Tom Witman on fri 4 aug 06
I have been using a basic clear glaze for about a year on my Cone 6
Stoneware 'Sandpiper' clay from Kickwheel Pottery. It contains 4%
whiting, 40.5% 3195 frit, 16.5% Custer feldspar, 2.5% bentonite, 6.5% EPK,
and 30% flint. To that I add 15% zircopax for a gloss white and 5% RIO
for a dark brown. I've had no problems at all using a firing schedule for
my Olympic kiln with auto control with a 5 and 10 minute soak just before
the peak and then at the peak temp.
I experimented with a couple of other colors using the white glaze already
mixed. I've added a picture of a pot to my website at
www.pensandcalls.com/blistering_green.jpg that shows the problem. The
color on the right shows smoothness with no blistering but the green is
severely blistering. To the white base, I added 5% copper carbonate for
the green. The color on the right is actually a pale blue that didn't
show up well in the picture. For that color I added 1% copper carbonate
and 5% rutile.
What could be causing the blistering? Nothing else in the kiln showed any
blisters. I always slow bisque to 04. Clearly it's a glaze problem or
incompatibility between the copper carbonate and something in the base
glaze. Any help would be appreciated.
I got the base glaze recipe from the internet quite a while ago, in case
you wonder.
Thanks
Tom
Lynn Goodman Porcelain Pottery on fri 4 aug 06
On Aug 4, 2006, at 9:10 PM, Tom Witman wrote:
The color on the right shows smoothness with no blistering but the
green is
> severely blistering. To the white base, I added 5% copper carbonate
> for
> the green. The color on the right is actually a pale blue that didn't
> show up well in the picture. For that color I added 1% copper
> carbonate
> and 5% rutile.
> What could be causing the blistering? Nothing else in the kiln showed
> any
> blisters.
If this is a glaze combo that you use regularly, and it was limited to
one pot, my guess is an impurity in the clay. If that is the problem,
there really isn't any way you could have prevented it. If it was a
materials incompatibility, you would have seen it in more of the
firing.
I'd be glad it was so limited!
Lynn
Lynn Goodman
Fine Porcelain Pottery
548 Court St.
Brooklyn, NY 11231
718-858-6920
Cell 347-526-9805
www.lynngoodmanporcelain.com
Tom Witman on sat 5 aug 06
Thanks for the help. I doubt that it was a clay problem because the other
half of the pot shows no signs of blisters. It's the same glaze except
for the addition of copper carbonate (blisters) or cobalt carbonate and
rutile on the right. This was just one piece that I used the green on to
test it.
Carol, in another answer, said "The glaze without the 5 % copper carb is a
mature glaze at ^6, add 5 % copper
carb, and it upped the maturation temperature. The pitting you show in
that
picture is characteristic of not quite mature glaze. You could hold for
half
an hour, you could go to ^7, could reduce the copper carb to say 3
percent. " She suggested adding more flux to get it to mature at a lower
temp. I have another load almost ready to go so I'll try that suggestion
with the sample green.
This is part of the 'fun', isn't it??? :o)
Thanks for the interest and support.
Tom
Ron Roy on mon 7 aug 06
Hi Tom,
I had a look at the example on your web site - looks to me like the rutile
was just enough more refractory material to underfire that glaze.
The Seger formula for that glaze is
*CaO - 0.63
*MgO - 0.02
*K2O - 0.10
*Na2O - 0.26
B2O3 - 0.72
Al2O3 - 0.55
SiO2 - 5.69
Ratio is 10.38
Expansion is 301 with out the Zircopax and 283 with it included.
This glaze is unusual in that it has high boron, high alumina and high
silica for a cone 6 glaze - and the expansion is very low - I suggest you
raise the expansion for starters.
If you want more melting you should run a line blend taking some of the
clay out - and I would recommend changing the clay to a ball clay because
there is so little clay to start with.
I also seem to see some small blisters on the clay surface - that could be
blebbing - which could indicate the clay is being overfired.
Are you sure of what temperature you are reaching - like are you using cones?
Let me know if you need more on this - RR
>I have been using a basic clear glaze for about a year on my Cone 6
>Stoneware 'Sandpiper' clay from Kickwheel Pottery. It contains 4%
>whiting, 40.5% 3195 frit, 16.5% Custer feldspar, 2.5% bentonite, 6.5% EPK,
>and 30% flint. To that I add 15% zircopax for a gloss white and 5% RIO
>for a dark brown. I've had no problems at all using a firing schedule for
>my Olympic kiln with auto control with a 5 and 10 minute soak just before
>the peak and then at the peak temp.
>I experimented with a couple of other colors using the white glaze already
>mixed. I've added a picture of a pot to my website at
>www.pensandcalls.com/blistering_green.jpg that shows the problem. The
>color on the right shows smoothness with no blistering but the green is
>severely blistering. To the white base, I added 5% copper carbonate for
>the green. The color on the right is actually a pale blue that didn't
>show up well in the picture. For that color I added 1% copper carbonate
>and 5% rutile.
>What could be causing the blistering? Nothing else in the kiln showed any
>blisters. I always slow bisque to 04. Clearly it's a glaze problem or
>incompatibility between the copper carbonate and something in the base
>glaze. Any help would be appreciated.
>I got the base glaze recipe from the internet quite a while ago, in case
>you wonder.
>Thanks
>Tom
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Tom Witman on tue 8 aug 06
Hi Ron,
Thanks for your expert advice, as always.
The green glaze doesn't have any rutile in it. It is the light blue on the
right side of the pot that has the rutile.
