Ron Roy on tue 28 jan 97
If the following is the cause of your blisters you will know - because when
you refire the glaze to get rid of them it won't work and may even make it
worse.
Clay bodies with iron in them, if reduced in the BISQUE firing, can cause
over fluxing of the clay. If there are specks of iron it can result in
local over-firing of the body and blistering of the glaze.
Here is what happens - Carbon is released from clay bodies during the
bisque - if there is not enough oxygen in the kiln to combine with the
carbon (making CO2) then the carbon takes the oxygen from the iron (Fe2O3
to FeO) - in other words reduces the red iron to FeO. This can happen in an
electric kiln - in fact does so most of the time. It can happen quite
easily in a gas kiln as well but for different reasons.
It is essential to fire iron bearing bodies slow enough, during the bisque
firing, IN THE PRESENCE OF OXYGEN to make sure the Fe2O3 is not reduced.
The critical temperatures are from 700C to 900C. Fact - once the iron is
reduced it is very difficult to reoxidize - certainly firing the bisque
higher is of no consequence.
So what you say - well Fe2O3 is not much of a flux at all. FeO however is a
major flux which starts melting at 900C. The kind of blistering I am trying
to describe is the result of the FeO fluxing the clay too much, the clay
starts to break down - producing gases - which then try to come through the
glaze. The reason the problem is so persistent is because the gases
continue to be produced - because we think they are gases produced by the
glaze (causing pinholes) we apply more heat to make the glaze flow more or
refire. This breaks down the clay even more and produces more gas.
As you can imagine - if there are iron specks in the body - this fault will
happen sooner because the iron is concentrated at the speck and causes more
melting.
The moral of the story is make sure there is an oxidizing atmosphere in
your kiln while bisque firing between 700C (1290F) to 900C (1650F). One way
to ascertain if there is oxygen in a kiln is to introduce something that
will burn like paper or wood. If it burns inside there is oxygen.
If you suspect the problem and are using iron bearing clays fire your
bisque slower (50C per hr) during the critical time and open a spy. If
firing a fuel burning kiln make sure all your burners and damper are set to
oxidize.
I know many of you are buying your clay so you cannot change the recipe so
- either try other bodies - as has been suggested before - or fire in such
a way as to eliminate the causes of blistering etc.
Apparently most clay mines test for carbon and sulfur.
Keep in mind - carbon (coal) in clay adds to plasticity.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Tom's E-mail on sat 14 sep 02
I have a very nice red/brown glaze that breaks black over texture =
[Litman's Red/Brown]. I've fired it dozens of time and it always comes =
out beautiful without defects of anykind. While unloading my kiln a few =
weeks ago I sustained a small chip in a large flower pot; it was hardly =
noticeable but I thought I would just touch it up and fire again. =
Yesterday, I unloaded my kiln and that pot was so badly blistered, I =
just hammered it. My question is why should I get blistering in a very =
stable glaze with a second firing? Thanks for any insight.
Tom Sawyer
tsawyer@cfl.rr.com
Timakia@AOL.COM on sun 15 sep 02
Tom, that chip that you saw first, was poberly also because of blistering. It
tells me that your clay/glaze fit is wrong. How I understand it is that the
shrinkage on one of the two is too big for the other. To correct it if you
want to keep using te glaze, will be to change your clay or to adjust the
glaze. In the chemistry part I do not want to even try put my foot and maybe
it would not suit the purpose at all.
What you can maybe try, is to bisque a little higher. If the clay did not
shrink enough before, this may help.
Take care.
Antoinette
Antoinette Badenhorst
http://hometown.aol.com/timakia
105 Westwood circle
Saltillo, MS
38866
Lily Krakowski on sun 15 sep 02
The theory is this; that every time you fire a glaze or a pot something else
happens in the firing. At the beginning gases escape, carbonaceous matter
burns out, diverse ingredients start to melt. And so on. But upon refiring
all this stuff has happened already, and new activities start EARLIER than
they did before. So, as your glaze melted the first time, some of it may
have been absorbed by the pot--some had to be for the glaze to stick; and
that material that seeped into the pot may now be missing on the surface.
In my admitedly limited experience glazes can and do 'separate out'in the
firing enough so that when I refire --which I rarely do--I put some new
glaze on the old.
I am eager to hear what the specialists say....
