cobalt colors again

Here is one more glaze, zincSr_Z3U_2, in three firings; my regular firing, and two "invert" firings, which drop,
then come back up.

A related glaze in these firings is seen here .

All three firings have the same upfire protocol.

Firing 1:

The downfire is my regular down firing, with holds at 1850 degF, 1750 degF, 1700 degF and 1650 degF.

Firing 2:

The downfire drops with a hold at 1650 degF, then rises back up with a hold at 1850 degF, then back down
with a hold at 1700 deg F.

Firing 3:

The downfire drops with a hold at 1550 degF, then rises to 1650 with a hold, rises to 1750 with a hold,
and finally rises to 1900 degF with a hold, and that's it.

The reasoning behind inversion of the order of the downfire holds is discussed here .

The design is created by adhering a mask and then applying the main glaze. After drying,
the mask is removed and the open area filled in with a second glaze, using a bulb syringe
with a needle applicator.

Image of the glaze zincSr_Z3U_2 in Firing 1

full view

Image of the glaze zincSr_Z3U_2 in Firing 2

full view

Image of the glaze zincSr_Z3U_2 in Firing 3

full view

bowls are ~6 inches in diameter.

oxidation firing to cone 10 in an electric kiln

Firing profiles

Firing 1 profile

Up Fire profile cone 10

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 cone 10

300 deg F an hour to 1850 deg F then a 2 hr hold at 1850 deg F

300 deg F an hour to 1750 deg F then a 1 hr hold at 1750 deg F

300 deg F an hour to 1700 deg F then a 3 hr hold at 1700 deg F

25 deg F an hour to 1650 deg F then a one hour hold at 1650 deg F

Firing 2 profile

Up Fire profile cone 10

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 cone 10

300 deg F an hour to 1650 deg F then a 3 hr hold at 1650 deg F

300 deg F an hour to 1850 deg F then a 2 hr hold at 1850 deg F

300 deg F an hour to 1700 deg F then a 2 hr hold at 1700 deg F

Firing 3 profile

Up Fire profile cone 10

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 cone 10

300 deg F an hour to 1550 deg F then a 1 hr hold at 1550 deg F

300 deg F an hour to 1650 deg F then a 1 hr hold at 1650 deg F

400 deg F an hour to 1750 deg F then a 2 hr hold at 1750 deg F

400 deg F an hour to 1900 deg F then a 3 hr hold at 1900 deg F

Clay body is a grolleg porcelain from Clay Art Center in Tacoma, WA.

glaze composition

Empirical Formula zincSr_Z3U_2:

K2O        0.15
Na2O       0.32
CaO        0.01
MgO        0.18
SrO        0.15
ZnO        0.19

Al2O3      0.52

SiO2       3.24

molecular percent Silica 67.9%

Added:

2.5% Cobalt Carbonate

Inlay Glaze:

Empirical Formula inlay hiAlk_Z3P_Z3V_0:

K2O        0.18
Na2O       0.41
CaO        0.31
MgO        0.10

Al2O3      0.64

SiO2       3.54
P2O5       0.07
TiO2       0.21

molecular percent Silica 64.9%

Remarks

The pitting seen in the second and third bowl is the result of an overly thick glaze application.

The reversal of the temperature of the holds has had a strking effect. The third firing, in which
there is one drop to 1550 degF with a hold, followed by successive increases in temperature, has had the most
pronounced effect. This is seen in the interaction region between the background glaze
and the inlay glaze. The Titanium in the inlay glaze surely contributes to this as the strong effect of
an inverted firing on titanium is seen here .

The size of the micro-crystalline clusters in the background glaze increased in both "invert" firings,
and more in the third firing (second invert firing).

Perhaps the significant change in the glaze appearance that occurs when the holds are inverted is primarily
due to the level of zinc in the background glaze.

That the zinc is strongly involved is suggested by the results seen here ,
where the second glaze, zincSr_Z3W_1, with the least zinc, shows the least difference in appearace
between the regular firing and the "invert" firing.

Carol's Home Page