zinc anomaly

Here are a family of high zinc glazes derived from the glaze #404 in Emanuel Coopers book
"The Potters Book of Glazes".

The first five glazes are the successive steps in the line blend between the first glaze
cooper_404_1Na_Z4A and cooper_404_1Na_Z4A_2. The first, middle and last of these glazes
are seen here:

The tween glazes, #2 and #4, were tested in an effort to understand the dunting of the middle
glaze cooper_404_1Na_PZn.

As these are high zinc glazes and it is commonly thought that zinc causes dunting, the last three
glazes shown, cooper_404_1Na_Z4A_MZn, cooper_404_1Na_Z4A_2_MZn, and
cooper_404_1Na_PMZn are the three glazes from the prior note, each with less ZnO. None of those
glazes dunted.

However, the empirical Formulae for all of these glazes are similar, so I can't conclude that ZnO
is the cause of the dunting from the work shown here, though I certainly assumed so earlier.

It is seen that lowering the ZnO shifts the colors toward yellow from blue.

Here I give the molecular percent of ZnO in these glazes, and it is seen that these vary little
as well. This gives the fraction of ZnO as a function of total oxide composition, rather than only
as a fraction of the composition of the first column oxides. It didn't clarify things.

molecular percent ZnO these glazes

cooper_404_1Na_Z4A 11.86%

cooper_404_1Na_Z4A_2_1 11.19%

cooper_404_1Na_PZn 11.5%

cooper_404_1Na_Z4A_1_2 11.04%

cooper_404_1Na_Z4A_2 11.13%

cooper_404_1Na_Z4A_MZn 10.07%

cooper_404_1Na_Z4A_2_MZn 10.18%

outside

bowls is ~6 inches in diameter

except otherwise noted bowls are ~3.5 inches in diameter

oxidation firing to cone 10 in an electric kiln

Firing profiles

Up Fire profile cone 10

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 cone 10

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

Clay body is a grolleg porcelain from Clay Art Center in Tacoma, WA.

glaze compositions

Empirical Formula cooper_404_1Na_Z4A:

K2O        0.07
Na2O       0.27
CaO        0.04
MgO        0.01
BaO        0.18
ZnO        0.43

Al2O3      0.31

SiO2       2.34

molecular percent Silica 64%

added:

1.5% Nickel Oxide

Empirical Formula cooper_404_1Na_Z4A_2_1 :

K2O        0.07
Na2O       0.27
CaO        0.04
MgO        0.01
BaO        0.19
ZnO        0.42

Al2O3      0.32

SiO2       2.39

molecular percent Silica 64.5%

added:

1.5% Nickel Oxide

Empirical Formula cooper_404_1Na_PZn :

K2O        0.07
Na2O       0.26
CaO        0.04
MgO        0.01
BaO        0.20
ZnO        0.42

Al2O3      0.32

SiO2       2.37

molecular percent Silica 64.2%

added:

1.5% Nickel Oxide

Empirical Formula cooper_404_1Na_Z4A_1_2 :

K2O        0.07
Na2O       0.26
CaO        0.04
MgO        0.01
BaO        0.20
ZnO        0.42

Al2O3      0.33

SiO2       2.42

molecular percent Silica 64.5%

added:

1.5% Nickel Oxide

Empirical Formula cooper_404_1Na_Z4A_2:

K2O        0.07
Na2O       0.25
CaO        0.04
MgO        0.01
BaO        0.21
ZnO        0.41

Al2O3      0.33

SiO2       2.39

molecular percent Silica 64.2%

added:

1.5% Nickel Oxide

Empirical Formula cooper_404_1Na_Z4A_MZn :

K2O        0.07
Na2O       0.29
CaO        0.04
MgO        0.01
BaO        0.20
ZnO        0.39

Al2O3      0.34

SiO2       2.49

molecular percent Silica 65.1%

added:

1.5% Nickel Oxide

Empirical Formula cooper_404_1Na_Z4A_2_MZn :

K2O        0.07
Na2O       0.26
CaO        0.05
MgO        0.01
BaO        0.22
ZnO        0.39

Al2O3      0.34

SiO2       2.48

molecular percent Silica 64.8%

added:

1.5% Nickel Oxide

Empirical Formula cooper_404_1Na_PMZn :

K2O        0.07
Na2O       0.27
CaO        0.04
MgO        0.01
BaO        0.22
ZnO        0.39

Al2O3      0.33

SiO2       2.47

molecular percent Silica 65%

added:

1.5% Nickel Oxide

Remarks

The magnitude of the changes seen in these glazes (e.g. color, viscosity, and tendency to dunt) is large
compared to the differences in their oxide compositions. I conclude that all of these glazes are near
a singular point in the phase space to which all these glazes belong.

To get a stable glaze, it is necessary to get some distance from that singular point. That is, a glaze
with an exact eutectic composition is like a top on its point, and will fall one way or the
other easily.

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