Tom's E-mail writes:
> I have a very nice red/brown glaze that breaks black over texture [Litman's Red/Brown]. I've fired it dozens of time and it always comes out beautiful without defects of anykind. While unloading my kiln a few weeks ago I sustained a small chip in a large flower pot; it was hardly noticeable but I thought I would just touch it up and fire again. Yesterday, I unloaded my kiln and that pot was so badly blistered, I just hammered it. My question is why should I get blistering in a very stable glaze with a second firing? Thanks for any insight.
> Tom Sawyer
> tsawyer@cfl.rr.com
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Lili Krakowski
P.O. Box #1
Constableville, N.Y.
(315) 942-5916/ 397-2389
Be of good courage....
Mondloch on mon 16 sep 02
>My question is why should I get blistering in a very stable glaze with a
second firing?
>
Tom,
I think sometimes a glaze becomes overfired when it's refired. It seems to
help to apply a thin coat of more glaze and put the piece in a cooler area
of the kiln the second time around.
Now my question for anyone- is why one glaze will blister if overfired and
another may become simply runny. I understand that if you have a glaze
material that becomes volatile at some certain high temp it will release
gasses and can cause blistering- but why then not all the time? why will a
material cause blistering in one recipe but not another? Does more alumina
in a recipe prevent the glaze from running so then it boils in place
instead? is it a matter of how much it is being involved with the melt of
other materials?
oops. I guess I answered a question with a question.
Sylvia
---
Mark & Sylvia Mondloch
Silver Creek Pottery & Forge
W6725 Hwy 144
Random Lake ,Wi 53075
HotArt@silvercreekpottery.com
http://www.silvercreekpottery.com
Wanda Holmes on mon 16 sep 02
Tom, I have had a cone 04 glaze that worked just fine on a tile when
fired to cone 6 blister when I fired it to cone 6 on a bowl. The tile
and the bowl were of the same claybody and bisqued the same way. I
repaired the bowl by refiring it to cone 05 and it came out just fine.
I don't know exactly what this means, but I offer the experience in
hopes that you gain a clue of some sort from it.
Wanda
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG] On
Behalf Of Tom's E-mail
Sent: Saturday, September 14, 2002 11:08 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Blistering
I have a very nice red/brown glaze that breaks black over texture
[Litman's Red/Brown]. I've fired it dozens of time and it always comes
out beautiful without defects of anykind. While unloading my kiln a few
weeks ago I sustained a small chip in a large flower pot; it was hardly
noticeable but I thought I would just touch it up and fire again.
Yesterday, I unloaded my kiln and that pot was so badly blistered, I
just hammered it. My question is why should I get blistering in a very
stable glaze with a second firing? Thanks for any insight. Tom Sawyer
tsawyer@cfl.rr.com
________________________________________________________________________
______
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
Moderator of the list is Mel Jacobson who may be reached at
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iandol on tue 17 sep 02
Dear Sylvia,
I have not read the original post so I do not know what the two glaze =
recipes were.
But using information from CM about the production of Oil Spot Glazes it =
would seem that Red Iron Oxide can decompose at high temperature with =
the evolution of Oxygen. If this gas were trapped between layers of =
glaze due, say to differing viscosities, the result might be some form =
of blistering.
Best regards,
Ivor Lewis
Chris Schafale on wed 18 sep 02
Tom,
I have also had this experience with high-iron glazes in particular,
also to some degree with glazes that have very high boron levels.
I'm guessing that on the refiring, the glaze is somehow interacting
with the body in a way that releases additional gases. Out of
curiosity, do you use a white body, or one with much iron in it? I
vaguely think this problem has been worse on brown clays for
me....
Chris
My question is why should I get blistering in a very
> stable glaze with a second firing? Thanks for any insight. Tom Sawyer
> tsawyer@cfl.rr.com
>
> Light One Candle Pottery
Fuquay-Varina, North Carolina, USA
(south of Raleigh)
candle@intrex.net
http://www.lightonecandle.com
Mondloch on wed 18 sep 02
Hello Ivor,
Ok, so would it also be reasonable to say that -
If one finds that there is less of a blistering problem when refiring at a
lower top temperature then the original firing, it could be because iron( or
whatever?) in the glaze or underlying body had already finished the
outgassing it would do at that particular temperature during the first
firing.
??