How would be the best method to raise the expansion? Increase the
EPK/substitute ball clay/add ball clay? At the same time, you suggest
taking clay out to get more melting..... Yep, I'm confused, but I confess I
knew I would be when I started playing with glazes. The programs out there
to analyze glazes seem very technical but well beyond my skills and they do
NOTHING to help the novice analyze his/her problems. I also confess I have
yet to get your book. It's on my list now for sure. :o)
To be honest, I haven't used cones for quite a while after I satisfied
myself that the electronic control was accurate. Probably not a good
assumption. :o( I'll check it next firing. The clay I was using is good
for cone 6-10 according to Kickwheel, so that shouldn't be the problem. Of
course, it would be for the glaze, though.
Thanks again,
Tom
----- Original Message -----
From: "Ron Roy"
To:
Sent: Monday, August 07, 2006 2:09 AM
Subject: Re: Cone 6 glaze puzzle??
> Hi Tom,
>
> I had a look at the example on your web site - looks to me like the rutile
> was just enough more refractory material to underfire that glaze.
>
> The Seger formula for that glaze is
>
> *CaO - 0.63
> *MgO - 0.02
> *K2O - 0.10
> *Na2O - 0.26
> B2O3 - 0.72
> Al2O3 - 0.55
> SiO2 - 5.69
>
> Ratio is 10.38
> Expansion is 301 with out the Zircopax and 283 with it included.
>
> This glaze is unusual in that it has high boron, high alumina and high
> silica for a cone 6 glaze - and the expansion is very low - I suggest you
> raise the expansion for starters.
>
> If you want more melting you should run a line blend taking some of the
> clay out - and I would recommend changing the clay to a ball clay because
> there is so little clay to start with.
>
> I also seem to see some small blisters on the clay surface - that could be
> blebbing - which could indicate the clay is being overfired.
>
> Are you sure of what temperature you are reaching - like are you using
> cones?
>
> Let me know if you need more on this - RR
>
>>I have been using a basic clear glaze for about a year on my Cone 6
>>Stoneware 'Sandpiper' clay from Kickwheel Pottery. It contains 4%
>>whiting, 40.5% 3195 frit, 16.5% Custer feldspar, 2.5% bentonite, 6.5% EPK,
>>and 30% flint. To that I add 15% zircopax for a gloss white and 5% RIO
>>for a dark brown. I've had no problems at all using a firing schedule for
>>my Olympic kiln with auto control with a 5 and 10 minute soak just before
>>the peak and then at the peak temp.
>>I experimented with a couple of other colors using the white glaze already
>>mixed. I've added a picture of a pot to my website at
>>www.pensandcalls.com/blistering_green.jpg that shows the problem. The
>>color on the right shows smoothness with no blistering but the green is
>>severely blistering. To the white base, I added 5% copper carbonate for
>>the green. The color on the right is actually a pale blue that didn't
>>show up well in the picture. For that color I added 1% copper carbonate
>>and 5% rutile.
>>What could be causing the blistering? Nothing else in the kiln showed any
>>blisters. I always slow bisque to 04. Clearly it's a glaze problem or
>>incompatibility between the copper carbonate and something in the base
>>glaze. Any help would be appreciated.
>>I got the base glaze recipe from the internet quite a while ago, in case
>>you wonder.
>>Thanks
>>Tom
>
> Ron Roy
> RR#4
> 15084 Little Lake Road
> Brighton, Ontario
> Canada
> K0K 1H0
>
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Ron Roy on tue 8 aug 06
Hi Tom,
I had a look at the example on your web site - looks to me like the rutile
was just enough more refractory material to underfire that glaze.
The Seger formula for that glaze is
*CaO - 0.63
*MgO - 0.02
*K2O - 0.10
*Na2O - 0.26
B2O3 - 0.72
Al2O3 - 0.55
SiO2 - 5.69
Ratio is 10.38
Expansion is 301 with out the Zircopax and 283 with it included.
This glaze is unusual in that it has high boron, high alumina and high
silica for a cone 6 glaze - and the expansion is very low - I suggest you
raise the expansion for starters.
If you want more melting you should run a line blend taking some of the
clay out - and I would recommend changing the clay to a ball clay because
there is so little clay to start with.
I also seem to see some small blisters on the clay surface - that could be
blebbing - which could indicate the clay is being overfired.
Are you sure of what temperature you are reaching - like are you using cones?
Let me know if you need more on this - RR
>I have been using a basic clear glaze for about a year on my Cone 6
>Stoneware 'Sandpiper' clay from Kickwheel Pottery. It contains 4%
>whiting, 40.5% 3195 frit, 16.5% Custer feldspar, 2.5% bentonite, 6.5% EPK,
>and 30% flint. To that I add 15% zircopax for a gloss white and 5% RIO
>for a dark brown. I've had no problems at all using a firing schedule for
>my Olympic kiln with auto control with a 5 and 10 minute soak just before
>the peak and then at the peak temp.
>I experimented with a couple of other colors using the white glaze already
>mixed. I've added a picture of a pot to my website at
>www.pensandcalls.com/blistering_green.jpg that shows the problem. The
>color on the right shows smoothness with no blistering but the green is
>severely blistering. To the white base, I added 5% copper carbonate for
>the green. The color on the right is actually a pale blue that didn't
>show up well in the picture. For that color I added 1% copper carbonate
>and 5% rutile.
>What could be causing the blistering? Nothing else in the kiln showed any
>blisters. I always slow bisque to 04. Clearly it's a glaze problem or
>incompatibility between the copper carbonate and something in the base
>glaze. Any help would be appreciated.
>I got the base glaze recipe from the internet quite a while ago, in case
>you wonder.
>
>Thanks
>
>Tom
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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