Firing temperature
I show the effect of decreasing the heat work on a glaze with metallic surface crystals.
The first firing is a hot cone 10 with cone 11 tipped and almost at 10 oclock.
The second firing is a middle cone 10.
The look of my cones had migrated. This is an attempt to recover my original firing.
Movement within the glaze at the temperature at which crystals form can interfere with the crystal growth.
The crystal growth in the bottom of the bowl fired to the higher temperature
is impressive. The glaze on that pot is seen to be
thin on the side walls
which show little crystal formation. That suggests that the deficit of
crystals on the higher fired pot
are the result of the greater heat work.
oxidation firing to cone 10 in an electric kiln
Firing profiles
The difference between the first and second upfire profiles is the top temperature, 2310 deg F for the first, 2300 deg F for the second.
Up Fire profile 1
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
There are two distinct down-fire protocols, given here:
There are two distinct down-fire protocols. Both had distinctive holds below
1700 deg F. This information is included for completeness,
but I do not
consider the differences below 1700 deg F relevant for this glaze.
Down Fire Profile 1
300 deg F hr to 2210 deg F then hold 30 minutes
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 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
Down Fire Profile 2
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 is a grolleg porcelain from Tacoma Clay Art Center.
glaze composition
Empirical Formula glaze satIron_ZG_4 :
K2O .10
Na2O .50
Li2O .18
CaO .16
MgO .06
Al2O3 .56
Fe2O3 .20
SiO2 3.53
P2O5 .04
molecular percent Silica 67%
First Firing
Hot cone 10, nearly cone 11
Inside of pot:
Outside of pot:
Second Firing
lower top temperature
Inside of pot:
Outside of pot:
bowls are respectively ~6 inches and ~ 4 inches in diameter
Remarks
The glaze that was fired to a lower top temperature has a darker background in
addition to showing more metallic crystals. The outside shows
impressive
crystal formation only at the bottom of large drips. Perhaps overly generous
glaze application allowed excess movement in the glaze at crystal
growth
temperature.
The pot fired to a higher temperature shows more crystals where the glaze is
thicker, both on the inside bottom, on the foot drips, and on
the upper part
of the outside wall.