Shifting Edges

A descendant of the glaze seen here .

The glaze shown in this note, iron_Alex_0PSiMAlk, is derived from iron_Alex_0. In its empirical formula,
some Na2O is replaced by MgO, and it has a bit more silica.

Several small changes, yet while iron_Alex_0PSiMAlk clearly shows its heritage, it's a noticeably different glaze.
It doesn't flow particularly, even if applied thickly. The background remains a full gloss,
and its color has shifted from midnight black to brown-black.

The major difference however, is in the appearance of the various crystals visible on the surface of both glazes.
On this glaze, iron_Alex_0PSiMAlk, the acicular surface crystals are thicker, and with a smaller diameter than in
iron_Alex_0. Because there are many more of these crystals, they cover a greater proportion of the surface.

The most prominent change in iron_Alex_0PSiMAlk is the replacement of the dusting of copper-colored micro-crystals
by matte orangey-red micro-crystals. Those scattered, brightly colored reddish micro crystals add an
almost garish look to this glaze.

Image of the glaze iron_Alex_0PSiMAlk

full view

inside

full view

outside

bowl is ~ 3.5 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 iron_Alex_0PSiMAlk :

K2O        0.07
Na2O        0.33
Li2O        0.15
CaO        0.33
MgO        0.12

Al2O3        0.57
Fe2O3        0.16

SiO2        3.31
P2O5        0.04

molecular percent Silica 65%

For reference the parent glaze:

Empirical Formula iron_Alex_0 :

K2O        0.08
Na2O        0.37
Li2O        0.14
CaO        0.32
MgO        0.09

Al2O3        0.57
Fe2O3        0.15

SiO2        3.0
P2O5        0.03

molecular percent Silica 63%

Remarks

The acicular crystals seen in this glaze are similar to those seen in satIron_P_Alex, and in iron_Alex_0,
from which it is derived. The appearance of these acicular crystals is different in each of these glazes.
Particularly, the thickness of the crystals and their diameter varies, as does the color.

In this glaze, iron_Alex_0PSiMAlk, the acicular crystals will not grow large independent of the glaze application thickness.
Higher silica and lower alkali metals both contribute to a more viscous melt. In a viscous melt, movement
of oxides within the glaze is slower, which in turn inhibits the size to which crystals can grow.

It is interesting that visible crystals can manage to grow in a glaze which flows as little as this one.

Perhaps the slight increase in P2O5 in iron_Alex_0PSiMAlk is partially responsible for the shift in colors.

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