Teresa Wooden on tue 8 mar 05
Hello to everyone! I am new to the list and look forward to all the =
wonderful tips and information offered here by other members.
I am currently having a problem and was hoping someone could shed some =
light for me... I have been applying a cobalt oxide slip to porcelain =
for sgraffito, fired to cone 6 as recommended for that clay body. No =
matter how thinly I apply the slip, I can't get rid of those darned =
pinholes that occur in the slipped area. I know it's not dusty, I rib =
my work smooth after trimming, and it's cooled slowly....the slip is =
made from the same clay body.... there are no pinholes in the rest of =
the object under the clear glaze, so I know it's associated with the =
cobalt slip. I am working with a "master potter" and she has no clue as =
to why it's happening.=20
Is cobalt oxide known for this problem? Would cobalt carbonate give =
better results? I'd be incredibly grateful for any advice as it's =
driving me nuts and ruining kiln after kiln of beautiful and tedious =
work.
Thanks,
Teresa
David Hewitt on wed 9 mar 05
Teresa,
When you say that you are using a cobalt slip, what exactly do you mean?
Obviously it contains cobalt oxide, but what else?
I regularly use a cobalt slip on porcelain and a white stoneware and do
not have your problem. I make my own lips by using the same clay as the
body. You can see details of this on my web site if it is of interest
to you under Pottery Techniques / Making Slip for Decorating.
Web:- http://www.dhpot.demon.co.uk
David
In message , Teresa Wooden writes
>Hello to everyone! I am new to the list and look forward to all the
>wonderful tips and information offered here by other members.
>
>I am currently having a problem and was hoping someone could shed some
>light for me... I have been applying a cobalt oxide slip to porcelain
>for sgraffito, fired to cone 6 as recommended for that clay body. No
>matter how thinly I apply the slip, I can't get rid of those darned
>pinholes that occur in the slipped area. I know it's not dusty, I rib
>my work smooth after trimming, and it's cooled slowly....the slip is
>made from the same clay body.... there are no pinholes in the rest of
>the object under the clear glaze, so I know it's associated with the
>cobalt slip. I am working with a "master potter" and she has no clue
>as to why it's happening.
>
>Is cobalt oxide known for this problem? Would cobalt carbonate give
>better results? I'd be incredibly grateful for any advice as it's
>driving me nuts and ruining kiln after kiln of beautiful and tedious
>work.
>
>Thanks,
>
>Teresa
--
David Hewitt
Web:- http://www.dhpot.demon.co.uk
Craig Martell on wed 9 mar 05
Hi:
I sent this off yesterday but I don't think it came thru to the list. I
checked the Archives, and no dice.
Teresa sez:
>Is cobalt oxide known for this problem? Would cobalt carbonate give
>better results?
Hello Teresa:
Pinholes are caused by some sort of gas escaping thru claybodies and slips
etc. If you aren't having a problem anywhere else other than where the
pots are coated with the cobalt slip you are correct to start working on a
solution concerning the slip.
I don't think that cobalt carb would give better results but it might be
worth a try just to see if there's a difference. If gassing thru the slip
is a problem it might get worse with the carbonate because it will outgas
a bit when it's fired. The end result being cobalt oxide. You lose some
CO2 during the fire. This would happen in the bisque.
One thought I had was that cobalt will act as a flux. If there's a good
percentage of cobalt in the slip it may fire a bit denser than the claybody
and retard the passage of gasses thru the slip. At what temp are you
bisque firing? Raising the bisque temp or lengthening the fire may allow
for complete purging of gasses thru the slip. You could also run some test
tiles of lower percentages of the cobalt in the slip and find a point where
the color is still good but no pinholes. You could also run some tests of
the slip with other colorants. This would give you an idea about problems
with colorants other than cobalt and would give more credence to whether or
not it was actually the cobalt that is causing the problem. You might find
some other colors of slip that you like too.
regards, Craig Martell Hopewell, Oregon
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