Increasing Silica

The effect of increasing Silica.

An indifferent bottle brown glaze with a dense yet diffuse dusting or orangey markings is transformed into
a striking oil spot glaze.

The two glazes iron_satIron_Z2T_0, and iron_satIron_Z2T_0LSi have the same alkali metal:alkaline earth ratio
of 0.67:0.33; and the same alumina, iron and phosphorus. iron_satIron_Z2T_0 with higher silica has sharply defined
circular orange oil spots, while iron_satIron_Z2T_0LSi has a heavy dusting of those same orange particles, which
occasionally coalesce into wispy clouds. Both glazes have a similar diffuse dusting of metallic coppery particles.

It's as if there are two phases present, the glassy brown tortoise shell phase of the matrix, and the orangey
microcrystalline phase. It appears that higher silica decreased the solubility of the orangey microcrystalline phase
in the glassy brown phase, forcing the consolidation of the orangey microcrystalline phase.

Image of the low silica glaze iron_satIron_Z2T_0LSi

full view

inside

full view

outside

bowl is ~ 4 inches in diameter.

Image of the high silica glaze iron_satIron_Z2T_0

full view

inside

full view

outside

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 50 min hold at 1750 deg F

50 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 iron_satIron_Z2T_0LSi :

K2O        0.18
Na2O       0.49
CaO        0.21
MgO        0.12

Al2O3      0.66
Fe2O3      0.23

SiO2       3.61
P2O5       0.05

molecular percent Silica 65%

Empirical Formula iron_satIron_Z2T_0 :

K2O        0.23
Na2O       0.44
CaO        0.22
MgO        0.11

Al2O3      0.66
Fe2O3      0.22

SiO2       4.19
P2O2       0.05

molecular percent Silica 68.4%

Remarks

In addition to the difference in level of silica, the first glaze, iron_satIron_Z2T_0LSi, has lower
potassium, and correspondingly higher sodium than iron_satIron_Z2T_0.

More testing is required to determine whether, higher silica, relative fractions of potasium/sodium, or both
cause the difference in phase structure and resulting appearance of the two glazes.

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