Magnesium vs. alkali metals
A dry matte glaze develops a glossy second phase with increased Magnesium.
The second glaze has more Magnesium and less alkali metals than the first
glaze. Both glazes are variegated with multiple, yet different, phases.
The glaze with high Alkali metals, low MgO.
A dry matte glaze with varigated mixture of browns and pink in a pale ivory matrix.
glaze with lower Alkali metals, and higher MgO
glaze is a bright mixture of glossy orange, pale green and ivory.
150 deg F an hour to 250 deg F
400 deg F an hour to 1850 deg F
120 deg F an hour to 2130 deg F
13 deg F an hour to 2180 deg F / hold 2 hours
13 deg F an hour to 2220 deg F / hold 3 hours
300 deg F an hour to 1750 deg F then a half hour hold at 1750 deg F
300 deg F an hour to 1700 deg F then a Three hour hold at 1700 deg F
25 deg F an hour to 1650 deg F then a one hour hold at 1650 deg F
K2O .27
Al2O3 .71
SiO2 3.62
molecular percent Silica 68%
K2O .24
Al2O3 .71
SiO2 3.61
molecular percent Silica 68%
The effect on the appearance of the glaze of replacing some Alkali metals is a
surprise. The second glaze with higher MgO is more fused,
The low MgO higher alkali metals glaze, copperYellow_Z26_1, has large pinholes
and crazing where thickly applied.
There is no glassy phase in the first glaze, which is dry, and almost
underfired if thickly applied.
Glaze copperYellow_Z26_1
bowl is ~4 inches in diameter
Glaze copperYellow_Z26_1MAlk
bowl is ~4 inches in diameter
oxidation firing to cone 10 in an electric kiln
Firing profiles
Up Fire profile
Down Fire Profile
Clay body is a grolleg porcelain from Tacoma Clay Art Center.
glaze compositions
Empirical Formula glaze copperYellow_Z26_1 :
Na2O .27
CaO .25
MgO .12
ZnO .09
Empirical Formula Glaze copperYellow_Z26_1MAlk
Na2O .24
CaO .26
MgO .16
ZnO .09
Remarks
not the expected
result from either increasing MgO, nor decreasing Alkali metals.
These defect can be
attributed to the high viscosity of a nearly underfired glaze.