Sylvia
---
Mark & Sylvia Mondloch
Silver Creek Pottery & Forge
W6725 Hwy 144
Random Lake ,Wi 53075
HotArt@silvercreekpottery.com
http://www.silvercreekpottery.com
----- Original Message -----
From: "iandol"
To:
Sent: Monday, September 16, 2002 2:16 PM
Subject: Re: Blistering
Dear Sylvia,
I have not read the original post so I do not know what the two glaze
recipes were.
But using information from CM about the production of Oil Spot Glazes it
would seem that Red Iron Oxide can decompose at high temperature with the
evolution of Oxygen. If this gas were trapped between layers of glaze due,
say to differing viscosities, the result might be some form of blistering.
Best regards,
Ivor Lewis
Tom's E-mail on thu 19 sep 02
Chris,
I use a tan stoneware body [some iron]. I'm just puzzled if its due
outgassing which is a good bet why it didn't outgas at the time of the first
cone 6 fire instead of the second refiring after I only touched up the rim.
Tom Sawyer
tsawyer@cfl.rr.com
Christena Schafale on thu 19 sep 02
Tom,
I don't know the answer, and I've wondered the same thing myself. What I
*speculate* is that in the refiring, with the additional heat work, the
flux in the glaze, or perhaps the iron in the glaze, is "attacking" the
clay-glaze interface area in such a way that additional reactions take
place, or reactions take place at a lower temperature. So let's say that
there is a reaction that would ordinarily take place at 1250C. When you
fire the first time, and only hit 1200C, it doesn't take place. When you
refire, though, because of the previous heatwork, maybe the reaction takes
place at 1195C, so you get the blistering.
That's one possibility, anyway, even if it has no scientific evidence
whatsoever to back it up.
Chris
At 02:31 AM 9/19/02 -0700, you wrote:
>Chris,
>I use a tan stoneware body [some iron]. I'm just puzzled if its due
>outgassing which is a good bet why it didn't outgas at the time of the first
>cone 6 fire instead of the second refiring after I only touched up the rim.
>Tom Sawyer
>tsawyer@cfl.rr.com
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at
>melpots@pclink.com.
Consultation and Referral Specialist
Resources for Seniors
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http://www.resourcesforseniors.com
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Raleigh, NC 27609
Snail Scott on fri 20 sep 02
At 02:31 AM 9/19/02 -0700, you wrote:
>...why it didn't outgas at the time of the first
>cone 6 fire instead of the second refiring after I only touched up the rim.
Some of my blistering problems seem due to the
viscosity of some melted glazes, which don't
let the air escape. Unmelted glazes let the air
through, but when melted will seal the surface,
until the get pretty runny.
I have this problem most often with high-frit
matte glazes. The reading I've done seems to
indicate that frits melt earlier in the firing
cycle than most raw-materials glazes, and many
matte glazes tend to have a high viscosity. (I
figure that means they're actually underfired.)
I have occasionally gotten blisters on just
isolated parts of a piece. When I refired the
piece, they only got worse. When I ground the
bubbles off and reapplied the glaze, it still
happened. The only thing that got rid of it was
to use a similar-appearing glaze with a different
formulation on those spots. Fortunately, this
particular piece actually looked better for the
variation, and the price range justified the
prolonged 'messing around' with it.
I usually rib my surfaces smooth, but on this
piece, I noticed a few contours that didn't please
me, after the piece had mostly dried. I scraped
them down to suit me, but re-ribbing was no longer
possible without re-wetting, and I thought, "I'm
going to glaze this piece anyway; it won't matter."
Well, those were the spots that blistered - over
the rough, groggy surface. None on the smooth,
rib-finished parts.
This may be mostly irrelevant to your situation,
but the tendence of 'sealed' glazes to trap
bubbles when they get viscous but not fully runny,
even on refiring, may be a factor.
-Snail
Corinne en Ben on wed 23 oct 02
I have a clear satin glaze commercial, so I don't know its formula.
the first time I used it it gave a milky result with some bubbles when I =
fired it to cone 05
I refired it to 1100 degr C and the bubbles and milkyness were gone and =
the piece was nice and smooth.
So I used the glaze again and a few other pieces same clay and different =
clay. Fired it up to 1100 C with a long soak to make sure the glaze =
would smooth out. All the pieces came out with bubbles and blisters, =
especially on parts where the glaze is a bit thicker.
I fired it again to 1100 C with a long soak, it did not smooth out the =
bubbles. I thought maybe the temp is too high so I fired it to a lower =
temp to cone 04 (1060 C) but the bubbles are still there.
Why did the bubbles dissapear the first time and not the second time, I =
don't understand.
