perturbations
firing to cone 10 in oxidation
slow downfire at 50 deg F an hour in the interval 1850 deg F to 1700 deg F
slow downfire at 25 deg F an hour in the interval 1700 deg F to 1650 deg F
One hour hold at 1650 deg F
We show the effect of one small changes in glaze composition.
We observe the effect of a moderate increase in silica in two calico glazes, then the effect of changing the balance of the alkali metals in an alkaline/alumina matte glaze, and in a calico glaze.
Effect of Only An Increase in Silica
cooper_404_1Sr vs cooper_404_1SrPSi
The second glaze has more silica, with silica:Alumina ratio 8 compared to 7 for the first glaze. Both glazes have alumina ~.33 with moderate alkali metals, and minimal calcia and magnesia.
glaze cooper_404_1Sr bowl ~3 inch diameter
glaze cooper_404_1SrPSi bowl ~3 inch diameter
cooper_404_1Zn vs cooper_404_1ZnPSi
The second glaze has more silica, with silica:Alumina ratio 8.2 compared to 7 for the first glaze. Both glazes have alumina ~.32 with moderate alkali metals, and minimal calcia and magnesia.
glaze cooper_404_1Zn bowl ~3 inch diameter
glaze cooper_404_1ZnPSi bowl ~3 inch diameter
In both cases the effect of the increase in silica is a decrease in the prominence of the calico markings, and a shift from a near stony matte glaze to a full gloss glaze.
Of course as expected, the increased silica cured the dunting.
Effect of a only change in the balance of the alkali metals
ti_z3_5 vs ti_z3_4
Both these glazes have alkali metals ~.5, alumina ~.5 and silica:alumina ratio of 6. The differ in the first having high potassia, low soda, the second, high soda, low potassia.
glaze ti_z3_5 bowl ~3 inch diameter
glaze ti_z3_4 bowl ~3 inch diameter
We note that the second glaze, with relatively higher soda, and lower potassia is more matured, less pitted, and where thin, nearly a full gloss glaze.
cooper_404_1Al vs cooper_404_1AlLi
Both these glazes have moderate alkali metals , alumina ~.37 and silica:alumina ratio of 7. The second glaze has lithia substituted for some of the soda and potassia of the first.
glaze cooper_404_1Al bowl ~3 inch diameter
glaze cooper_404_1AlLi bowl ~3 inch diameter
Observe the change in the calico markings, a second pale turquoise color has appeared, and the background color shifts to a lighter pale yellow shade.