Jon Byler on fri 20 jan 06
Hi,
We have a problem with pinholing in some of our glazes. I think it is a clay body thing, because it
doesn't happen over whiteware.
I was told maybe the problem is due to iron pyrite (sulfur offgassing) in the clay body. any
suggestions about this?
we fire glazes primarily to cone 9-10 reduction. bisque ware is fired to cone 06 with the slow
bisque program on a super-duper bailey electric computer fired kiln.
I have a list of things to try, such as keeping the spyholes open on the bisque firing, soaking the
bisque firing at the end and/or raising the bisque temperature, more oxidization after body
reduction during the glaze firing before a final reduction cycle around cone 8, and possibly a soak
at the end of the glaze firing. We are also considering buying one of bailey's kiln ventilator kits so
as to reduce damage to the kiln form the sulfiric acid that forms when bisque firing ( this should
also assure lots of oxygen to the bisque ware).
anything else that could solve this problem?
many thanks,
jon byler
3D Studio Art Building Technician
Auburn University
Auburn, AL 36849
Bob Santerre on sat 21 jan 06
Hello Jon,
I too have struggled with pinholing problems ( cone 10, white stoneware
body, no problems with same glazes over porcelain body). Tried all the
suggestions about bisque firing (slower firing schedule, higher temp,
etc. ... no help!).
Here are 3 things that have worked for me:
1. Fire to higher temp,. cone 11-12 (if your clay body and glazes can
stand it).
2. Re-fire (after 1st glaze firing). Can change glaze "look" and can
cause glazes to run off pots onto shelves. Essentially the 2nd firing
to cone 10 is roughly equivalent to a 1st firing to cone 11-12. Have
tried re-firing to cone 06. This often drastically changes to "look" of
the glaze, but it does eliminate most of the pinholes.
3. Over-spray the bone-dry greenware with a porcelain-type slip
(typically you only need to spray the upper half of the pot where 95% of
the pinholes occur).
Firing down from 2300 to 2000 at 100 degrees per hr helps some, but does
not fully eliminate the pinholes.
Hope this helps, Bob
/////////////////////////////////////////////////////////////////////////////////////////////////////////////////
Jon Byler wrote:
>Hi,
>
>We have a problem with pinholing in some of our glazes. I think it is a clay body thing, because it
>doesn't happen over whiteware.
>
>I was told maybe the problem is due to iron pyrite (sulfur offgassing) in the clay body. any
>suggestions about this?
>
>we fire glazes primarily to cone 9-10 reduction. bisque ware is fired to cone 06 with the slow
>bisque program on a super-duper bailey electric computer fired kiln.
>
>
>I have a list of things to try, such as keeping the spyholes open on the bisque firing, soaking the
>bisque firing at the end and/or raising the bisque temperature, more oxidization after body
>reduction during the glaze firing before a final reduction cycle around cone 8, and possibly a soak
>at the end of the glaze firing. We are also considering buying one of bailey's kiln ventilator kits so
>as to reduce damage to the kiln form the sulfiric acid that forms when bisque firing ( this should
>also assure lots of oxygen to the bisque ware).
>
>anything else that could solve this problem?
>
>
>many thanks,
>
>jon byler
>3D Studio Art Building Technician
>Auburn University
>Auburn, AL 36849
>
>______________________________________________________________________________
>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.
>
>
>
>
>
Dave Finkelnburg on sat 21 jan 06
Jon,
I am sorry to hear about this problem. Pinholes in glazes can be really aggravating! :-(
I'm going to offer a theory on what's going on...but I'd appreciate input on this, too. It's possible you have iron oxide in your clay body. The iron oxide decomposes, giving off iron, beginning around cone 9. Your glaze is well-sealed over then. It's possible that is the source of the pinholes.
A standard technique for high firing stoneware clay is to reduce (body reduction) early in the firing to remove oxygen from the iron, then hold near neutral to milder reduction to the end of the firing, and possibly oxidize at the end. I have always speculated this was to prevent the problem you see, but I have never proven this. I'd like to know if I'm on the wrong track on this.
Based on my experience, the sulfur will burn off quite early in the firing, back before cone 04, so it should not be a problem for your glazes unless they melt very early and you don't have clean bisque. However, as the iron sulfide converts to iron oxide in the bisque you now have a potential source of oxygen to make bubbles late in your glaze firing.
Ultimately I think iron oxide decomposition may be what you have to deal with.
