Firing

cone 10 oxidation

One glaze satIron_ZG_0 seen in two different firings.

One way to distroy a fabulous glaze with an ill conceived firing.

The two firings:

Firing profile One

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 2310 deg F with a hold of 20 minutes at 2310 deg F

Down Fire Profile

Two hour hold at 1850 deg F

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 half hour hold at 1650 deg F

Firing profile Two

Up Fire profile

150 deg F an hour to 250 deg F

400 deg F an hour to 2100 deg F

120 deg F an hour to 2240 deg F

60 deg F an hour to 2270 deg F with a 20 minute hold at 2270 deg F

30 deg F an hour to 2300 deg F with a hold of 30 minutes at 2300 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

The cones show the first firing as cone 10, the second as cone 11. There is a crossover , in that the second firing
has a lower cut off temperature, yet reaches a higher cone.

The purpose of this second firing had been to slow the firing near the top temperature, the lower firing temperature
was chosen to prevent a higher cone falling, as a result of the extra time near the top of the firing.

This firing was designed for different glazes, this glaze was a control to verify the behavior of a favorite glaze
of a different type in this firing.

The difference in these two firings is primarily in the altered up firing.

This glaze is high in alkali metals, so that once it starts to melt, it goes fast, not withstanding that it is relatively high in Alumina.
The extra time near the final firing temperature caused this glaze to
"flow like water", therby thinning to the point where crystals did not form, nor grow.

For this glaze, a slower upfiring overly melts the glaze at a temperature which does not allow its full development.

Nigel Wood in "Chinese Glazes" notes that Magnesium Oxide rather abruptly becomes a powerful flux and brings many
chineese glazes to life when it does. This glaze has some magnesium, enough for the slightly lower final temperature to be felt by this glaze.

Clay body is a grolleg porcelain from Tacoma Clay Art Center.

The pasta bowl in the first firing shows a more varied pattern of micro-crystalline growth.

glaze composition

Emperical Formula satIron_ZG_0:

K2O        0.1
Na2O        0.49
Li2O        0.19
CaO        0.16
MgO        0.06

Al2O3        .59
Fe2O3        .2

SiO2        3.14
P2O5        .04

molecular percent Silica 63 %

The pasta bowl is ~ 9 inches in diameter. The deap bowl shown second is ~8 inches in diameter.

There is a leaf print visible at the lower left of the pasta bowl, a leaf is embedded in the wet glaze, waxed with an emulsion wax, removed, and the resulting image stained with a contrast glaze.

The markings on the outside of the deap bowl are a second glaze inlay. The glaze is filled in to an area that had been masked prior to
the pouring of first layer of glaze. It was then waxed with an emulsion wax prior to
stripping the mask.

pasta bowl with firing One

Note the rich growth of a variety of surface crystals, with a hint of luster. Note also that the glaze layer is thick
and shows a few pinholes as a result.

inside of deap bowl with firing Two

outside of deap bowl with firing two

Note that the glaze has run through the inlaid design. The glaze is thin. At the top of both the inside and outside
one sees a large border with no metallic crystals. Below that one sees a scattering of crystals, which are nowhere
massed as they are on the first pasta bowl.

It is possible that a lower cone achieved with a higher faster firing produces turbulance within the
melting glaze that promotes random varied effects. That is speculation.