Firing time vs. temperature
I show the effect of a slower colder firing, and more time in the approach
to Cone 10, with a lower firing temperature on a saturated iron glaze.
Previously I'd shown that contrast
Now I show a different glaze in two cone 10 firings.
The first firing is to 2310 deg F,
faster firing to cone 10 at 2310 deg F.
Inside of pot:
slower firing to cone 10 at 2230 deg F
Inside of pot:
Outside of pot:
The differences between the first and second upfire profiles are:
The top temperature, 2310 deg F for the first, 2230 deg F for the second.
The temperature ramp is steeper for the first firing, slower for the second.
Faster and hotter:
150 deg F an hour to 200 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
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
Slower and cooler
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 2150 deg F
13 deg F an hour to 2200 deg F / hold 1 hr
13 deg F an hour to 2230 deg F / hold 1 hr
300 deg F an hour to 1900 deg F then hold 2 hours
300 deg F an hour to 1850 deg F then hold 2 hours
K2O .10
Al2O3 .66
SiO2 3.21
molecular percent Silica 63%
This glaze in my usual firing is a full gloss, and has a near dense coverage
of golden metallic microcrystals. It's a purplish black
In the variant firing, to a lower temperature with a slower ramp, it is a full
gloss, dark brown, with an adventurine sparkle,
I attribute the lack of metallic micro-crystals to the lower final firing
temperature, although the
Next is an even slower ramp to cone 10, with an even lower final firing
temperature.
the second slower but only to 2230 deg F.
First Firing
Second Firing
oxidation firing to cone 10 in an electric kiln
Firing profiles
Up Fire profile 1
Down Fire Profile 1
Up Fire profile 2
Down Fire Profile 2
Clay body is a grolleg porcelain from Tacoma Clay Art Center.
glaze composition
Empirical Formula glaze satIron_ZG_H_2 :
Na2O .50
Li2O .20
CaO .14
MgO .06
Fe2O3 .21
P2O5 .04
bowls are ~ 4 inches in diameter
Remarks
in the interstices
between the micro-crystals, and also in the bottom of the bowl, where the
glaze is thickly applied.
This is seen in the first picture.
and has no metallic
microcrystals. As seen in the second pair of pictures.
slower temperature ramp may be relevant as well.