Magnesium vs. Calcium
Increasing Magnesium transforms a dry matte glaze into one which is transparent
and full gloss.
The second glaze was generated from the first by replacing some whiting and silica
with talc.
The glaze with high CaO, low MgO.
A dry matte glaze with varigated mixture of olive green, bronze and pale yellow.
glaze with low CaO, high MgO.
Glaze is pale green and transparent.
150 deg F an hour to 250 deg F
400 deg F an hour to 1800 deg F
300 deg F an hour to 2050 deg F
120 deg F an hour to 2310 deg F with a hold of 20 minutes at 2310 deg F
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 .13
Al2O3 .8
SiO2 3.7
molecular percent Silica 67%
K2O .13
Al2O3 .81
SiO2 3.8
molecular percent Silica 68%
The effect on the appearance of the glaze of replacing CaO with MgO is
substantial, and surprising. The second glaze with higher MgO
The low MgO higher CaO glaze, copperYellow_Z25_0, has large pinholes where thickly
applied, and crazing is visible.
The higher MgO glaze copperYellow_Z25_1 is slightly pinholed and shows some
crazing, yet it is a more fluid melt.
A subtly varigated multiphase glaze melts completely into a
uniform glossy pale green.
Glaze copperYellow_Z25_0
bowl is ~4 inches in diameter
Glaze copperYellow_Z25_1
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_Z25_0 :
Na2O .42
CaO .25
MgO .1
ZnO .1
Empirical Formula Glaze copperYellow_Z25_1
Na2O .43
CaO .1
MgO .24
ZnO .1
Remarks
is more
fused, not what one would expect from increased MgO. In order to rule out a
weighing error,
I reweighed the glaze and retested to get the same result.
These defect can be attributed to high
viscosity, and a glaze on the edge of being underfired.