Glaze Group Meeting

11 July 2006

This month's agenda dealt with iron oxide in glazes and continuing experimentation with transmutation glazes.  Mike reported some tests using both natural and synthetic red iron oxide in 'orange/red' glazes described by Michael Bailey in his Ceramic Handbook on Cone 6 Glazes.  Using a progression of red iron oxide from 8% to 16% the glazes went from dark brown, to dark brown with orange crystalline spots through to the 'orange/red' band and then into dark brown again.

Mike found that the orange/red band with synthetic oxide was some 2% of iron oxide concentration lower than that for the naturally occurring oxide and he suggested the reason for this was that natural Fe2 O3 was deficient in iron and more like Fe1.8 O3.  The need to test new batches of raw materials when used in a previous recipe was clearly highlighted.

Following Di Wren's work (reported earlier) on transmutation glazes, Mike has looked at the interaction of 3 matt glazes containing colouring oxides with 3 transparent glazes containing potentially reactive systems.  All tests were fired in oxidation to 1230C with 1 hour soak.

Basic Matt Recipe:  Potash Feldspar 44; China Clay 6; Bentonite 2; Lithium Carbonate 4; Whiting 14; Silica 24.  To this was added 2% Cobalt Carbonate; 2% Chromium Carbonate and 10% Copper Oxide to make Matt Co, Matt Chr, and Matt Cu glazes.

Basic Transmutation Recipte: Soda Feldspar 49; Lithium Carbonate 5; Whiting 15 and Silica 25.  To this was added 20% Alkaline Frit (giving Trans Fr); 8% Tin oxide (giving Trans Tin) and 6% Titanium Oxide (give Trans Tit).

The matt glazes were applied with 2 coats thick over 2/3rds of the tiles with the Transmutation glazes applied over the other 2/3rds of the tile.  This produced 3 bands: Matt, Matt overlaid with transmutation and transmutation only.  The most interesting appeared to be the Chromium and Copper combinations with Tin and Titanium.

Tile with basic matt and Trans Fr only resulted in little colour reaction.  Tile featuring Trans Tin had produced good reaction with a strong chromium/tin pink with very pale pink/white tinge.  Perhaps most interesting was the reaction produced with the Trans Tit used.  Here the colours included hues or orange, salmon pink, creams and smokey blues.

It was decided to try further tests using different thicknesses of applied glaze and different percentages in glaze formulation.