Paul B on mon 13 dec 04
I have been getting sporadic glaze blisters and what looks like "bubbles"
on the surface of the glaze. Basically, it looks like a big bubble was
pushing through the glaze, evidently from the body, and was just in the
middle of bursting when it froze. Soaking the kiln at top temp has not
completely solved the problem, although i have noticed that in the hotter
parts of the kiln it does not seem to happen as much. Also, on thinner
glazes, at least in the last firing, none at all came out that way. Then
again, on very thick copper reds it still does not occur, unless i have a
few glazes layered on top of each other, then the glazes are put on very
thick.
A slow, cone 04 electric bisque has not solved the problem. Has anyone else
had this happen, and if so, how did you fix it? I am almost certain it has
to do with the body (mostly EPK, OM-4, hawthorne bond and goldart). I have
tried it without the goldart, and even without the hawthorne, and it still
seems to occur sometimes, so now i am focusing on the OM-4 because i have
heard of potters having problems with it. Any suggestions? thanks!
Paul B
Blanket Creek Pottery
Ama Menec on tue 14 dec 04
Paul,
I don't know the clay you are using, (being in the UK), but I wondered if it
had molochite in it? My partner Lea is having similar problems, mostly of
'snow' as bubbles are brought through the surface of a coloured overglaze
from a white underglaze, and I think it happens most on the stoneware clays
she is trialing which have molochite in them. Smoothing the surface seems to
make no difference, 'tho thinner walled pots don't seem so badly affected.
She's tried higher bisque, longer bisque and longer glost soaking, none of
these make a lot of difference. She's also tried tweaking the glaze recipes
to no great effect.
What is OM4 exactly? What temperature are you firing to? I hope other
Clayarters can illuminate.
Ama, Totnes, Devon.
Ron Roy on tue 14 dec 04
Hi Paul,
Could be the glaze or glazes - under or overfired - tell me what cone and
I'll compare them to other glazes in that range that I know work.
Send the recipies to:
ronroy@ca.inter.net (Ron Roy)
It will help if I can compare the ones that blister to the ones that don't.
You bisquing in a gas kiln?
When you refire the blisters - they get worse or better?
I can check out the clay body as well - I have lots of data to compare to.
RR
>I have been getting sporadic glaze blisters and what looks like "bubbles"
>on the surface of the glaze. Basically, it looks like a big bubble was
>pushing through the glaze, evidently from the body, and was just in the
>middle of bursting when it froze. Soaking the kiln at top temp has not
>completely solved the problem, although i have noticed that in the hotter
>parts of the kiln it does not seem to happen as much. Also, on thinner
>glazes, at least in the last firing, none at all came out that way. Then
>again, on very thick copper reds it still does not occur, unless i have a
>few glazes layered on top of each other, then the glazes are put on very
>thick.
>A slow, cone 04 electric bisque has not solved the problem. Has anyone else
>had this happen, and if so, how did you fix it? I am almost certain it has
>to do with the body (mostly EPK, OM-4, hawthorne bond and goldart). I have
>tried it without the goldart, and even without the hawthorne, and it still
>seems to occur sometimes, so now i am focusing on the OM-4 because i have
>heard of potters having problems with it. Any suggestions? thanks!
>Paul B
>Blanket Creek Pottery
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
mmondloch1 on tue 14 dec 04
Paul,
I was frustrated with this problem for awhile. My clay body also has
Goldart, Hawthorn and sometimes EPK but no ball clay. Very slow firing to
bisque temp (I single fire) and a soak at peak temp helped some but didn't
cure it. I've kept those things but what has seemed to really fix it is
soaking at a slightly lower temp after peak temp and/or slowly firing down
from peak.
My personal-don't really know what I'm talking about- theory on this:
Certain body constituents are breaking down and gassing-off at specific
temps as peak temp is reached. If I hold at the hottest temp, this breaking
down process is still in progress and if I quickly drop from there, the
bubbles freeze in place. By dropping the temp a bit and then soaking, the
gassing-off process is stopped (because I've already been through that temp)
but the glazes are still molten enough to heal over.
OK. Now I'd be interested to hear what's really going on. ;)
Sylvia
---
Mark & Sylvia Mondloch
Silver Creek Pottery & Forge
W6725 Hwy 144
Random Lake ,Wi 53075
HotArt@silvercreekpottery.com
http://www.silvercreekpottery.com
>I have been getting sporadic glaze blisters and what looks like "bubbles"
> on the surface of the glaze. Basically, it looks like a big bubble was
> pushing through the glaze, evidently from the body, and was just in the
> middle of bursting when it froze. Soaking the kiln at top temp has not
> completely solved the problem, although i have noticed that in the hotter
> parts of the kiln it does not seem to happen as much. Also, on thinner
> glazes, at least in the last firing, none at all came out that way. Then
> again, on very thick copper reds it still does not occur, unless i have a
> few glazes layered on top of each other, then the glazes are put on very
> thick.
> A slow, cone 04 electric bisque has not solved the problem. Has anyone
> else
> had this happen, and if so, how did you fix it? I am almost certain it has
> to do with the body (mostly EPK, OM-4, hawthorne bond and goldart). I have
> tried it without the goldart, and even without the hawthorne, and it still
> seems to occur sometimes, so now i am focusing on the OM-4 because i have
> heard of potters having problems with it. Any suggestions? thanks!
