Fired slipware, a UK tradition of terra cotta and lead glaze

The traditional UK slipware is possible because of the brilliant gloss and hyper transparency of glaze made using lead bisilicate frits. The lead glaze interacts with the colors in underlying slips, dissolving and feathering them (as enabled by the time and temperature of the kiln). Interactions with iron produce warm colors. Ware is bisque fired after the decorating and drying (lower left), then dipped in the leaded glaze. Photo courtesy of Russell Kingston, Lynmouth Slipware Pottery.

Related Pictures

Is it impossible to get a good clear over-glaze for cone 6 brush decorated stoneware?

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The mug on the right is terra cotta slipware firing at cone 04 using underglazes and a leaded transparent over-glaze (lead glazes are still commonly used in many parts of the world and considered safe). Mug on the left: This potter wants to use the same technique on cone 6 stoneware. Pretty well all cone 6 transparents depend on a boron frit (or Gerstley Borate or Ulexite) to melt them, this is no different. But it produces micro-bubble clouding and boron blue even though it is only slightly thick. Also, there are many more surface defects. The colors are washed out, partly because of the clouding but also because these commercial underglazes are overfired by cone 6, they are beginning to melt and diffuse into the clear overglaze (lightening their color). Achieving the warm brown color thus requires a more refractory underglaze (possibly with an addition of some terra-cotta colored stain). A better transparent glaze, better able to shed bubbles coupled with a drop-and-hold firing would also help. Finally, careful control of the glaze thickness and quality of laydown would also help. Preparing it as a brushing glaze, at least for application on the outsides, would enable tight control of thickness being applied and enable dense laydown.

The secret of the higher gloss glaze on the right? A lead frit addition.

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These cone 04 glazes have the same recipe (a version of Worthington Clear sourcing B₂O₃ from Ulexite instead of Gerstley borate). But the one on the right is more glassy, more transparent. Why? It has 10% added lead bisilicate frit. Lead bisilicate produces dazzling transparent glazes. no other method matches it. While potters gasp at the thought of using lead consider this: They thrive on unstable flux-deprived, glass-deprived and alumina-deprived base stoneware glazes with additions of large percentages of toxic colorants like chrome and manganese!

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URLs	http://www.russellkingstonceramics.co.uk/ Russell Kingston, Lynmouth Slipware Pottery Russell Kingston makes Devonshire Slipware (roots in medieval North Devon). Slip is dipped, poured, brushed, trailed and splashed with animated movements. Colour is achieved using oxides in slips. The terra cotta pots are then fired in a large homemade gas kiln in oxidation about 1100c (cone 03).
Glossary	Slipware Slipware, in the UK, is terra cotta pieces decorated at leather hard with thixotropic high ball clay slips, then bisque fired and clear glazed with lead bilisicate.

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