Paul Borian on fri 13 jul 07
Can anyone explain, in technical terms, what actually happens to a glaze
which has been overfired to the point where it has blisters on it? What
actually causes these surface defects to occur? I have a few glazes which
will blister at cone 11 if i don't add about 10% more silica to the batch,
then it almost never happens.
If anyone can point me to a technical explanation of this somewhere on
line please do.
thanks,
Paul
John Hesselberth on sun 15 jul 07
Hi Paul,
More than likely one of your ingredients or something in your clay
body is decomposing and giving off gas at the wrong time. For
example, under the right circumstances iron oxide can thermally
reduce at about cone 8 giving off oxygen, but it can do it a bit
higher or lower depending on the overall composition of the glaze/
body. I have a glaze that blisters at cone 7, but is fine if only
fired to cone 6. Many glazes will do this if fired too hot. There is
always some chemistry happening in hot silica soups--you have to pick
a temperature to fire at where the glaze is fully melted, but no
gases are being evolved at the moment.
Regards,
John
On Jul 13, 2007, at 12:54 PM, Paul Borian wrote:
> Can anyone explain, in technical terms, what actually happens to a
> glaze
> which has been overfired to the point where it has blisters on it?
> What
> actually causes these surface defects to occur? I have a few glazes
> which
> will blister at cone 11 if i don't add about 10% more silica to the
> batch,
> then it almost never happens.
> If anyone can point me to a technical explanation of this somewhere on
> line please do.
> thanks,
> Paul
John Hesselberth
www.frogpondpottery.com
"Man is a tool-using animal....without tools he is nothing, with
tools he is all" .... Thomas Carlyle
Ivor and Olive Lewis on sun 15 jul 07
Dear Paul Borian,=20
The phenomenon you describe may be laid at the door of Chemical =
Reaction. Therefore, to formulate an explanation it is necessary to know =
something about the composition of your clay and glaze and your firing =
atmosphere.
Without that information my suggestion would be that even if there is no =
added Iron Oxide, this compound might be responsible as a residual =
contaminant in your raw materials. Fe2O3 has the ability to self =
decompose releasing oxygen even in the absence of a reducing agent. A =
reaction with silica may suppress this autolysis.
For a potted version of potential causes of gas evolution I suggest you =
read the entry "Breakdown" in Frank and Janet Hamer, "The Potter's =
Dictionary of Materials and Techniques".
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Ron Roy on sun 15 jul 07
Hi Paul,
Most glazes go through boiling stages - if you stop firing during one of
those stages you will get blisters. Probably some material going through a
gassing stage. Maybe even the clay underneath - have you tested your clay
for absorption at cone 11? If you know the makeup of the body I can compare
with other cone 10 clays for instance.
If you send me your glaze recipe perhaps the molecular formula will give us
more clues.
It's always helpful to compare the molecular formulas of other glazes with
the troublesome one to get more clues about what is happening.
RR
>Can anyone explain, in technical terms, what actually happens to a glaze
>which has been overfired to the point where it has blisters on it? What
>actually causes these surface defects to occur? I have a few glazes which
>will blister at cone 11 if i don't add about 10% more silica to the batch,
>then it almost never happens.
>If anyone can point me to a technical explanation of this somewhere on
>line please do.
>thanks,
>Paul
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
WJ Seidl on mon 16 jul 07
Ron, you made a very good point. I came to the same conclusion watching
pancakes cook on a grill the other day.
The bubbles come up through from the "hot" side. If the cooking is fast
enough (high enough heat) the batter
hardens leaving the bubbles in place. I looked at it hard, and DP asked
what I was staring at. "Pinholing" I said.
Interesting. Blistering could simply be pinholing with a hard surface
on the glaze overhead, not allowing gases to escape.
Blunting might be more a "delamination" of the glaze from the surface of
the body?
Best,
Wayne Seidl
Ron Roy wrote:
> Hi Paul,
>
> Most glazes go through boiling stages - if you stop firing during one of
> those stages you will get blisters. Probably some material going through a
> gassing stage. Maybe even the clay underneath - have you tested your clay
> for absorption at cone 11? If you know the makeup of the body I can compare
> with other cone 10 clays for instance.
>
> If you send me your glaze recipe perhaps the molecular formula will give us
> more clues.
>
> It's always helpful to compare the molecular formulas of other glazes with
> the troublesome one to get more clues about what is happening.
