John Hesselberth on sun 15 aug 04
Carbonate
On Saturday, August 14, 2004, at 11:26 PM, Ivor and Olive Lewis wrote:
> I would have to test Strontium Carbonate to establish if behaves in
> the same manner. Does any one know if this work has been done? Or once
> again are we relying on Folk Lore to inform our technology.
Hello Ivor,
I have tested a few high strontium glazes using my standard test (load
them with 5% copper carbonate and 6% rutile and test for copper
leaching). I have published the results from one which is very
stable--Pete's Weathered Bronze Green. You can see the recipe, unity
formula and results at
http://www.frogpondpottery.com/glazestability/glaze0022.html
Others I have tested have been both stable and unstable. Sometimes they
behaved as I predicted, other times not. As a very general statement, I
seem to be able to predict when they would be unstable, but at least
one I would have predicted to be stable was not. My experiments
continue with strontium based glazes. They fascinate me for several
reasons. For example, you seem to get an alkaline color response from
copper without as much high expansion materials such as Na and K.
Therefore I think I have a better chance of making a craze-free
turquoise. They also sometimes give some unexpected gloss where one
would think the glaze would be matte. My experiments to date lead me to
believe it is a slightly better flux than calcium. Overall, strontium
is a fascinating flux which needs to be explored more fully. I think
there is lots of opportunity there for some beautiful (and stable) new
glazes.
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Ivor and Olive Lewis on tue 17 aug 04
Carbonate
Dear John,
I will have a look at that site and archive the information.
I have several concerns.
The first is to distinguish between the instability of copper
compounds and the instability of the basic glaze. I say this because
there may be more than one reason for any element or compound to be
leached from a mature glaze.
The second is to define quite clearly what functions Alkali Earth
Elements perform and the degree to which they achieve these. To say
that one of these elements is a better flux than another seems to me
to be meaningless without a definition of the purpose for
incorporation in a batch recipe. Each may have several functions, none
of which are concerned with causing melting.
The third is that we should, whenever possible, isolate the variable
component by eliminating what is unnecessary to the creation of a
silicate solution.
Tests I have done with Barium Carbonate suggest to me that it has a
greater range of solubility than described in popular text books I
will attempt to duplicate these tests substituting Strontium Carbonate
for the Barium compound.
Rejecting glazes as unstable when Copper ions are leached may cause us
to discard samples which might otherwise, with other colouring agents,
have highly desirable properties.
So perhaps we do need to reassess, by trial, what has been written to
get an objective viewpoint.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
----- Original Message -----
From: "John Hesselberth"
To:
Sent: Sunday, 15 August 2004 11:08
Subject: Re: ^5 Oxidation Matt Turquoise help. Questions about
Strontium Carbonate
> On Saturday, August 14, 2004, at 11:26 PM, Ivor and Olive Lewis
wrote:
>
> > I would have to test Strontium Carbonate to establish if behaves
in
> > the same manner. Does any one know if this work has been done? Or
once
> > again are we relying on Folk Lore to inform our technology.
>
> Hello Ivor,
>
> I have tested a few high strontium glazes using my standard test
(load
> them with 5% copper carbonate and 6% rutile and test for copper
> leaching). I have published the results from one which is very
> stable--Pete's Weathered Bronze Green. You can see the recipe, unity
> formula and results at
>
> http://www.frogpondpottery.com/glazestability/glaze0022.html
>
> Others I have tested have been both stable and unstable. Sometimes
they
> behaved as I predicted, other times not. As a very general
statement, I
> seem to be able to predict when they would be unstable, but at least
> one I would have predicted to be stable was not. My experiments
> continue with strontium based glazes. They fascinate me for several
> reasons. For example, you seem to get an alkaline color response
from
> copper without as much high expansion materials such as Na and K.
> Therefore I think I have a better chance of making a craze-free
> turquoise. They also sometimes give some unexpected gloss where one
> would think the glaze would be matte. My experiments to date lead me
to
> believe it is a slightly better flux than calcium. Overall,
strontium
> is a fascinating flux which needs to be explored more fully. I think
> there is lots of opportunity there for some beautiful (and stable)
new
> glazes.
>
> Regards,
>
> John
>
> John Hesselberth
> http://www.frogpondpottery.com
> http://www.masteringglazes.com
>
>
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