightly
thick pale green glaze, which welded the pots to their wadding and actually
ate through the work (see photograph at top left). This problem was no doubt
exacerbated by my use of high flux clays. Generally the clay bodies that I
mix have 15-20% nepheline syenite added (see clay recipes on page 19). During
firing this soda flux combines with the fly ash and other flame borne alkalis
to produce very glassy surfaces, with good natural fly ash glaze runs and
matt flashed areas in the lea of the flame or ash fall. The added action of
the calcium in the shells (a secondary flux), combined with the primary flux
of nepheline syenite and the ash, created this thick welding and corrosive
glaze. Shells, I concluded, were not appropriate for this extreme degree of
heat, and so I am having to re-examine more conventional wadding methods.
This research was carried out with another woodfire potter, Stephen Parry,
who has collaborated with me on many firings. His particular interest is in
achieving wadding marks on side-fired pots that do not stand out too starkly
against the flashed wood-fired surface, but generate further subtleties upon
We began to look more closely at the qualities that can be achieved aesthetically,
as well as practically when using wadding. I have been woodfiring my work
in anagama type kilns for the past ten years, Steve for the same length of
time, although he has been firing other types of wood kilns for much longer.
I regularly fire in two kilns, one in the ceramics area at Loughborough University
School of Art and Design1, and the second at Wysing Arts in Cambridgeshire2.
The kiln at Loughborough fires hotter, to about 1400ºC at the front and
approximately 1350ºC at the back. The Wysing kiln fires slightly cooler,
in that it will fire to about 1320ºC at the front, and about 1300ºC
at the back. The closer the work is to the front of the kiln and the full
effect of fly ash and other fluxing alkalis, the more resilient the wadding
needs to be in terms of being able to resist these elements, and still retain
enough bulk to support the work off the floor or shelf.
There are several other important qualities that a good wadding mix must possess:
it should be relatively easy to apply to the pots as they are stacked in the
kiln; allow the pots to be removed fairly easily from the kiln after firing,
and release easily from the pots themselves. The cost involved should also
be considered, as a lot of wadding may be used to place the pots in one firing,
and if using expensive materials this will add to the overall overheads. Other
properties which are desirable from wadding include: the promotion of flashing
around the wadded areas, and leaving an aesthetically pleasing mark which
can be regarded as a decorative feature.
The first wadding I developed consisted of one part fine sawdust, one part
high alumina fireclay, and one part silica sand. This mix worked well in several
respects. It was easy to apply when stacking pots and kept its bulk through
the firing, thus keeping the pots proud of the floor. However, it was not
that easy to remove from the fired pots. After several experimental mixes
in which I played around with differing proportions of these materials, I
came up with a slightly altered recipe which consisted of two parts silica
sand, one and a half parts sawdust, and one part fireclay - or just enough
fireclay to make the mix usable. This is the recipe I continue to use as it
works well in all parts of the kiln.
Each of the three elements of the wadding bring their own attributes: the
sawdust (potentially any fine combustible material) renders the fired wadding
crumbly, and combined with the refractory sand make for a mix that will readily
separate from the fired work. Finally the fireclay addition acts as a binder
for the mix, aiding its application and adhesion to the unfired work and also,
due to the larger particle sizes it adds to the openness. This mix becomes
problematic however, if the wadding becomes saturated with fly ash glaze.
When this happens the voids created by the burnt out organic material act
a bit like a sponge and absorb the ash glaze, making a very hard surface that
can only be removed with a grinder, and even then it is with difficulty that
it comes away from the pot.
A potential solution then would be to reduce the amount of combustible material
in favour of more refractory sand in order to reduce the number of voids in
the mix, or to use a wadding which resists the ash glaze by early fusion of
its elements, making it less pervious to ash glaze, yet still easy to remove
from pots, not by crumbling but just by popping off. In recent firings Steve
has been experimenting with various wadding mixes that incorporate less combustible
material, while adding iron in the form of low firing red clays and iron bearing
The aim of this initial exploration was to achieve a darker wadding mark rather
than a stark body coloured mark (we both use white firing stonewares and porcelains
which flash very well in the firing, however the areas which are wadded can
be an extreme contrast). This research has been fruitful in that the wadding
mix which contained a higher amount of red clay not only came away from the
pots, even at the front of the hottest firing, but also left a pleasing, subtle,
darker mark. This went against my initial fear that the fluxing action of
the iron in the red clay would bind the wadding to the pots. It seemed as
though it was working as a result of the iron fusing on the outside of the
wad and resisting the ash. We also tried high alumina wadding more akin to
that used in salt-glaze firings. These worked well for ease of application
and ease of removal, but left a very stark mark, and due to the relatively
low combustible content and lack of iron did not contribute to flashing around
the wadding mark itself. Another negative aspect of a high alumina wadding
is its cost: alumina is an expensive material and a fair amount would be required
to stack an entire kiln. This therefore ruled it out of the equation, except
for use on porcelain bodies that one would not wish to have contaminated by
These investigations are ongoing and we have achieved some very interesting
results. With each firing our understanding of the complex relationships between
materials in these types of firing increases. As successes and failures are
removed from the kilns, conclusions are drawn and built upon, equipping us
with more information to prepare for the next foray into fire and ash.
1. See article in issue 14 of The Log Book.
2. See article in issue 5 of The Log Book.
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