karen gringhuis on mon 17 feb 03
To Garry and anyone else following this thread -
several points:
The 2/12 posting of "feldspar" analysis from Johnson
Matthey termed "frit" - this indeed looks like a frit,
NOT a spar. It also appears to contain a major error
in listing Ba2O3 28.46 - I think this should probably
be B2O3 BORON. (If it is in fact a high BARIUM frit,
bring it on and where can we get it?!!) With boron in
mind, one could test a sub of Ferro frits #3185 or
#3195.
The more basic problem with the orig. glaze is in the
oxides. It is way overloaded with Ti and Zn. Drop
the zinc to around 8%. Try a MAX of 2 -3% titanium
-10% is what's making the glaze muddy.
For blue, test with a combo of cobalt and chrome
starting with 0.25% of each increasing to 1% of each.
You could also add in 0.25% of an appropriate Mason
stain with some combo of cobalt and chrome.
Stay with a soda feldspar -- you could try half and
half neph. syenite and Kona F-4. For more breakup,
test with around 4% lithium.
Firing oxidation, keep in mind your rate of heating
and COOLING will significantly affect the final glaze
surface i.e. take it SLOW both ways. Before changing
the ingredients of the glaze, depending on how fast
your kiln cools, consider firing down to slow the
cooling to keep mattness.
=====
Karen Gringhuis
KG Pottery
Box 607 Alfred NY 14802
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