iandol on sun 25 may 03
Dear Friends,
I have a question for the Glaze Chemists, Teachers and other Gurus who =
advise us about the behaviour and stability of glaze recipes. I think, =
because of the reported instability of Copper bearing glazes we all =
would like to know more about the nature of the interaction between =
Copper Oxides, either Black, Red or Oxide derived from Copper =
"Carbonate" and molten silicate glaze compositions regardless of the =
temperature to which we fire our work.
There seem to be several possible ways of explaining what might occur.
1. Copper oxides react like many other metallic oxides. They decompose, =
releasing oxygen which assists in providing terminating oxygen atoms in =
silicate chains. Copper atoms becomes ions which then embed in the =
silicate lattice to become "Colour Centres". Because of the contribution =
of Oxygen, the melting point of the silicate glass is reduced. In this =
way, copper oxides not only colour the glaze but work as active fluxes.
2. Copper oxides dissolve in molten silicate mixtures without =
decomposing. They can be affected by changes in kiln atmosphere to give =
blue, green or red colouration which is called a "Solution colour". =
Since Copper oxides are soluble in acids the element will leach from any =
glaze which contains them.
3. Copper Oxides enter into reactions with other species of oxides to =
give distinct chemical compounds such as Calcium Cupro Silicate which =
are coloured chemicals.
4. Copper oxides do not decompose or dissolve in the molten silicate =
mixture but distribute themselves through the molten glaze as discrete =
particles and form a "colloidal colour"
I favour No 2.
What makes Copper different from the other Transitional Elements, =
including Zinc? How are general observations to be explained? Does =
Copper have special qualities which have gone undiscovered? Or are there =
facts which have been ignored when people have written about the way we =
use Copper as a colourant in our glazes?
Any Answers Anyone?
Best regards,
Ivor Lewis
Earl Brunner on sun 25 may 03
Ivor, Does only have to be one? Could it be more complex and a
combination of your choices? Could it be one way under some
circumstances and another way under different circumstances?
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of iandol
Sent: Saturday, May 24, 2003 9:51 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Copper Oxides in silicate melts,
There seem to be several possible ways of explaining what might occur.
1. Copper oxides react like many other metallic oxides. They decompose,
releasing oxygen which assists in providing terminating oxygen atoms in
silicate chains. Copper atoms becomes ions which then embed in the
silicate lattice to become "Colour Centres". Because of the contribution
of Oxygen, the melting point of the silicate glass is reduced. In this
way, copper oxides not only colour the glaze but work as active fluxes.
2. Copper oxides dissolve in molten silicate mixtures without
decomposing. They can be affected by changes in kiln atmosphere to give
blue, green or red colouration which is called a "Solution colour".
Since Copper oxides are soluble in acids the element will leach from any
glaze which contains them.
3. Copper Oxides enter into reactions with other species of oxides to
give distinct chemical compounds such as Calcium Cupro Silicate which
are coloured chemicals.
4. Copper oxides do not decompose or dissolve in the molten silicate
mixture but distribute themselves through the molten glaze as discrete
particles and form a "colloidal colour"
I favour No 2.
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