Silica and crystal formation

Reducing the silica in a saturated iron glaze causes an increase the formation of metallic golden crystals.

The second glaze has less silica than the first.

Both of these glazes are high phosphorus, saturated iron glazes, with an orangey brown to black background, and golden metallic crystals.

The first glaze has a black phase which dominates its appearance where the glaze is thick. This is seen in the black puddle
in the bottom of the bowl. Yet where thin, as seen on the outside of the bowl, that glaze is brown with almost no metallic crystals.

The metallic crystals dominate the appearance of the second glaze.

Glaze satIron_ZG_1S_1Munge

The glaze with high silica.

Black where thick, brown where thin. Golden metallic crystals present elsewhere.

full view

bowl is ~4 inches in diameter

Glaze satIron_ZG_1S_1Munge_1

glaze with lower silica

Glaze is covered in golden metallic crystals, though thickness of glaze varies greatly. There are scattered bits of the
dense black phase seen in glaze satIron_ZG_1S_1Munge, above.

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 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 satIron_ZG_1Munge :

K2O        .06
Na2O        .51
Li20        .21
CaO        .21
MgO        .01

Al2O3        .53
Fe2O3        .2

SiO2        3.3
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        .16

molecular percent Silica 61%

Remarks

The effect of a small decrease in silica on the nucleation of the golden metallic crystals is impressive.
Likely the glaze satIron_ZG_1S_1Munge with higher silica is slightly underfired, with resulting high viscousity. The high viscosity would impede
the movement of the crystal component, in turn slowing crystal growth.

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