Oil Spot

Exploring in the vicinity of a saturated iron glaze, I encountered an oil spot glaze. A classic oil spot, a glossy brownish black
background with persimmon-colored disks.

The oil spot glaze, iron_satIron_Z2T_0, is derived from the saturated iron glaze, iron_8_R_Plus_satIron_ZG, by dropping
the Lithium Carbonate from its recipe.

The glazes

iron_8_R_Plus_satIron_ZG

The saturated iron glaze:

full view

Inside of bowl:

full view

Outside of bowl:

bowl is ~4 inches in diameter



glaze iron_satIron_Z2T_0

The oil spot glaze:

full view

Inside of bowl:

full view

Outside of bowl:

bowl ~3 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_8_R_Plus_satIron_ZG :

K2O        0.18
Na2O        0.34
Li2O        0.24
CaO        0.17
MgO        0.07

Al2O3        0.52
Fe2O3        0.18

SiO2        3.25
P2O5        0.04

molecular percent Silica 65.1%



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
P2O5        0.05

molecular percent Silica 68.4%



Remarks

The explanation of the formation of oil spot glazes below is my condensed version of that given
in Nigel Wood's book "Chinese Glazes." He is not responsible for mistakes in transmission.

Oil spot glaze forms in a series of steps:

After the glaze has melted, a high-iron second phase forms, droplets of which coalesce to form a
thin iron rich layer within the molten glaze. The remaining glaze is deficient in iron.

Bubbles rising through the maturing glaze carry some of the iron-rich layer to the glaze surface.

Dissolved iron oxides in these iron-rich surface spots crystallized out at high temperatures (and in cooling)
to give various iron-rich minerals.

I deduce from the above, that the first essential step in the formation of an oil spot glaze, is the separation of the glaze
into two distinct phases. Both of these phases are liquid, one of which is iron-rich, the other iron poor. This is
distinguished from the more common form of phase separation, where a solid second phase forms by crystallizing
out of the melt.

I conjecture that decreasing the alkali metals in the migration from the glaze iron_8_R_Plus_satiron_ZG
to iron_satIron_Z2T_0 caused the 'unmixing' of the later glaze. This is the liquid-liquid phase separation,
described above, that is required as the first step of the formation of an oil spot glaze.

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