Good potting!
Dave Finkelnburg
Jon Byler wrote:
We have a problem with pinholing in some of our glazes. I think it is a clay body thing, because it
doesn't happen over whiteware.
---------------------------------
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Jacqui Kruzewski on sat 21 jan 06
I had this problem. I found, after a process of elimination (and trying all
the suggestions I received), that it was because I had used an old batch of
potash feldspar that had been given to me, along with the rest of his
pottery, by a potter who had left the business. Clearly it was a different
formulation - and of course reinforces the message about materials from
different sourses. I ordered more potash feldspar from my usual supplier,
made up a new batch of claze - and hey presto. Of course this is not the
same cause (claybody) as you seem to have, but my solution may be worth
trying.
Not wanting to waste the old batch - I noted that my old and slow kiln
produced a nice glaze fisnish with no pinholes. I narrowed it down to very
slow heating at the end of the firing - pretty much as Ron and John advocate
in their book. I might note here that a long soak caused the glaze to change
in character and run. Now I set my controller to heat at 50 centegrate an
hour for the last 200 centegrade. You could even go as low as 30. I add my
usual 20 min soak.This works.
Sorry my temps are all in centegrade, I don't have a conversion chart to
hand.
Jacqui
North Wales
>Here are 3 things that have worked for me:
>1. Fire to higher temp,. cone 11-12 (if your clay body and glazes can
>stand it).
>2. Re-fire (after 1st glaze firing). Can change glaze "look" and can
>cause glazes to run off pots onto shelves. Essentially the 2nd firing
>to cone 10 is roughly equivalent to a 1st firing to cone 11-12. Have
>tried re-firing to cone 06. This often drastically changes to "look" of
>the glaze, but it does eliminate most of the pinholes.
>3. Over-spray the bone-dry greenware with a porcelain-type slip
>(typically you only need to spray the upper half of the pot where 95% of
>the pinholes occur).
>
>Firing down from 2300 to 2000 at 100 degrees per hr helps some, but does
>not fully eliminate the pinholes.
>
>Hope this helps, Bob
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terryha on sun 22 jan 06
jacqui,
don't use a conversion chart. and, it's only in Liberia and USA. the world
uses metric system.
multiply 1.8 to convert difference in centigrade to in fahrenheit,
at 90 degrees F an hour for the last 360 degrees F. You could even go as low
as 54 degrees F.
or you may suggest,
at 100 degrees F an hour for the last 400 degrees F. You could even go as
low as 60 degrees F.
terry
>Date: Sat, 21 Jan 2006 20:32:16 +0000
>From: Jacqui Kruzewski
>Subject: Re: pinholing problem (probably claybody/firing issues)
>
>..... Now I set my controller to heat at 50 centegrate an
>hour for the last 200 centegrade. You could even go as low as 30. I add my
>usual 20 min soak.This works.
>
>Sorry my temps are all in centegrade, I don't have a conversion chart to
>hand.
>
>Jacqui
>
>North Wales
terry hagiwara
terryha@sbcglobal.net
http://www.geocities.com/terry.hagiwara
http://www.terryha.com/pottery
Ron Roy on sun 22 jan 06
Hi Jon,
Sounds like it could be your bisque firing is not "clean" enough - slow
down from red heat till the end - say 100C per hour tops and leave a spy
open - or better still have a vent running.
I recommend cone 04 for bisque for several reasons - and a short soak at
the end - say 10 min to even out the kiln.
If you care to send me the recipies for the offending glazes I can tell you
how to lower the melting a little - stiff glazes are part of the pinholing
problem usually.
Refiring might work as you are really going a cone higher when refiring to
the same cone - but If it's a clay body problem it could also get worse.
You are welcome to send any recipes directly to me at -
ronroy@ca.inter.net
I don't keep them or publish them in any way.
RR
>We have a problem with pinholing in some of our glazes. I think it is a
>clay body thing, because it
>doesn't happen over whiteware.
>
>I was told maybe the problem is due to iron pyrite (sulfur offgassing) in
>the clay body. any
>suggestions about this?
>
>we fire glazes primarily to cone 9-10 reduction. bisque ware is fired to
>cone 06 with the slow
>bisque program on a super-duper bailey electric computer fired kiln.
