In 2000, I completed my Diploma of Art (Ceramics) at ANU Canberra School of Art with the goal to produce simple thrown forms with an interesting glazed surface. I was initially attracted to the high felspar iron reduction glazes of the Chinese Song dynasty (12th-13th century). I chose eight glazes.

The Testing Procedure
The initial tests were 500 gram batches based on potash felspar with a yellow iron oxide line blend from base to 3% in 0.5% increments. Glaze was brushed on to tiles with thickness built up towards the top. Tests were fired in a gas kiln to Orton Cone 8-9 over eight to ten hours with moderate reduction commencing at 1000C and maintained for at least four hours.

Glazes 1 - 3 showed bluish iron tones, especially at the 1.5% concentration. Glaze 2 developed a typical ‘fish scale’. Glaze 5 was very fluid. Glaze 6 was more satin and opaque with the dolomite content contributing a ‘buttery’ quality. Glazes 7 & 8 were a typical celadon. The next step was to fire the glazes at different temperatures. Three iron variants of each glaze (base, 1.5 & 3%) were fired to cones 6 & 7 with the results considerably more satin. At Cone 6 all glazes were immature. Glazes 4 to 8 crawled where the application was thicker, and colour development was poor. Cone 7 firing gave a smoother satin finish - particularly glazes, 5, 7 & 8 which had celadon qualities earlier.

LHS Top - Glaze 5
LHS Bottom - Glaze 6
RHS Top - Glaze 7
RHS Bottom - Glaze 8
(all base glazes blended with 0.5 -
3% iron oxide in
0.5% intervals)

The glazes chosen for further testing were Glaze 3 with its three variants - base, 1.5% & 3% iron increments to give white, medium and dark chun blues; Glaze 6 (closest to the satin guan style) and Glaze 7 for a deep green celadon.

These glazes were tested with different fluxes by substituting soda felspar, nepheline syenite and wollastonite for the potash felspar in each recipe. To each flux substitution 5% barium carbonate, then 5% Frit 4194 was added. The wollastonite produced a dry glaze in glazes 3 & 7 becoming slightly more satin with the addition of the frit. Glaze 6 changed to a clear, fluid glaze with a stringy almost ash effect. All flux substitutions in glaze 6 altered the satin quality to a transparent glaze. Glaze 3 lost its blue colouration with the use of soda felspar & nepheline syenite, reverting to a transparent iron green. Fluidity was increased with the barium and frit additions. Glaze 7 was least affected by the flux substitutes.

The final testing were line blends of talc and dolomite to glaze 3 (base, 1.5% & 3% iron) to gauge the effects of magnesia. Materials were added in 3% increments up to 12%. Talc had little effect on the iron bearing variations of glaze 3 until 9% when the glaze became distinctly satin and lost its blue colouration. In the base, additions of 3 - 6% gave a slightly luminous grey-blue which became a satin flat white at increments of 9% and above.

Glaze 3 base with a 4% talc addition gave a glaze with a distinct pale blue cast - even on white porcelain and intensifying on a red body - due to the interaction of the magnesia and bone ash. The dolomite line blend, which contributed the flux, calcium, as well magnesia, gave more fluid, though similar, results.