The effect of distinct Firings on a Glaze
A glaze seen in two distinct firing protocols.
Both firings are to Cone 10 in an electric Kiln, though the first
firing is a hot cone 10,
with the cone 11 slightly more than tipped.
The first firing is slowed on the way up with an hour hold at 2170 deg F, and
an additional half-hour hold at
2270 deg F. In addition, it has a slower
temperature rise for the last segments of the up part of the firing.
The intent had been to increase the time available for slower reacting
materials to enter the glaze melt,
without firing to a higher cone. This
result was only partially achieved.
It is interesting that the result of firing the glaze longer seems to have had
the effect of preventing the
development of the coppery colored metallic
crystals seen in the second firing.
The holds on the way down in the first firing differ as well, an artifact of
the inconvenience of programming
more than 8 ramps for the kiln
controller. Perhaps the differing holds on the way down, rather than the
extra time at the top is the cause of the loss of the features. I doubt this
as those crystals did appear in
other glazes in that first firing. I
conjecture that the neucli, "seeds" for the crystal growth did not
form
in that first firing.
Image of the glaze satIron_ZG_0_PK_MLi in Firing One
section of glaze without distracting decoration
Image of the glaze satIron_ZG_0_PK_MLi in Firing Two
inside
outside
bowl is ~ 4 inches in diameter.
oxidation firing to cone 10 in an electric kiln
Firing profiles
First Firing
Up Fire profile Firing 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 2170 deg F with a hold of 1 hour at 2170 deg F
120 deg F an hour to 2250 deg F
60 deg F an hour to 2270 deg F with a hold of 30 minutes at 2270 deg F
60 deg F an hour to 2310 deg F with a hold of 20 minutes at 2310 deg F
Down Fire Profile Firing 1
300 deg F an hour to 1750 deg F then a 4 hour hold at 1750 deg F
25 deg F an hour to 1650 deg F then a one hour hold at 1650 deg F
Second Firing
Up Fire profile Firing 2
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 Firing 2
300 deg F an hour to 1800 deg F then a 2 hr hold at 1800 deg F
300 deg F an hour to 1750 deg F then a 1 hr hold at 1750 deg F
50 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 satIron_ZG_0_PK_MLi :
K2O 0.12
Na2O 0.69
CaO 0.18
MgO 0.01
Al2O3 0.64
Fe2O3 0.22
SiO2 3.23
P2O5 0.10
molecular percent Silica 62.2%