Active Wadding
This text was published in The Log book. Issue 28
Active Wadding
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 it.

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 ball clays.

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 iron marks.

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.

Link to recipes page