Silvery Oil Spot Glaze
We show two variants, mashiko_ZJ_0 and mashiko_ZJ_1, of silvery oil spot
glazes. As these glazes are thickness dependant, we show thick and thin glaze
for both of these glazes on similar bowls. In each case the glaze is thicker
on the inside. We also show mashiko_ZJ_1 on a plate with a black oil spot
inlay glaze decoration.
cone 10 oxidation
Firing profile
Up Fire profile
150 deg F an hour to 250 deg F
400 deg F an hour to 2050 deg F
120 deg F an hour to 2250 deg F
60 deg F an hour to 2310 deg F with a hold of 20 minutes at 2310 deg F
Down Fire Profile
A half hour hold at 1750 deg F
A three hour hold at 1700 deg F
slow downfire at 25 deg F an hour in the interval 1700 deg F to 1650 deg F
A one hour hold at 1650 deg F
Clay body is a grolleg porcelain from Tacoma Clay Art Center.
The glaze mashiko_ZJ_0, and mashiko_ZJ_1 were seen here:
and mashiko_ZJ_1 here:
glaze composition
mashiko_ZJ_0 and mashiko_ZJ_1 are high alumina, moderate silica, saturated iron Glazes:
Emperical Formula mashiko_ZJ_0
K2O 0.12
Na2O 0.1
Li2O 0.1
CaO 0.26
MgO 0.42
Al2O3 .82
Fe2O3 .23
SiO2 5.68
Emperical Formula mashiko_ZJ_1
K2O 0.14
Na2O 0.14
Li2O 0.12
CaO 0.2
MgO 0.4
Al2O3 .82
Fe2O3 .23
SiO2 5.73
discussion of composition
The glaze mashiko_ZJ_0 is observed to have lower viscosity, and is a
more fusible glaze. It does have slightly lower silica,
but seems to
have substantially lower Alkali metals, which one would have expected to give
a less fusible glaze,
yet it is the opposite.
Both bowls are ~5 inches in diameter, the plate is ~9 inches in diameter.
The inlay second glaze design in the plate is produced by first masking the
design on the bisque. the inside of the pot is then glazed.
Then an
emulsion wax is brushed over both glaze and design. When the wax is
dry,
the design is peeled off, and the second glaze applied to the now
bare bisque with an ear syringe.
Glaze mashiko_ZJ_0
inside of bowl with thick glaze application
outside of bowl with thin glaze application
Glaze mashiko_ZJ_1
inside of bowl with thick glaze application
outside of bowl with thin glaze application
plate with inlay decoration
Commentary
Where glaze is thick, both these glazes show dimpling at the oil spots, though mashiko_ZJ_1 shows stronger dimpling than mashiko_ZJ_0.
Where the glaze is thin, both glazes migrate toward a transparent brown,
loosing the metallic luster and eventually the oil spots.
mashiko_ZJ_0 becomes browner and thinner for a given thickness.
These are both stiff/viscous glazes, so that a thick application leads to dimpling where the eruptions that created the oil spots haven't healed.
If Too thick these glazes show dimpling, however if thin, these glazes loose their metallic luster.
mashiko_ZJ_1 is more viscous, so dimples more substantially, and is correspondingly more resistant to loss of metallic surface luster.