Technical Information
A variety of clay bodies is used for the work. Some pieces are made entirely of porcelain, others of a mixture of porcelain and C material, and some from Earthstone Original. I may use the wheel, moulds or hand-build depending on my mood or desired outcome. Some pieces are covered with a slip before the bisque firing in order to give a very white background to the final decoration. Work is burnished to ensure a smooth surface on the final piece after the resist slip and resist glaze is removed following the raku firing. Terra sigilata ( ‘sealed earth’ - a very fine liquid clay) may be applied when the piece is dry and the surface gently ‘buffed’ to bring up the shine.
The work is bisque fired to ^08 after which the resist slip and resist glaze are applied. The process is sometimes known as ‘naked raku’ as these slips and glazes are designed to ‘shell’ off the work after the raku firing. Marks drawn through the glaze and slip or other resist methods are used to provide the decorative effects required on the work.
Each piece is fired in a ‘top-hat’ raku kiln to around 850° C. (exact temp is usually dependent on the glaze recipe). At this temperature the kiln is lifted off the work and the piece transferred to a non-flammable chamber containing sawdust and newspaper. The chamber is sealed removing the source of oxygen and allowing the sawdust to create thick smoke in the chamber. It is this smoke which penetrates the exposed clay to provide the grey/black/brown markings on the final work. Those parts totally covered with slip and glaze resist the smoke and stay white. Where the glaze cracks the smoke penetrates leaving crack markings on the fired body. The piece is removed from the chamber after about 5 minutes and gently cooled allowing the glaze and slip to ‘shell’ off.
The pink/red work is a similar process but uses the fumes of ferric chloride instead of smoke to deposit traces of red iron oxide in the porous body of the clay. A saggar (enclosure) of tinfoil is used to contain and direct the fumes into the clay.
The work is finally cleaned and a coat of Renaissance wax polish applied to bring out the contrast in the monochrome markings. As the work is low-fired and unglazed it will be porous and relatively fragile. It is not suitable for food-use or for holding water unless a plastic liner is used. From time to time a light coating of natural beeswax polish should be reapplied to maintain the appearance. If kept in strong sunlight the carbon markings, like those on a charcoal drawing, will fade over a long period of time.
Click here for my article in Ceramic Review Issue230
Click here for images from the article
Please email if you would like further information on the technical processes.