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alkalinity and ^10 turquoise gloss

updated wed 27 feb 02


Mert & Holly Kilpatrick on tue 26 feb 02

Last year at an Ian Currie workshop at Gable Young's studio, we made several
^6 oxidation glaze test grids that were turquoise glazes. I would like to
get a workable one also, I need to continue working on these but haven't
taken the time. But the following C corner (lower left) recipe yielded some
rich bright turquoises.

Neph Sy 30
Frit 3134 30
Dolomite 15
Strontium Carb 25
Copper Carb 3%

I put a (poor, sorry) scan of the grid tile and two test tiles on my website

The colors are richer than it looks in the picture. This is a high
strontium glaze.

The two I tested on tiles were glaze 12 and 29. To find them on the grid
you count left to right, starting in the top left (A) corner. The grid
increases the alumina going from bottom to top, and increases the silica
going left to right. Therefore, glaze 12 is low silica and glaze 29 is low
alumina. So these are not John's balanced glazes for functional ware, but
could be used on the nonfood surfaces. 29 is very glossy and 12 is more
towards satin. Standing to reason based on their composition.

When I tried them on test cups I think I glazed too thin, because they are
too transparent. If you notice specs in glaze 12 it is because the Sr
needed more sieving.

One interesting thing is that the area of more balanced glazes, the diagonal
trough rising in the center from left to right, is not turquoise, it is dark
green. Why is that?

Glaze 12:
Nepheline syenite 19.5%
Frit 3134 19.5%
Dolomite 9.8%
Strontium carbonate 16.3%
EPK 24.2%
Flint 10.8%

Copper carbonate 3.0%
On test tile, Turquoise satin with lighter feathering, some tiny blisters.
Chemical Analysis

Na2O 0.15 Al2O3 0.33 SiO2 1.68
K2O 0.03
B2O3 0.15
MgO 0.12
CaO 0.29
SrO 0.26

Alumina:Silica ratio is 1.00 : 5.09
Alkali:Neutral:Acid ratio is 1.00 : 0.33 : 1.68

Glaze 29:
Nepheline syenite 17.7%
Frit 3134 17.7%
Dolomite 8.8%
Strontium carbonate 14.7%
EPK 4.8%
Flint 36.3%

Copper carbonate 3.0%
On test tile, translucent to clear gloss with turquoise opalescence.
Chemical Analysis
Na2O 0.15 Al2O3 0.16 SiO2 2.48
K2O 0.03
B2O3 0.15
MgO 0.12
CaO 0.29
SrO 0.26

Alumina:Silica ratio is 1.00 : 15.68
Alkali:Neutral:Acid ratio is 1.00 : 0.16 : 2.48

in Pennsylvania

> on 2/26/02 7:57 AM, Christena Schafale at christenas@RFSNC.ORG wrote:
> > At risk of being wrong -- What I have understood from reading and a few
> > tests is that copper turquoise and crazing are inseparable partners. If
> > you decrease the high-expansion fluxes enough to get rid of crazing,
> > you lose the turquoise color and get a green instead.
> John Hesselberth" wrote:
> At cone 10 I think you may be right, but at lower cones it may be
> There you can use boron to offset the high expansion of Na and K. One of
> the glazes that will be published in Clay Times in the issue about to be
> released is a cone 04 turquoise and, at least on a calculated basis, it
> not have high expansion.
> I think I will try to develop one and see if I can get there at cone 6.
> John