The use of ash as a glaze ingredient has long historical antecedents; it has been an integral part of the evolution of pottery making in China, and from there played a similar role in the development of high fired pottery in Japan and through other centres in Asia.
As a glaze ingredient ash has the value of bringing a complex array of minerals to the mix in a very simple manner. It is primarily a source of calcium and silica, providing both flux and glass matrix to the melt that forms our glazes, as well as variable amounts of alkalis (potash and soda), magnesium and other minerals such as iron and manganese which contribute to the eventual fired colour of the glaze.
The attraction of ash from a potters perspective comes from the known and the unknown, as it were, or even the known knowns and the known unknowns (thank you Mr. Rumsfeld)…and it is precisely the unknowns which contribute to the fusibility, or amount of melt, the fired colour, and the quality of the eventual glaze.
Australian ashes tend to reflect what we burn in our fireplaces, which would be mainly hardwood (Eucalyptus species). Hardwood ashes are usually high in silica with varying amounts of calcium as the principal flux; these ashes vary considerably depending on species type, location and treatment (washed or unwashed). Most ashes that emerge from these sources (fireplaces and stoves) are likely to be mixed or not attributable to a single species.
It is rare to be able to obtain a single species ash unless one sets out with the deliberate intent to do so, either through particular management of what goes in the fire or by cutting and burning the selected wood. This is even more applicable to softwoods in Australia, as commercial wood merchants supply hardwood as a matter of course, due to its greater calorific value and availability.
Early in the development of my Ash glazes, I was fortunate enough to obtain a single bucket of Blackwattle ash (Acacia decurrens) from a colleague who cleared an amount of it from her land in the Hawkesbury Valley. After washing, sieving through 25 mesh and drying, I made a number of small batch tests before deciding on the preferred glaze and then made up a single bucket of about 20 litres of glaze. I used a white clay (Puggoon110 or ‘Clayceram’ ) as the clay content, so that the resultant colour was due only to the ash component of the glaze.
These type of softwood ash glazes are quite common in Britain, where they can be seen in the work of potters such as Richard Batterham, Mike Dodd and Phil Rogers. They are not seen so often here in Australia due to our geology and geography, so that it has been an enjoyable exercise for me to have a dalliance with a material that I last made use of in the time that I worked in the UK.
Of course, the continuing challenge with Ash glazes is their temporal nature: once a source of ash is exhausted it is not replaceable - the next ash is always different. By 2009 the Blackwattle ash was gone: it was followed by a 'Falls Ash' which was my fireplace ash from 2 winters of 06-07, containing a reasonable amount of willow ash from a willow in the garden that died. This was replaced by a Music Room ash, gleaned from the demolition of a part of our house (mainly cypress pine, some western red cedar and hardwood) and the rebuilding of a new Music Room (hardwood and Merbau, thanks to RFS fire regulations), though due to efficient builders there were minimal offcuts.
This provided an ash that produces a rich, dark mottled yellow/green/black range of colours with a semi-matt surface.
My fireplace ash continues to provide a supply of mixed ash, principally hardwood, and other ashes are sourced from friends who know I ‘collect’ ashes
A current supply is a mix of cherry and plum, from the removal of an orchard. This ash gives a rather lovely soft green, buttery surface that can develop a more glassy nature when in the hotter parts of the kiln, or when pooling in the bottom of bowls or dishes.