Exploring in the vicinity of a saturated iron glaze, I encountered an oil spot
glaze. A classic oil spot, a glossy brownish black
The oil spot glaze, iron_satIron_Z2T_0, is derived from the saturated iron
glaze, iron_8_R_Plus_satIron_ZG, by dropping
The saturated iron glaze:
Inside of bowl:
Outside of bowl:
The oil spot glaze:
Inside of bowl:
Outside of bowl:
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
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
K2O 0.18
Al2O3 0.52
SiO2 3.25
molecular percent Silica 65.1%
K2O 0.23
Al2O3 0.66
SiO2 4.19
molecular percent Silica 68.4%
The explanation of the formation of oil spot glazes below is my condensed
version of that given
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
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)
I deduce from the above, that the first essential step in the formation of an
oil spot glaze, is the separation of the glaze
I conjecture that decreasing the alkali metals in the migration from the glaze
iron_8_R_Plus_satiron_ZG
background with
persimmon-colored disks.
the Lithium Carbonate from its
recipe.
The glazes
iron_8_R_Plus_satIron_ZG
bowl is ~4 inches in diameter
glaze iron_satIron_Z2T_0
bowl ~3 inches in diameter
oxidation firing to cone 10 in an electric kiln
Firing profiles
Up Fire profile
Down Fire Profile
Clay body is a grolleg porcelain from Tacoma Clay Art Center.
glaze compositions
Empirical Formula iron_8_R_Plus_satIron_ZG :
Na2O 0.34
Li2O 0.24
CaO 0.17
MgO 0.07
Fe2O3 0.18
P2O5 0.04
Empirical Formula iron_satIron_Z2T_0 :
Na2O 0.44
CaO 0.22
MgO 0.11
Fe2O3 0.22
P2O5 0.05
Remarks
in Nigel Wood's book "Chinese Glazes." He is not
responsible for mistakes in transmission.
thin iron rich layer within the molten glaze. The
remaining glaze is deficient in iron.
to give various iron-rich minerals.
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.
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.