Ron Philbeck on tue 3 jun 97
I just tested the following glaze in my salt kiln and need some insight on
some things that happened:
Rob's Green
Cornwall Stone 63.5
Whiting 15.2
Gerstley 4.3
Bentonite 4.0
Copper Carb. 8.5
I used this on some bowls and mugs on the inside only...I fire in a gas
hard brick kiln to cone 9 in a neutral atmosphere and lightly salt...the
pots were glazed leather hard.
The color I got was fine, a nice dark trans. green. Yet on some pieces,
there were areas of black or small black spots.... I thought this was great
too, until I rubbed with my finger and found that I had picked up very tiny
specks of green/black residue...not so good after all.
What caused this and how can I prevent this "residue" from forming?
I like the color, even with the black areas. I am looking for a green to
use as a liner in my functional pots and thought I had found it until this
happened.
Any help or suggestions is appreciated.
(Oh yeah, the orginial recipe called for 3% Rutile, but I left that out of
my test.)
Thanks,
Ron Philbeck
rphilbeck@connectu.net
http://www.cclay.com/rphilbeck
Brooks Burgess on wed 4 jun 97
At 09:44 AM 6/3/97 EDT, you wrote:
>----------------------------Original message----------------------------
>I just tested the following glaze in my salt kiln and need some insight on
>some things that happened:
>
>Rob's Green
>Cornwall Stone 63.5
>Whiting 15.2
>Gerstley 4.3
>Bentonite 4.0
>Copper Carb. 8.5
>
>I used this on some bowls and mugs on the inside only...I fire in a gas
>hard brick kiln to cone 9 in a neutral atmosphere and lightly salt...the
>pots were glazed leather hard.
>The color I got was fine, a nice dark trans. green. Yet on some pieces,
>there were areas of black or small black spots.... I thought this was great
>too, until I rubbed with my finger and found that I had picked up very tiny
>specks of green/black residue...not so good after all.
>What caused this and how can I prevent this "residue" from forming?
>I like the color, even with the black areas. I am looking for a green to
>use as a liner in my functional pots and thought I had found it until this
>happened.
>
>Any help or suggestions is appreciated.
>(Oh yeah, the orginial recipe called for 3% Rutile, but I left that out of
>my test.)
>
>Thanks,
> Ron Philbeck
>rphilbeck@connectu.net
>http://www.cclay.com/rphilbeck
>
>Is this the same green glaze that someone on list earlier said was
unstable? Leached cu carb ? Faded in dishwasher? If so , I too would like to
know how to "fix it" as I love the look but dont want to be messing with
unstable glazes on food oriented pots. So any glaze gurus please reply to
list. TIA
Brooks
Ron Roy on wed 4 jun 97
Ron Philbeck asked
>Rob's Green
>Cornwall Stone 63.5
>Whiting 15.2
>Gerstley 4.3
>Bentonite 4.0
>Copper Carb. 8.5
>
>I used this on some bowls and mugs on the inside only...I fire in a gas
>hard brick kiln to cone 9 in a neutral atmosphere and lightly salt...the
>pots were glazed leather hard.
>The color I got was fine, a nice dark trans. green. Yet on some pieces,
>there were areas of black or small black spots.... I thought this was great
>too, until I rubbed with my finger and found that I had picked up very tiny
>specks of green/black residue...not so good after all.
>What caused this and how can I prevent this "residue" from forming?
>I like the color, even with the black areas. I am looking for a green to
>use as a liner in my functional pots and thought I had found it until this
>happened.
Hi Ron,
The usual amount of copper in a glaze to get a strong green would be around
2% - at 5% you can expect some of that copper to be on the surface of the
glaze and uncombined - it is readily dissolved by some foods in this state.
May I suggest you lower the amount of copper till it stays in your glaze.
Keep in mind that copper is known to increase the solubility of glazes. My
limits say this glaze just has enough silica to be durable at cone 9 so you
might want to check with vinegar for 24 hours at room temperature (keep
covered) to see if the glaze is affected.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/ronroy.htm
Craig Martell on wed 4 jun 97
At 09:44 AM 6/3/97 EDT, you wrote:
>----------------------------Original message----------------------------
>I just tested the following glaze in my salt kiln and need some insight on
>some things that happened:
>
>Rob's Green
>Cornwall Stone 63.5
>Whiting 15.2
>Gerstley 4.3
>Bentonite 4.0
>Copper Carb. 8.5
Hi Ron:
The black spots that also make the residue on your fingers are caused by too
much copper in the glaze. I think that you can safely cut the percentage of
copper to 5.0 or lower and still get the desired hue. This glaze is also
low in alumia and silica. If you want a glaze for functional ware, you will
have to put this one into the Seger Formula and bring up the silica and
alumina to get a stable glass. You are also adding soda, through the vapor
glazing process and that is compounding the problem. You should have about
..35 to .50 moles of alumina and at least 3.0 moles of silica to 1.0 moles of
flux in the seger formula to get a stable glass. These guidelines vary,
according to what texts you have read and I'm sure there are others on the
list that will advise you as to the silica and alumina limits. You might
want to get rid of the Gerstley Borate too.
Check out the copper oxide section in Frank Hamer's book The Potters
Dictionary of Materials and Techniques if you have it.
Regards, Craig Martell-Oregon
Grimmer on wed 4 jun 97
Ron,
I used to use this glaze a lot in grad school. It looks killer if you fire
it to ^11-12 and salt it silly. Unfortunately, it has very little silica
at all and the nice, black crystals are copper oxide precipitating
out of the solution during the cooling. That stuff will go right into
any food that contacts it. Not so good. I switched to another glaze,
but I bet with the addition of flint, you could make it more stable.
steve grimmer
carterville illinois
> I just tested the following glaze in my salt kiln and need some insight on
> some things that happened:
>
> Rob's Green
> Cornwall Stone 63.5
> Whiting 15.2
> Gerstley 4.3
> Bentonite 4.0
> Copper Carb. 8.5
>
> I used this on some bowls and mugs on the inside only...I fire in a gas
> hard brick kiln to cone 9 in a neutral atmosphere and lightly salt...the
> pots were glazed leather hard.
> The color I got was fine, a nice dark trans. green. Yet on some pieces,
> there were areas of black or small black spots.... I thought this was great
> too, until I rubbed with my finger and found that I had picked up very tiny
> specks of green/black residue...not so good after all.
> What caused this and how can I prevent this "residue" from forming?
> I like the color, even with the black areas. I am looking for a green to
> use as a liner in my functional pots and thought I had found it until this
> happened.
>
> Any help or suggestions is appreciated.
> (Oh yeah, the orginial recipe called for 3% Rutile, but I left that out of
> my test.)
>
> Thanks,
> Ron Philbeck
> rphilbeck@connectu.net
> http://www.cclay.com/rphilbeck
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