Cobalt in a glaze containing Rutile

A varigated cobalt glaze with an inlay

A closeup of the background glaze and several closeups of the interaction region are shown and, for context, a picture of the pot.

The background glaze is paperWhite_ZV_3_Z2H; the foreground design is glazed with mashiko_ZJ_1_N. paperWhite_ZV_3_Z2H contains cobalt.

The design is created by adhering a mask and then applying the main glaze. After drying, the entire pot is coated in wax,
then the mask is removed and the open area filled in with the design inlay glaze.

Close up images of the background and interaction region

full view

Image of the piece

full view

bowl with background glaze paperWhite_ZV_3_Z2H

and inlay mashiko_ZJ_1_N

bowl is ~7 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

Background glaze

Empirical Formula paperWhite_ZV_3_Z2H :

K2O        0.05
Na2O        0.15
Li2O        0.20
CaO        0.02
MgO        0.02
BaO        0.28
SrO        0.28

Al2O3        0.6

SiO2        3.2
TiO2        0.16

molecular percent Silica 64.6%

Added:
1% cobalt carbonate

Inlay Glazes

Empirical Formula mashiko_ZJ_1_N :

K2O        0.14
Na2O        0.14
Li2O        0.12
CaO        0.20
MgO        0.40

Al2O3        0.82
Fe2O3        0.23

SiO2        5.73

molecular percent Silica 73.6%

Remarks

The background glaze with no additions is multiphase. It is comprised of a translucent waxy matte matrix
with islands of a dry white micro-crystalline phase.

The cobalt colors the two phases differently. The background is a classic cobalt blue. Rutile, in a glaze containing cobalt,
usually provides a green tint. The only green here is in the second opaque phase, which is an intense yellow green
as a result of the concentration of the rutile in a small fraction of the glaze.

That the concentration of the rutile is higher in the micro-crystalline matte phase, is shown by the coloring effect of cobalt.

Both glazes are viscous and have high surface tension, so they react minimally with each other. When melted,
they flow together retaining their individual identity.

The surface of the inlay cooled before the bulk, forming a skin of fixed size. The bulk solidified at a lower temperature,
with a smaller volume, causing the wrinkling of the skin.

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