Alumina in Calico Glazes
Increased alumina suppresses the calico markings in the high zinc, low alkaline
earth glazes derived
The calico markings are a non-uniform mixture of distinct materials differing
both in color and texture,
I've tested in excess of one hundred calico glazes, and found that increasing
alumina suppresses the
Three glazes are shown here:
glaze 1 cooper_404_1Na_PZn_v5
Both glazes 2 and 3 have higher alumina than glaze 1, and diminished calico
markings.
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
300 deg F an hour to 1850 deg F then a 2 hr hold at 1850 deg F
300 deg F an hour to 1750 deg F then a 1 hr hold at 1750 deg F
300 deg F an hour to 1700 deg F then a 3 hr hold at 1700 deg F
25 deg F an hour to 1650 deg F then a 1 hour hold at 1650 deg F
K2O 0.07
Al2O3 0.33
SiO2 2.43
molecular percent Silica 64.7%
added:
1.5% Nickel Oxide
K2O 0.07
Al2O3 0.36
SiO2 2.4
molecular percent Silica 63.8%
added:
1.5% Nickel Oxide
K2O 0.06
Al2O3 0.36
SiO2 2.44
molecular percent Silica 64%
added:
1.5% Nickel Oxide
The first two glazes (cooper_404_1Na_PZn_v5, and cooper_404_1Na_PZn_PAl) are
dunted. The glaze
I do not know the cause yet can speculate. It is possible that the many
micro-crystals act as boulders,
from Hans Coopers glaze #404. I've characterized the
puzzle-like geometric markings in these glazes
as calico, which to me are
reminiscent of the fanciful floral calico fabrics of my youth.
i.e. separate phases within the glaze, analogous to
the state of oil and vinegar in salad dressing.
calico markings. Alumina at 0.35 molecular equivalents
seems an upper limit for prominent calico markings.
glaze 2 cooper_404_1Na_PZn_PAl
glaze 3 cooper_404_1Na_Z4E_0
Image of the glaze cooper_404_1Na_PZn_v5
inside
outside
the baseline glaze with lower alumina
Image of the glaze cooper_404_1Na_PZn_PAl
inside
Image of the glaze cooper_404_1Na_Z4E_0
inside
outside
bowls are ~3.5 inches in diameter
oxidation firing to cone 10 in an electric kiln
Firing profiles
Up Fire profile cone 10
Down Fire Profile cone 10
Clay body is a grolleg porcelain from Clay Art Center in Tacoma, WA.
glaze compositions
Empirical Formula cooper_404_1Na_PZn_v5:
Na2O 0.23
CaO 0.04
MgO 0.01
BaO 0.20
ZnO 0.45
Empirical Formula cooper_404_1Na_PZn_PAl:
Na2O 0.26
CaO 0.04
MgO 0.01
BaO 0.20
ZnO 0.42
Empirical Formula cooper_404_1Na_Z4E_0 :
Na2O 0.14
CaO 0.04
MgO 0.01
BaO 0.32
ZnO 0.43
Remarks
cooper_404_1Na_PZn_PAl broke apart into many pieces
and, for photographic purposes, is held together
by duct tape. The
transverse section (i.e., cross section), of the crack is revealed on the
pieces,
and is seen to be nearly entirely crystalline, rather than the
glassy surface one expects of a glaze.
preventing thermal contraction of the
cooling glaze.