Silica and Texture

Decreasing silica decreased the prominence of a dense black second phase.

The second glaze was generated from the first by decreasing silica, which increased the surface coverage by metallic micro-crystals,
while simultaneously markedly decreasing the occurance of a lusterous black phase.

Glaze satIron_ZG_1S_1Munge

The original glaze with high silica.

Thickly applied, this glaze is a dense black. With a medium thick application, it has a dense cover of metallic micro-crystals
in a glossy orangy brown matrix with small patches of black. With a thin application, it is a lusterous brown
with regions of metallic micro-crystals.

full view

bowl is ~4 inches in diameter

Glaze satIron_ZG_1S_1Munge_1

glaze with less silica.

This glaze is covered with metallic micro-crystals, the glossy orangy brown matrix is only minimally visible. There are a few small fragments
of the dense black phase prominent in the higher silica glaze satIron_ZG_1S_1Munge seen above.

The glaze application on the outside of the pot was half the thickness of that on the inside, the effect on its appearance is not significant.
With less silica, this glaze is less dependant on the thickness of the application.

full view

bowl is ~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 2300 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 satIron_ZG_1S_1Munge :

K2O        .06
Na2O        .51
Li2O        .21
CaO        .21
MgO        .01

Al2O3        .53
Fe2O3        .19

SiO2        3.28
P2O5        .16

molecular percent Silica 63%

Empirical Formula Glaze satIron_ZG_1S_1Munge_1

K2O        .06
Na2O        .53
Li2O        .2
CaO        .2
MgO        .01

Al2O3        .54
Fe2O3        .21

SiO2        3
P2O5        .17

molecular percent Silica 61%

Remarks

The decrease in silica is small, but the effect on the appearance of the glaze is substantial.

The high silica glaze satIron_ZG_1S_1Munge has pinholes, and the inside of the bowl, where the glaze is thickly applied has collapsed bubbles.
Both of these defects can be attributed to high viscosity, and a glaze on the edge of being underfired.
A slight decrease in silica rectified that defect.

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