VIP on wed 17 sep 97
I had a strange thing happen to a glaze lately maybe someone can help?
The recipe is:
cone 6 black
Kaolin 4.91
Pot Spar 78.86
Whiting 5.48
Colemanite 10.75
Copper Carb. .04
Mang Dioxide .04
Cobalt Carb. .02 This is usually a very nice lustery black but for
some reason it fired more a midnight blue. Someone put apples in this
bowl that had this glaze and the glaze lightened to a light blue
without luster where ever the apples were.
Why would this happen? Is this glaze unsafe for food? Any advise would
be appreciated. thanka in advance Susan Zaluski
Karl P. Platt on fri 19 sep 97
Did you change lots of Colemanite? The glaze, for whatever reason,
probabl having to do with the fugitive nature of Colemanite, became
somewhat soluble.
Ron Roy on fri 19 sep 97
Hi Susan,
This is a puzzelment - I need more information. The only conclusion I can
come to at the moment is you left something out but I don't know if you
just mixed up a new batch.
Are the colouring oxides really only .04,.04 and .02 - I can't see how that
could give a black glaze. If they are 4.0, 4.0 and 2.0 it would make more
sense.
>----------------------------Original message----------------------------
>I had a strange thing happen to a glaze lately maybe someone can help?
>The recipe is:
>cone 6 black
>Kaolin 4.91
>Pot Spar 78.86
>Whiting 5.48
>Colemanite 10.75
>Copper Carb. .04
>Mang Dioxide .04
>Cobalt Carb. .02 This is usually a very nice lustery black but for
>some reason it fired more a midnight blue. Someone put apples in this
>bowl that had this glaze and the glaze lightened to a light blue
>without luster where ever the apples were.
>Why would this happen? Is this glaze unsafe for food? Any advise would
>be appreciated. thanka in advance Susan Zaluski
Ron Roy
93 Pegasus Trail
Scarborough,Canada
M1G 3N8
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm
VIP on sat 20 sep 97
Karl P. Platt wrote:
>
> ----------------------------Original message----------------------------
> Did you change lots of Colemanite? The glaze, for whatever reason,
> probabl having to do with the fugitive nature of Colemanite, became
> somewhat soluble.It is possible the colemanite was subsituted with gerstley bo
would that make to much of a difference?
thanks susan
Karl P. Platt on tue 23 sep 97
> > Did you change lots of Colemanite? The glaze, for whatever reason,
> > probabl having to do with the fugitive nature of Colemanite, became
> > somewhat soluble.It is possible the colemanite was subsituted with gerstley
> would that make to much of a difference?
Absolutely. It is likely that the new lot of Colemanite (here I'm assuming
that you changed lots) is markedly more rich in B2O3 (as it were). This
would tend to reduce the chemical durability of the glaze markedly - all
else being equal. Colemanite, like Ghastly Borate or the stuff they call
Rutile are all highly fugitive materials -- expect composition variations.
It's worth it to make a test of some sort when changing lots (or even
little bags within a single lot). Adding CuO at 1 wt% or so in a simple
clear glaze based on these materials, when fired in an electric kiln, will
give colors slightly different depending on the nature of the material.
This will reflect any differences between lots.
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