> Paul B
> Blanket Creek Pottery
John Hesselberth on tue 14 dec 04
Hi Sylvia, Paul and others,
Sylvia, you might be right on with your guess. Particularly if a fair
amount of iron is involved. Iron oxide can thermally reduce (yes, in an
oxidation atmosphere) giving off gas. This reaction is normally thought
to occur at cone 8 or 9, but I am becoming convinced it can occur as
low as cone 6 or 7 under some circumstances. I don't know what cone you
fire at but, if it is 6, perhaps one of the minor constituents of your
glaze or your clay body catalyzes the reaction. If you drop the
temperature a bit from peak and give time for the bubbles/blisters to
heal that would be an excellent solution if thermal reduction of iron
(or some similar reaction) is involved. So are the glazes causing the
problem for you ones that have a lot of iron? Wish I understood all
this better.
Regards,
John
On Tuesday, December 14, 2004, at 12:37 PM, mmondloch1 wrote:
> Certain body constituents are breaking down and gassing-off at specific
> temps as peak temp is reached. If I hold at the hottest temp, this
> breaking
> down process is still in progress and if I quickly drop from there, the
> bubbles freeze in place. By dropping the temp a bit and then soaking,
> the
> gassing-off process is stopped (because I've already been through that
> temp)
> but the glazes are still molten enough to heal over.
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Paul B on tue 14 dec 04
Actually, my clay does have 5%-10% mullite grog, which i believe is
mulochite. It is sold under the name "mulcoa mulgrain" and hopefully
someone here can confirm they are the same thing. I will do some research
to find out for sure. I was actually thinking that this could have
something to do with it because i don't recall having this happen about a
year ago when i was using brick grog. I will have to do some tests without
this stuff right away and see if that does it.
OM-4 is just a type of ball clay known for being quite variable. There is a
potter in north carolina who got a bad batch of it a few years ago and it
ruined almost every pot he made until he figured out the problem.
thanks,
Paul
Ivor and Olive Lewis on wed 15 dec 04
Dear John Hesselberth,
Given a Reaction Equation, values for the Entropies of both Reactants
and Products and Standard Free Energies of Formation you would be able
to calculate the Equilibrium Temperature at Standard Pressure.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
<giving off gas. This reaction is normally thought to occur at cone 8
or 9, but I am becoming convinced it can occur as low as cone 6 or 7
under some circumstances>>
Silver Creek Pottery on wed 15 dec 04
Hi John,
I fire to cone 10, but my firings are slow and probably don't reach as high
a top temp that cone 10 firings do for most. My pyrometer often only tops
out at about 2250F. I don't know how accurate the readings are on it but I
would think that I'm under 2300F for sure.
My firing schedule would probably be considered weird. Slow and fully
oxidized to cone 08 (I single fire). Then a 10 minute reduction with another
short redux at about cone 6-8 or so and a short redux at top temp with wood.
Firing slightly oxidized in-between. After lots of playing around with the
schedule, this is what I feel gives me fairly bright colors I want but still
enough depth and variation from the redux. Overall I guess I'd say it's a
light reduction schedule.
The iron oxide reaction you talk about is interesting. Are you referring to
the same reaction used to get oil spot glazes?
One of the glazes I was having problems with was Reitz Green- a fairly
common shino type cobalt green glaze. The other one that had been giving me
lots of problems, I hate to even say- is a rutile blue that's high in zinc
oxide- OK now you'all can throw your arms up in disgust and run screaming
from the room- "Oh no, not zinc oxide in reduction. ;) I've got it working
good for me now and just for the record, I have tested the recipe without
the zinc and found that it does need it.
Neither of these is really high in iron, but they do have some from the
rutile and my stoneware claybody would add some too. All I know is that
adding the slow cool-down from peak and/or slightly lower temp soak is what
seems to have eliminated the blister problems for me.
When you get this stuff figured out explain to us ok?
Sylvia
---
Mark & Sylvia Mondloch
Silver Creek Pottery & Forge
W6725 Hwy 144
Random Lake ,Wi 53075
HotArt@silvercreekpottery.com
http://www.silvercreekpottery.com
> Hi Sylvia, Paul and others,
>
> Sylvia, you might be right on with your guess. Particularly if a fair
> amount of iron is involved. Iron oxide can thermally reduce (yes, in an
> oxidation atmosphere) giving off gas. This reaction is normally thought
> to occur at cone 8 or 9, but I am becoming convinced it can occur as
> low as cone 6 or 7 under some circumstances. I don't know what cone you
> fire at but, if it is 6, perhaps one of the minor constituents of your
> glaze or your clay body catalyzes the reaction. If you drop the
> temperature a bit from peak and give time for the bubbles/blisters to
> heal that would be an excellent solution if thermal reduction of iron
> (or some similar reaction) is involved. So are the glazes causing the
> problem for you ones that have a lot of iron? Wish I understood all
> this better.
>
> Regards,
>
> John
> On Tuesday, December 14, 2004, at 12:37 PM, mmondloch1 wrote:
>
>> Certain body constituents are breaking down and gassing-off at specific
>> temps as peak temp is reached. If I hold at the hottest temp, this
>> breaking
>> down process is still in progress and if I quickly drop from there, the
>> bubbles freeze in place. By dropping the temp a bit and then soaking,
>> the
>> gassing-off process is stopped (because I've already been through that
>> temp)
>> but the glazes are still molten enough to heal over.
> John Hesselberth
> http://www.frogpondpottery.com
> http://www.masteringglazes.com
>
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