>
> RR
>
>
>> Can anyone explain, in technical terms, what actually happens to a glaze
>> which has been overfired to the point where it has blisters on it? What
>> actually causes these surface defects to occur? I have a few glazes which
>> will blister at cone 11 if i don't add about 10% more silica to the batch,
>> then it almost never happens.
>> If anyone can point me to a technical explanation of this somewhere on
>> line please do.
>> thanks,
>> Paul
>
Angela Davis on mon 16 jul 07
Anyone who has watched a raku glaze mature has witnessed
the process some glazes go through to maturity. Some glazes
actually go foamy and if your inexperienced you might think that piece
was ruined, but at maturity you will see the surface smooth out beautifully.
Sometimes this happens quite suddenly and it can be influenced by the
atmosphere in the kiln, reducing or oxygen.
It might benefit interested non raku potters to attend a raku firing just to
watch
the different stages a glaze goes through to maturity, can be very
informative.
If we could watch our glazes in our electric kilns we would be able to tell
when all
the nasty business is over and we have the surface we want.
Angela Davis
In Homosassa, too hot to go out. Wayne might be able to fry his cakes on
the drive.
----- Original Message -----
From: "WJ Seidl"
To:
Sent: Monday, July 16, 2007 8:12 AM
Subject: Re: What happens to an overfired glaze?
> Ron, you made a very good point. I came to the same conclusion watching
> pancakes cook on a grill the other day.
> The bubbles come up through from the "hot" side. If the cooking is fast
> enough (high enough heat) the batter
> hardens leaving the bubbles in place. I looked at it hard, and DP asked
> what I was staring at. "Pinholing" I said.
> Interesting. Blistering could simply be pinholing with a hard surface
> on the glaze overhead, not allowing gases to escape.
> Blunting might be more a "delamination" of the glaze from the surface of
> the body?
>
> Best,
> Wayne Seidl
>
>
> Ron Roy wrote:
>> Hi Paul,
>>
>> Most glazes go through boiling stages - if you stop firing during one of
>> those stages you will get blisters. Probably some material going through
>> a
>> gassing stage. Maybe even the clay underneath - have you tested your clay
>> for absorption at cone 11? If you know the makeup of the body I can
>> compare
>> with other cone 10 clays for instance.
>>
>> If you send me your glaze recipe perhaps the molecular formula will give
>> us
>> more clues.
>>
>> It's always helpful to compare the molecular formulas of other glazes
>> with
>> the troublesome one to get more clues about what is happening.
>>
>> RR
>>
>>
>>> Can anyone explain, in technical terms, what actually happens to a glaze
>>> which has been overfired to the point where it has blisters on it? What
>>> actually causes these surface defects to occur? I have a few glazes
>>> which
>>> will blister at cone 11 if i don't add about 10% more silica to the
>>> batch,
>>> then it almost never happens.
>>> If anyone can point me to a technical explanation of this somewhere on
>>> line please do.
>>> thanks,
>>> Paul
>>
>
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Dave Finkelnburg on wed 18 jul 07
Paul,
The blisters are the result of a combination of
vapor from some source plus low glaze viscosity.
The viscosity (runniness) of the glaze is easy to
deal with. Network formers increase glaze viscosity.
Thus, adding silica makes the glaze more viscous,
hence more resistant to blistering. Alumina is even
more effective at this. Thus, adding clay really does
the trick because it's just alumina and silica.
The vapor is the real problem. It can come from
iron oxide decomposing, from pore space in the glaze
trapped by the melt, or from pore space in the clay
body, or any combination of the above. Moisture in
frits can also be a source. Since it is virtually
impossible to eliminate all possible vapor sources,
the key to stopping the blistering is usually to make
the glaze more viscous.
Boron, by the way, is probably not a good glaze
ingredient at your firing temperature. It seems to
come from materials that may give off water vapor and
it also seems to make a less viscous melt than silica.
Good potting,
Dave Finkelnburg, listening to a fine cricket
chorus on a very warm summer night in Idaho
Date: Fri, 13 Jul 2007 11:54:41 -0500
From: Paul Borian
Can anyone explain, in technical terms, what actually
happens to a glaze
which has been overfired to the point where it has
blisters on it?
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Earl Krueger on thu 19 jul 07
On 7/18/07, Dave Finkelnburg wrote:
> The blisters are the result of a combination of
> vapor from some source plus low glaze viscosity.
>
Dave,
I'm having a little trouble getting around to your way
of thinking on this. It just seems counterintuitive to me.