Can I make these bubbles dissapear or should I better throw away this =
glaze and not use it anymore?
It drives me crazy and I can't give up because with the first piece the =
blisters and bubbles dissapeared.
Logically it could or should dissapear......or not?.....
Corinne Manintveld
(Clayartist in Brunei)
Lily Krakowski on wed 23 oct 02
Ok.
You fired your glaze to 05. 1040C in orton cones.
Then you refired it to 1100C which is close to a cone 03 in Orton--O3 being
listed at 1115.C
Then you refired again at cone 04 or 1060 and again blisters.
There is a lot of guessing one needs to do here, because blistering is, I
think one of the hardest problems to cure.
You say you soaked. But how slowly did you go up? how high was your bisque?
This bisque business is a real thing. If the clay is not bisqued high
enough a lot of gases generated by the burning off of different materials
will come out later. Blah, blah, blah. First thing I would try is higher
bisque.
Next I would go up really slowly. I would try to increase my firing cycle
by adding an hour even two to reaching cone temp.
According to Harry Fraser's CERAMIC FAULTS AND THEIR REMEDIES a book i think
should be in every hotel room alongside the Gideon Bible, speaks of whiting
and dolomite as particular bubble producers by their release of carbon
dioxide. In other words if your boughten glaze were to be high in these,
fire very slowly on the way up. Soaking smoothes over but soaking does not
stop active bubbling that is still going on.
Last; the explanation of why your cure worked once and not again may be
voltage drop! If you have an electric kiln it is affected by the voltage
provided by the supplier. If there was a voltage drop at the start of your
firing it actually may have slowed the firing at a specific stage, and been
a benefit.
Before you all get dirty rolling on the floor with laughter, I used to have
trouble firing in winter because the voltage dropped when the farmers milked
in the dark hours. The combo of lighting and milking machines on an already
burdened system lowered the voltage enough to give me trouble. I had to
start the kiln at night, see that it was on Medium during the morning
milking, and fire it to temp before 3:00 PM
i have no idea what the electricity supply is like where you are. Maybe
your lucky kiln was fired during a voltage drop? VERY hot day, MUCH
airconditioning going on?
Good luck
Corinne en Ben writes:
> I have a clear satin glaze commercial, so I don't know its formula.
> the first time I used it it gave a milky result with some bubbles when I fired it to cone 05
> I refired it to 1100 degr C and the bubbles and milkyness were gone and the piece was nice and smooth.
> So I used the glaze again and a few other pieces same clay and different clay. Fired it up to 1100 C with a long soak to make sure the glaze would smooth out. All the pieces came out with bubbles and blisters, especially on parts where the glaze is a bit thicker.
> I fired it again to 1100 C with a long soak, it did not smooth out the bubbles. I thought maybe the temp is too high so I fired it to a lower temp to cone 04 (1060 C) but the bubbles are still there.
> Why did the bubbles dissapear the first time and not the second time, I don't understand.
> Can I make these bubbles dissapear or should I better throw away this glaze and not use it anymore?
> It drives me crazy and I can't give up because with the first piece the blisters and bubbles dissapeared.
> Logically it could or should dissapear......or not?.....
>
> Corinne Manintveld
> (Clayartist in Brunei)
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Lili Krakowski
P.O. Box #1
Constableville, N.Y.
(315) 942-5916/ 397-2389
Be of good courage....
Ron Roy on sun 27 oct 02
Hi Corinne,
First - it is important to use large cones to make sure you are getting to
the right temperature pyrometers are not a good way of determining end
temperature because they don't measure heat work - time and temperature.
Most glazes go through a bubbling stage - sometimes its before maturation
and sometimes after - sometimes both.
When you refire a glaze you add to the heat work - it sounds like the glaze
should be fired higher. Make sure you are going to the recommended
temperature - the only way to do that is with cones.
Blistering is sometimes caused by firing bisque too fast and/or with not
enough ventilation (free oxygen) in the kiln.
Your comment about the blisters being worse where the clay is thicker is a
clue here - either the clay needs longer cooking or the bisque needs a
cleaner and longer firing. I recommend firing bisque to cone 04 by the way
- help with a number of problems.
RR
>I have a clear satin glaze commercial, so I don't know its formula.
>the first time I used it it gave a milky result with some bubbles when I
>fired it to cone 05
>I refired it to 1100 degr C and the bubbles and milkyness were gone and
>the piece was nice and smooth.
