Neighbor glazes
firing to cone 10 in oxidation
The Downfire profile
A half hour hold at 1750 deg F
A three hour hold at 1700 deg F
slow downfire at 25 deg F an hour in the interval 1700 deg F to 1650 deg F
A one hour hold at 1650 deg F
Clay body is a grolleg porcelain from Tacoma Clay Art Center.
composition of the two glazes
These are both high alkali metal saturated iron glazes
Emperical Formula satIron_ZJ_1:
K2O 0.01
Na2O 0.52
Li2O 0.25
CaO 0.15
MgO 0.07
Al2O3 .52
Fe2O3 .22
SiO2 3.26
P2O5 .038
Emperical Formula satIron_ZJ_1PP:
K2O 0.01
Na2O 0.47
Li2O 0.23
CaO 0.23
MgO 0.06
Al2O3 .47
Fe2O3 .2
SiO2 2.94
P2O5 .067
back story
The intent had been to test only the glaze satIron_ZJ_1, however, a weighing error,
adding Bone ash twice leaving
me with the glaze satIron_ZJ_1PP.
As I had the weighed glaze,
with known composition, it was too much temptation to think I'd throw it away
rather than test, so that's what I did.
Note that the second glaze with 3 % extra bone ash differs slightly in
several oxides, most notably, higher P2O5,
though the extra CaO increased the
base (flux) content of the glaze, giving lower Alumina and Silica as well.
In the first firing the fired difference between these two glazes was
spectacular, and ought to have been a warning -
the compositional
difference is insufficient to warrant that huge a change.
Therefore
one concludes that both glazes are greatly influenced by details of
glaze application and firing conditions.
As it happens in the first
firing, these two glazes were next to each other on the same shelf in the
kiln.
I show here the two glazes in the first firing, and the second glaze in a
subsequent firing.
Both firings were in the same kiln, with the same
firing protocol, with near identical fired orton cones.
The Two glazes in the first firing:
satIron_ZJ_1 on a 3.5 inch diameter bowl
satIron_ZJ_1PP on a 3.5 inch diameter bowl
The secon glaze only in the second firing
satIron_ZJ_1PP on a plate 9 inches in diameter
commentary
The first glaze, satIron_ZJ_1 has a waxy matt texture, and is a mixture of various shades of brown with only hints of bright red and orange.
The second glaze, satIron_ZJ_1PP, with the extra bone ash, in the first firing
is a full gloss,
and where the glaze application is sufficiently thick,
is a fiery mix of yellows, oranges and reds.
This same glaze, in the second firing is a waxy gloss, where the glaze
application is thick has a lusterous metallic surface film,
the fiery
oranges are nearly gone, except on the upper rim
where the plate is
thin.