Firing time vs. temperature
I show the effect of a slower, colder firing and more time in the approach
to Cone 10, on a semi-matte iron free glaze.
Previously I'd shown that contrast on a saturated iron glaze
Now I show a semi-matte iron free 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:
Outside 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 .06
Al2O3 .43
SiO2 2.7
molecular percent Silica 66%
This glaze in my usual firing is a textured gloss. It is translucent,
rather than transparent as a result of its textured surface.
In the variant firing, to a lower temperature with a slower ramp, it is a
Perhaps as suggested by Nigel Wood in "Chinese Ceramics", the nuclei for
micro-crystal growth are melted in the higher temperature firing.
Is the difference seen in the glaze primarily the result of the altered final
firing temperature, or a slower firing? To find out
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 hiCa_Z1Z_2 :
Na2O .08
CaO .6
MgO .26
bowls are ~ 4 inches in diameter
Remarks
Careful
inspection with a loupe fails to find surface crystals or opaque particles within
the glaze.
This is seen in the first pair of pictures.
matte glaze with a soft, lusterous surface and has a dusting of white
matte particles, as if dusted in snow.
This is seen in the second pair of
pictures, most clearly in the picture of its outside.
Alternatively a slower firing under non-equilibrium conditions produces a
different, more homogenized melt.
Phases that might form at a higher
tempearture don't form. The chemical components of the melt are determined
by
the conditions under which it is formed.
it will be necessary
to fire to the same temperature with two different firing speeds.