>So I used the glaze again and a few other pieces same clay and different
>clay. Fired it up to 1100 C with a long soak to make sure the glaze would
>smooth out. All the pieces came out with bubbles and blisters, especially
>on parts where the glaze is a bit thicker.
>I fired it again to 1100 C with a long soak, it did not smooth out the
>bubbles. I thought maybe the temp is too high so I fired it to a lower
>temp to cone 04 (1060 C) but the bubbles are still there.
>Why did the bubbles dissapear the first time and not the second time, I
>don't understand.
>Can I make these bubbles dissapear or should I better throw away this
>glaze and not use it anymore?
>It drives me crazy and I can't give up because with the first piece the
>blisters and bubbles dissapeared.
>Logically it could or should dissapear......or not?.....
>
>Corinne Manintveld
>(Clayartist in Brunei)
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Corinne en Ben on mon 28 oct 02
Well,
I fired the things again, in a cone 6 firing and the result
was a very smooth glossy glaze.
The clear satin glaze was supposed to be for firing between
1040 - 1100 C
Blisters are gone!
Corinne Manintveld
(Clayartist in Brunei)
_____________________________________________________________
_________________
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Ben on mon 18 oct 04
Does anyone have any experience or solutions with a rutile glaze =
blistering over white stoneware or porcelain? I think I've isolated it =
to a reaction between one clay and a new batch of rutile. The next kiln =
will tell but it's been kinda rough. I initially assumed it was firing =
but I don't think so. C10 oxidation basic =
Feldspar/kaolin/dolomite/silica/whiting with 7% Rutile.
Thanks,
Ben
Lili Krakowski on wed 6 dec 06
Kathy McDonald wrote:
"[Mugs glazed only with CM White] looked rather boring so I thought
I'll jazz them up a bit, and see what happens if I
refire them to C6 with some colorful overglazes (Mayco) and
a clear coat over the outside of the mug (also Mayco c6).
I have done this in the past
with mugs glazed only in the CM White and they were fine.
That's the "control" part of this experiment.
"They were fine with the exception of areas that had the
iron/rutile
glaze decoration. It was a crusty bubbled mess . ...
"All other areas are fine. It is highly unlikely that the
clay caused it because
it was fired to vitrification, and was a nice semi gloss
surface in all areas except those with the iron underglaze.
" I broke a few just to see...there was that characteristic
blackened area almost to the core of the clay right where
the iron decoration was.
I also broke a piece fired a few firings ago in the same
glaze combo but NOT refired
and it showed no evidence of any blackened areas in the
clay."
So. An always successful c.10 white glaze with some
rutile and iron painted atop, had underglazes and c.6 glaze applied.
Refiring to c.6 created nasty blisters on the surface, but,
examination showed, they occurred only where the overglaze/ c.6 glaze was
atop the rutile/iron decoration.
The question is : what happened?
Well, of recent, I have--through correspondence with a friend/ expert--begun
to think a lot more about the vertical behavior of glaze. We generally
focus on the horizontal behavior, and sort of ignore what I long have
suspected: that the vertical behavior is an equally important mover and
shaker.
What I am guessing is this: At c.6, the c.10 glaze experienced some
slight softening which allowed the release of some gases which may have been
simply ones that had not quite finished their escape in the original
firing. These gases made their escape safely where there was the c.10 base
and the c.6 overcoat. This because the interface reactions between the two
glazes was copasetic.
But where the gases were either under the rutile/iron layer, or inside it,
the gases could not elbow their way through the underglaze and the c.6
glaze. I am guessing the blackness is from the reduction of
whatever created the gases produced
It also might be possible that there was a reaction between the rutile/iron
and the overglazes, creating gases which could not make their escape
totally.
I think we tend to think of escaping gases as tornadoes, or gusts...I
somehow do not think so. I think they are very slight and gentle, just
enough to disturb the glaze surface.
If it were my problem I would check out the whole situation in layers. A:
the c.10 glaze alone. B.the c 10 glaze with rutile/iron
C. the c.10 glaze with rutile/iron and overglaze. D. The whole nine
yards--c.10., rutile/iron, overglaze. c.6 glaze E.. The cone 10 glaze with
just the c. 6 glaze atop. F. The cone 10 glaze with just underglaze on top.
And for good measure, G. the underglaze atop the clay, with c.6 glaze over
it.
Lili Krakowski
Be of good courage
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