Firing temperature vs. Time
I show the effect of more time above cone 5 together with lower firing temperature
on a saturated iron glaze.
Both firings are cone 10, with nearly identical cones.
The first firing is fast to 2310 deg F, the second firing is slower but only
to 2270 deg F.
Faster firing to cone 10 at 2310 deg F.
Inside of pot:
Outside of pot:
Slower firing to cone 10 at 2270 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, 2270 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 1600 deg F then a one hour hold at 1600 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 2170 deg F
15 deg F an hour to 2270 deg F / hold 20 min
300 deg F an hour to 1850 deg F then hold 3 hours
300 deg F an hour to 1600 deg F then a Three hour hold at 1600 deg F
The hold at 1850 deg F is to promote micro-crystals in Hank Murrows satin
Celedon glaze.
The hold at 1600 deg F, is my usual for the metallic microcrystals in my
saturated iron glazes.
K2O .06
Al2O3 .53
SiO2 3.28
molecular percent Silica 63%
The first pair of pictures shows the result of an iron-saturated glaze from my
usual firing profile. The result is a waxy matte glaze
The second pair of pictures shows the variant firing, to a lower temperature
and with a slower ramp. The result is a full gloss dark brown,
The hold at 1600 deg F is present as required by those glazes for the
development of the metallic micro-crystals.
I designed the second slow/low firing to test Hank Murrow's satin Celedon
glaze. Nigel Wood, in his book Chineese Glazes,
Saturated iron glazes also develop metallic micro-crystals, but of a different
species.
Initial results indicate this slower lower temperature firing has a great
impact on saturated iron glazes.
Next is an even slower ramp to cone 10, with a lower final firing temperature.
I am greatly indebted to Hank Murrow for his generosity in sharing his glazes
and knowledge of firing.
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_1S_1Munge :
Na2O .51
Li2O .21
CaO .21
MgO .01
Fe2O3 .19
P2O5 .16
bowls are ~ 4 inches in diameter
Remarks
that has a near dense
coverage of metallic micro-crystals. It's orange-y rather than brown.
with no
metallic micro-crystals. I attribute the lack of these metallic micro-crystals
to the lower final firing temperature,
though the slower temperature ramp
may be relevant as well. The glaze is fully mature, the visible pitting
on the outside
of this pot is the result of a thin application of a viscous
glaze.
remarks that celedon
glazes fired too hot won't develop the crystals that give a satin surface
texture. This firing was designed
to test that by having a lower final
temperature. In order to reach cone 10 at a lower temperature, this
firing
has a slower temperature ramp between cone 5 and cone 10.
I wondered about the effect of a slower/lower temperature
firing on iron-saturated glazes.
Several of my regular saturated iron
glazes were in this firing for that reason.
Yet there was a only a small
non-impressive effect on the non iron bearing glazes in that slow/low firing.