John Sankey on mon 14 may 07
Beginning with Hesselberth&Roy's High Calcium Semimatte Base 2
with Licorice colorants:
Wollastonite 25%
EPK Kaolin 25
Fusion Frit F2 20
Flint 15
Red Iron Oxide 8
Nepheline Syenite 3.5
Bentonite 1.8
Cobalt Carbonate 1.7
Four trials were run, with small sample mixes so they were
painted on rather than dipped. Adding 1/2% or 1% titanium oxide
decreased green where the glaze was thin, closer to black where
thick. Adding 2% changed the colour to uniform brown. Adding
1-1/2% manganese oxide changed the colour to black where thick,
brown where thin.
The manganese test shows, as I interpret it, that the iron was
being partially de-oxidized during firing. The titania tests
suggest that Weyl's theory about iron moving from
network-modifying to network forming role applies.
So, I plan the next two tests as:
1. add an additional 2% cobalt carbonate to the 2% titanium oxide
mix to see if that restores the blue absorption, and
2. add 1% manganese oxide to the 1% titanium oxide mix to see if
that converts the remaining green to brown without impacting
the cobalt blue.
Any other suggestions?
Gay Judson on tue 15 may 07
John, I'm very interested in your tests. Wish I could see the results. I am
especially interested in the last you mentioned: adding 1 1/2 percent
manganese dioxide (I am guessing that you meant dioxide and not oxide?). It
sounds like it is like a Tenmoku glaze--black breaking brown. Am I
interpreting it correctly?
Gay Judson
John Sankey on wed 16 may 07
Gay Judson wrote:
"John, I'm very interested in your tests. Wish I could see the results."
My progress blog with photos is at
http://sankey.ws/pottery2.html
Note: if you wish to answer me off list, you must include the
keyword 'Byrd' in the subject to get through my spam filter.)
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