Down Firing

I show the effect of a long, slow down-ramp that begins slightly below the final firing temperature of 2310 deg F.
This is a cone 10 firing. The slow down-ramp begins at 2210 deg F, which is 100 deg F below the final firing temperature.
A slow down-ramp that begins 25 deg F below the final firing temperature will result in a cone 11 firing. It is possible
that beginning the slow down-ramp somewhere between 25 deg F and 100 deg F will also produce a cone 10 firing,
but this is yet to be determined.

The glaze I am showing here, sh_1I, is 75% Imerys Mica, which is primarily Mica, with some odd bits
of other things thrown in. Mica is slow melting.

Image of the glaze sh_Z1I in Firing 1

full view

inside

full view

outside

bowl is ~ 4 inches in diameter.

Image of the glaze sh_Z1I in Firing 2

full view

inside

full view

outside

bowl is ~ 4 inches in diameter.

oxidation firing to cone 10 in an electric kiln

Firing profile 1

Up Fire profile

150 deg F an hour to 250 deg F

400 deg F an hour to 2050 deg F / hold 20 min

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 2210 deg F

80 deg F an hour to 1900 deg F

300 deg F an hour to 1750 deg F then a 30 minute hold at 1750 deg F.

300 deg F an hour to 1700 deg F then a two hour hold at 1700 deg F

300 deg F an hour to 1600 deg F then a two hour hold at 1600 deg F

Firing profile 2

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 of bowl in first firing is a an english grolleg from tacoma clay art center.
clay body of bowl in second firing is mixed english grolleg and some B-Mix.

glaze compositions

Empirical Formula sh_Z1I :

K2O        0.26
Na2O       0.03
Li2O       0.49
CaO        0.18
MgO        0.04

Al2O3      1.02
Fe2O3      0.04

SiO2       2.5
P2O5       0.05

molecular percent Silica 54%

Remarks

The glaze in firing 1, with the slow ramp near top temperature, is thoroughly melted, has visible sheen,
and is translucent where thin. By contrast, this glaze in firing 2 isn't matured sufficiently to have formed an interfacial bond with
the clay body. The result of this, as seen in the pictures, is that the glaze is flacking off of
the upper rim of the bowl.

I've previously had the experience that glazes high in Mica mature on the way down. I'd earlier used coarse
60 Mesh Mica and had thought that was the cause of the slow melting. Now I am using 300 mesh Mica,
and again seeing that effect.

Clearly if a glaze is sufficiently slow in melting, the longer it remains at a high temperature, the more likely it is to mature.

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