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devitrification of copper bearing glaze

updated sun 10 feb 02


iandol on sun 10 feb 02

Dear June,
So far I have not answered your query about the gungy appearance of the =
devitrified areas on your pots which went a dull grey instead of =
maintaining the bright blue green celadon hue.
The clue comes from W.A. Weyl who comments information given by F. =
Fouqu=E9. Fouqu=E9 isolated a double silicate by reacting Copper oxide, =
Calcium carbonate and Silica which formed into crystals which analysed =
as CaO.CuO.4SiO2. These could be identified by their peculiar property =
of being green when viewed from one direction and red from another, a =
quality called "Di-Chroism"
As you already know, red and green are complementary colours which, when =
mixed as pigments, mutually eliminate each other, becoming effectively =
black. I speculate that you have managed to make these in your glaze. It =
is possible they they are the least soluble of the substances in a fully =
molten silicate which might separate out during cooling.
Since you seem to enjoy translucent mat glazes here is a GIF file of a =
square blend tile by a separate posting, one of a series of three I am =
experimenting with. I like the interplay between magnesia and lime. The =
felspar is 12% Sodium, another of my idiosyncrasies.
I hope these notes shed some light on this issue for you.
Best regards,
Ivor Lewis. Redhill, South Australia