Effect of Increasing Alumina

The glaze wisskirchen-crystals-PAl is derived from the glaze wisskirchen-crystals by
increasing alumina. That is the major change in its empirical formula. Its silica alumina ratio decreases
from 7.3 to 6.3. A full gloss glaze with a small number of micro-crystals morphs into a micro-crystalline
matte glaze.

Image of the lower alumina glaze wisskirchen-crystals

full view

inside

full view

outside

bowl is ~ 4 inches in diameter.

Image of the higher alumina glaze wisskirchen-crystals-PAl

full view

inside

full view

outside

bowl is ~ 3 inches in diameter

oxidation firing to cone 10 in an electric kiln

Firing profiles

Same upfire profile for both glazes:

Up Fire profile cone 10

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 cone 10 for glaze wisskirchen-crystal

300 deg F an hour to 1650 deg F then a 3 hr hold at 1650 deg F

400 deg F an hour to 1850 deg F then a 2 hr hold at 1850 deg F

300 deg F an hour to 1750 deg F then a 1 hr hold at 1750 deg F

300 deg F an hour to 1700 deg F then a 3 hr hold at 1700 deg F

Down Fire Profile cone 10 for glaze wisskirchen-crystal-PAl

300 deg F an hour to 1650 deg F then a 3 hr hold at 1650 deg F

400 deg F an hour to 1900 deg F then a 3 hr hold at 1900 deg F

300 deg F an hour to 1750 deg F then a 1 hr hold at 1750 deg F

300 deg F an hour to 1700 deg F then a 3 hr hold at 1700 deg F

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

glaze compositions

Lower Alumina Glaze:

Empirical Formula wisskirchen-crystals:

K2O        0.10
Na2O       0.24
CaO        0.62
MgO        0.04

Al2O3      0.48
Fe2O3      0.3

SiO2       3.48
TiO2       0.03

molecular percent Silica 65.8%

Higher Alumina Glaze:

Empirical Formula wisskirchen-crystals-PAl

K2O        0.10
Na2O       0.24
CaO        0.62
MgO        0.04

Al2O3      0.55
Fe2O3      0.31

SiO2       3.45
TiO2       0.03

molecular percent Silica 62.3%

Remarks

The firing profiles for these two glazes differ in the down fire. The higher alumina glaze had a hold of 3 hours
at 1900 degF replacing a two hour hold at 1850 degF for the lower alumina glaze.

Several glazes were fired in both firings, none showed a substantial shift in degree of micro-crystalline development.
So I attribute the difference shown above to the increased alumina in the glaze.

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