copper Red with magnesium Part 1

Here is a local reduction copper glaze.

Previously I'd shown the effect of higher viscosity resulting from high alumina and low silica on local reduction copper glazes.

here:

Now I show the effect of increasing MgO in the glaze alexanderBowl_Z15_0. The increase in MgO is achieved by an equal decrease in CaO.

The surface of this glazes has, like a congealed sauce, separated into a viscous layer with a fluid component bubbling through.
The red is darker and greyer.

Glaze alexanderBowl_Z15_0

full view

derived glaze alexanderBowl_Z23_2 with higher MgO

full view

bowls are ~4 inches in diameter

oxidation firing to cone 10 in an electric kiln

Firing profiles

Up Fire profile

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

Down Fire Profile

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

Clay body is a grolleg porcelain from Tacoma Clay Art Center.

glaze compositions

Empirical Formula glaze alexanderBowl_Z15_0 :

The glaze with added .5% copper carbonate, 2% tin oxide, and 1% silicon carbide.

K2O        .08
Na2O        .05
CaO        .7
MgO        .17

Al2O3        .48

SiO2        2.65

molecular percent Silica 64%

Empirical Formula glaze alexanderBowl_Z23_2 :

The glaze with added .5% copper carbonate, 2% tin oxide, and 1% silicon carbide.

K2O        .08
Na2O        .05
CaO        .61
MgO        .26

Al2O3        .48

SiO2        2.66

molecular percent Silica 64%

Remarks

alexanderBowl_Z23_2, the higher MgO version of alexanderBowl_Z15_0, is a mixed lilac and grey-green similar to that of alexanderBowl_Z15_0.
However the balance of the two states has shifted toward the grey-green.

The red is present, yet darker and greyer. The more viscous grey component has gained prominence and has a definite green tint.

Is the shift in color the result of the increase in MgO, the decrease in CaO, or a side effect of the change in the viscosity of the glaze?

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