Doing a little google-ing to check my memory I found
the following:
"A low-viscosity magma, like basalt, will allow the
escaping gases to migrate rapidly through the magma
and escape to the surface. However, if the magma is
viscous, like rhyolite, its high polymerization will impede
the upward mobility of the gas bubbles."
( http://www.geology.sdsu.edu/how_volcanoes_work/Controls.html )
This would seem to contradict your argument.
However, I also found on the same website something
that might support your argument and explain some
cases of pinholing as well:
"Alternatively, as magma cools and anhydrous minerals
begin to crystallize out of the magma, the residual liquid
will become increasingly enriched in gas. In this case,
the increased vapor pressure in the residual liquid can
also lead to gas exsolution."
earl krueger
elmira, oregon, usa
Michael Wendt on fri 20 jul 07
Earl,
One key difference between a glaze and magma
is thickness. With any gas filled liquid, the bubbles
rise to the top depending on viscosity and the
density of the liquid. We see this effect clearly
with bubbles rising in a bottle of alcohol we have
inverted quickly. Do the same with a bottle of
syrup or dishwashing liquid and see how much
slower they rise.
Now consider a glaze only a few thousandths
of an inch thick. On flat horizontal surfaces,
this same force results in migration
of bubbles to the top surface. Bubbles will
also rise on the bottom, this time toward the
clay-glaze interface.
On vertical surfaces, there is no reason for
bubbles to exit a surface of a liquid except
expansion until they burst, resulting in a
crater. The only other mechanisms at work
to help heal the crater are the cohesive nature
of many liquids which pull the glaze surface
back together and gravity which causes
vertical flow to further obscure such defects.
Low surface tension and the resulting inability
to support a bubble seems to be the way to
get rid of blistering so if the addition of more
silica or silica+alumina will make it harder
for a bubble to maintain a surface, then this
would explain why it works with glazes.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
USA
208-746-3724
http://www.wendtpottery.com
wendtpot@lewiston.com
Earl wrote:
On 7/18/07, Dave Finkelnburg
wrote:
> The blisters are the result of a combination of
> vapor from some source plus low glaze viscosity.
>
Dave,
I'm having a little trouble getting around to your way
of thinking on this. It just seems counterintuitive to
me.
Earl Krueger on sat 21 jul 07
OK, Micheal, I got ya. Forgot all about
surface tension. Time to study some
more.
I'm starting to consider that high viscosity prevents
bubble formation, as Dave F. described, by
retarding the migration of gaseous elements so
that coalescence to form bubbles does not occur.
Thanks...
earl...
Dave Finkelnburg on mon 23 jul 07
Earl,
I apologize for being so slow to respond to your
question. I'm swamped at the moment!
Regarding your excellent question, and Michael
Wendt's response, bubbles DO NOT MOVE in our glazes.
Not shouting, just need to emphasize this important
point. I'm quite confident this is so.
Our glazes are like the rhyolite...highly
polymerized glass networks. Adding clay or silica
makes the glaze even more polymerized, thus more
resistant to bubbles blistering the surface.
Blisters tend to form when the glaze viscosity
drops due to higher temperature (overfiring) or high
flux content (runny ash glazes, for example).
Bubbles do not rise in a normal glaze, one that
will stay on a vertical surface. The glaze is too
viscous, the time it is molten is too short, and the
bubbles are too small for the bubbles to move. I
don't think you can say any of these are true for a
magma, so you can't compare our glaze to the magma.
You must be throwing. How else would you have time
to ponder such esoteric stuff? :-)
Good potting!
Dave
From: Earl Krueger
On 7/18/07, Dave Finkelnburg wrote:
> The blisters are the result of a combination of
> vapor from some source plus low glaze viscosity.
Dave,
I'm having a little trouble getting around to your way
of thinking on this. It just seems counterintuitive
to me.
Doing a little google-ing to check my memory I found
the following:
"A low-viscosity magma, like basalt, will allow the
escaping gases to migrate rapidly through the magma
and escape to the surface. However, if the magma is
viscous, like rhyolite, its high polymerization will
impede
the upward mobility of the gas bubbles."
(http://www.geology.sdsu.edu/how_volcanoes_work/Controls.html
)
This would seem to contradict your argument.
However, I also found on the same website something
that might support your argument and explain some
cases of pinholing as well:
"Alternatively, as magma cools and anhydrous minerals
begin to crystallize out of the magma, the residual
liquid
will become increasingly enriched in gas. In this
case,
the increased vapor pressure in the residual liquid
can
also lead to gas exsolution."
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