Increase both Alumina and Silica

A glaze with a dense covering of copper colored metallic crystals, satIron_ZG_4, with an increase
in both silica and alumina, turns into a black glaze with silvery oil spots, satIron_ZG_4PAlSi.

Images are shown of the precursor glaze, its derived glaze, with increased alumina and silica, and
closeups of the oil spot glaze.

Image of the precursor glaze, satIron_ZG_4

full view

butter dish, ~6 inches in length

Image of the glaze satIron_ZG_4PAlSi

The derived glaze, the glaze with additional alumina and silica

full view

inside

full view

outside

bowl is is 3 inches in diameter

Close-up Images of the surface of satIron_ZG_4PAlSi

full view

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 satIron_ZG_4 :

K2O        0.10
Na2O        0.50
Li2O        0.18
CaO        0.16
MgO        0.06

Al2O3        0.56
Fe2O3        0.20

SiO2        3.54
P2O5        0.04

molecular percent Silica 66%

Empirical Formula iron_satIron_Z2G_4_PAlSi :

K2O        0.33
Na2O        0.26
Li2O        0.21
CaO        0.16
MgO        0.04

Al2O3        0.82
Fe2O3        0.20

SiO2        5.09
P2O5        0.04

molecular percent Silica 71%

Remarks

The close-up pictures were taken with a digital microscope.

The oil spot glaze satIron_ZG_4PAlSi is dimpled and glossy black. It has various markings in addition to the
matte silvery oil spots. The texture of this glaze is intriguing for these other markings in addition to the oil spots.

As the spots are almost overshadowed by the glare off the dimples, it is hoped that the close-ups
show some of the additional markings in addition to a clearer image of the oil spots.

The glaze iron_satIron_ZG_4_PAlSi and iron_satIron_ZG_4 have nearly the same alkali metal:alkaline earth ratio of 4.
However, they have different balance of alkali metals.

I attribute the difference between these two glazes primarily to the increased alumina and silica.
Note that in the higher alumina/silica glaze the red color contributed by phosphorus is absent.

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