jonathan byler on sun 11 apr 10
I have asked this before and can't remember if I was satisfied with
the answer:
I have a metallic black glaze that is very handsome, but where thin
makes small bubbles that sort of ruin it for use as a liner glaze or
on the lips of pots. doesn't seem to matter much whether this is on
stoneware or whiteware.
recipe is
custer feldpar 70
gerstly borate 10
flint 10
epk 10
cobalt carb 2
copper carb 3
RIO 4.5 (or black iron 4.0)
before anyone starts to blame the poor old gerstly borate, may I
point out that I made a fritted recipe and it did the exact same thing?
is this getting over fired? under fired? I follow the reduction
firing schedule prescribed by cushing for the most part, with a period
of oxidation at the end of the firing for about ten minutes. any
thoughts?
thanks,
jon
jonathan byler on mon 12 apr 10
yes, I forgot to mention that it is ^10 reduciton.
I will try that, and see what happens.
thanks,
jon
On Apr 12, 2010, at 9:21 PM, ronroy@ca.inter.net wrote:
> Hi Jonathan,
>
> I'm guessing this is a cone 10 reduction glaze.
>
> The mols of both alumina and silica are very high - that could be
> the problem.
>
> Try adding 10 whiting and lower the spar by 10.
> Doing the same thing with Talc keeps the expansion down.
>
> Not sure if the result will keep all that copper in - maybe test
> with orange juice overnight and see if you can taste it in the
> morning.
>
> RR
>
>
> Quoting jonathan byler :
>
>> I have asked this before and can't remember if I was satisfied with
>> the answer:
>>
>> I have a metallic black glaze that is very handsome, but where thin
>> makes small bubbles that sort of ruin it for use as a liner glaze or
>> on the lips of pots. doesn't seem to matter much whether this is on
>> stoneware or whiteware.
>>
>> recipe is
>>
>> custer feldpar 70
>> gerstly borate 10
>> flint 10
>> epk 10
>>
>> cobalt carb 2
>> copper carb 3
>> RIO 4.5 (or black iron 4.0)
>>
>> before anyone starts to blame the poor old gerstly borate, may I
>> point out that I made a fritted recipe and it did the exact same
>> thing?
>>
>> is this getting over fired? under fired? I follow the reduction
>> firing schedule prescribed by cushing for the most part, with a
>> period
>> of oxidation at the end of the firing for about ten minutes. any
>> thoughts?
>>
>> thanks,
>> jon
>>
>
>
>
Ron Roy on mon 12 apr 10
Hi Jonathan,
I'm guessing this is a cone 10 reduction glaze.
The mols of both alumina and silica are very high - that could be the probl=
em.
Try adding 10 whiting and lower the spar by 10.
Doing the same thing with Talc keeps the expansion down.
Not sure if the result will keep all that copper in - maybe test with
orange juice overnight and see if you can taste it in the morning.
RR
Quoting jonathan byler :
> I have asked this before and can't remember if I was satisfied with
> the answer:
>
> I have a metallic black glaze that is very handsome, but where thin
> makes small bubbles that sort of ruin it for use as a liner glaze or
> on the lips of pots. doesn't seem to matter much whether this is on
> stoneware or whiteware.
>
> recipe is
>
> custer feldpar 70
> gerstly borate 10
> flint 10
> epk 10
>
> cobalt carb 2
> copper carb 3
> RIO 4.5 (or black iron 4.0)
>
> before anyone starts to blame the poor old gerstly borate, may I
> point out that I made a fritted recipe and it did the exact same thing?
>
> is this getting over fired? under fired? I follow the reduction
> firing schedule prescribed by cushing for the most part, with a period
> of oxidation at the end of the firing for about ten minutes. any
> thoughts?
>
> thanks,
> jon
>
Sumi von Dassow on mon 12 apr 10
jonathan
My response doesn't answer your question but I wonder if this is really
a suitable glaze to use inside pots that are intended to hold food. I'd be
a bit concerned about the cobalt and copper.
> I have asked this before and can't remember if I was satisfied with
> the answer:
>
> I have a metallic black glaze that is very handsome, but where thin
> makes small bubbles that sort of ruin it for use as a liner glaze or
> on the lips of pots. doesn't seem to matter much whether this is on
> stoneware or whiteware.
>
> recipe is
>
> custer feldpar 70
> gerstly borate 10
> flint 10
> epk 10
>
> cobalt carb 2
> copper carb 3
> RIO 4.5 (or black iron 4.0)
>
> before anyone starts to blame the poor old gerstly borate, may I
> point out that I made a fritted recipe and it did the exact same thing?
>
> is this getting over fired? under fired? I follow the reduction
> firing schedule prescribed by cushing for the most part, with a period
> of oxidation at the end of the firing for about ten minutes. any
> thoughts?
>
> thanks,
> jon
>
>
--
Sumi von Dassow
www.herwheel.com
sumi@herwheel.com
David Finkelnburg on tue 13 apr 10
Jonathan,
You don't mention the cone you are firing to but presumably it's cone 10
or so. This glaze contains over a mole of alumina and over 7 moles of
silica for every mole of flux. The 0.24 moles of boron oxide/mole of flux,
the high alkali level (0.75 moles/mole of flux), the iron fired in
reduction, the copper, and the cobalt allow it to melt.
Are you saying the glaze flows off the rims so the fired glaze is thin
there? If you are seeing that, then yes, in a sense, despite the very high
alumina and silica levels, the glaze is overfired.
Thermal decomposition of iron is not likely the problem or you would se=
e
it everywhere, not just where the glaze is thin.
I don't know what Cushing's firing schedule is, but one way to "heal"
glaze bubbles/blisters is to soak a glaze while it remains fluid. You may
want to fire test tiles only to explore various soak times and temperatures
to see if that helps. If the test tiles duplicate the problematic
dimensions and orientation of your ware that would tell you the most. If
the glaze is overfiring now, you may consider firing faster and soaking at =
a
reduced temperature by firing during cooling, say at 200 to 300 degrees F
below peak firing temperature.
Good potting!
Dave Finkelnburg
http://www.mattanddavesclays.com
-----------------------------
On Sun, 11 Apr 2010 18:35:09 -0500, jonathan byler
wrote:
I have a metallic black glaze that is very handsome, but where thin
makes small bubbles that sort of ruin it for use as a liner glaze or
on the lips of pots. doesn't seem to matter much whether this is on
stoneware or whiteware.
recipe is
custer feldpar 70
gerstly borate 10
flint 10
epk 10
cobalt carb 2
copper carb 3
RIO 4.5 (or black iron 4.0)
before anyone starts to blame the poor old gerstly borate, may I
point out that I made a fritted recipe and it did the exact same thing?
is this getting over fired? under fired? I follow the reduction
firing schedule prescribed by cushing for the most part, with a period
of oxidation at the end of the firing for about ten minutes.
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