>
>
>I have a list of things to try, such as keeping the spyholes open on the
>bisque firing, soaking the
>bisque firing at the end and/or raising the bisque temperature, more
>oxidization after body
>reduction during the glaze firing before a final reduction cycle around
>cone 8, and possibly a soak
>at the end of the glaze firing. We are also considering buying one of
>bailey's kiln ventilator kits so
>as to reduce damage to the kiln form the sulfiric acid that forms when
>bisque firing ( this should
>also assure lots of oxygen to the bisque ware).
>
>anything else that could solve this problem?
>
>
>many thanks,
>
>jon byler
>3D Studio Art Building Technician
>Auburn University
>Auburn, AL 36849
>
>______________________________________________________________________________
>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.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ivor and Olive Lewis on sun 22 jan 06
Dear Jon Byler,=20
<a clay body thing, because it doesn't happen over whiteware. I was told =
maybe the problem is due to iron pyrite (sulfur offgassing) in the clay =
body. Any suggestions about this?>>
If this is rather coarse clay I suggest you get hold of a Gold Pan.. Dry =
some of your clay, say a couple of pounds, break it down then slake in =
sufficient water to create a slurry and then "Pan it out" to get rid of =
the lighter and less dense fractions. Examine any residue with a X 10 =
hand lens. Pyrite will settle and can be recognised because of the =
bright yellow reflective lustre. It may pay to be patient and pan out =
this residue, clearing the Pyrites.....you never know your luck ! ! !
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Ivor and Olive Lewis on mon 23 jan 06
Dear Jon Byler,
I support Dave Finkelnburg's suggestion about Iron oxide decomposing at =
temperatures above cone 9. This has been described in work on Oil Spot =
Tenmoku glazes fired in the cone 10 range. Bubbles are due to Oxygen =
which is not mopped up by a reducing atmosphere. =20
I brought this to the attention of the group in the past when I =
discovered bubbles in glaze tests that were supposed to be clear with a =
high bright surface. The conclusion I drew was that some or the raw =
materials contained sufficient iron to promote this reaction and that it =
was not due to gas coming from the clay body, in my case a white smooth =
porcelain.
One observation that lead me to this conclusion was that although the =
glazes would normally have been thin and transparent the samples I fired =
were about 3 mm thick and showed a very pale green tinge, good enough to =
be considered Celadon.
I suggest you find out the oxide assay compositions of your glaze raw =
materials and look for residual iron oxide, stated as Fe2O3.=20
I do not think this can be prevented. It may be ameliorated by adjusting =
the firing schedule.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
=20
jonathan byler on mon 23 jan 06
hi everyone,
thanks for your replies. I am sort of getting an idea of where to go
with this from the discussion. I'll have to do a little experimenting
in a variety of areas and see what happens.
Until now, I have kept the kiln reducing so that there is a healthy
flame coming from the bottom spy hole from about 1800F until shut down,
so I doubt there is much problem from not enough reduction and the
FeO2/FeO3 breakdown issues. I am a bit loathe to run the reduction to
extra hard (heavy chimney flame, etc) , so as not to waste fuel, and
also to keep from having to much black-coring in the ware, and making
it too brittle, but it is always reducing nicely throughout the end of
the firing.
I am still convinced that it is a clay body problem that we are having
and not so much with the glaze ingredients. this is because I have not
had any trouble over white ware. I am going to try firing our bisque a
bit higher (say cone 05 or cone 04), and leave all the spy holes open
throughout the firing, then take it from there to see what could be the
next possible culprit.
My thoughts now are that it is not necessarily FeO3 or FeO2 that is the
problem, rather some form of iron pyrite or FeS2 /FeS3. i am not sure
of all the forms sulfur takes when it is in clay, so this is but a
guess. Maybe this is a job for some friendly chemistry students here
at auburn. We have pretty significant sulfur burnout during our bisque
firing, but towards the end of the firing it goes away (at least it can
no longer be smelled). I'll have too through a whole firing sometime,
so I can see exactly at what temperature we are getting initial sulfur
burnout, and figure out the sulfur compounds that are burned out in the
bisque firing.
The reason I think we are having some kind of Iron Pyrite problem is
that it does not melt until about 2193F. When this breaks down, you
get elemental iron and sulfur I am guessing. Since the boiling point
of sulfur is only 832F, it would off gas right away, oxygen or no. I
think it is just a matter of how completely these compounds can break
down, and at the moment they seem to be not all breaking down before
the end of the firing perhaps due to the short timeframe in which this
breakdown can occur (from about 2193F to about 2370F). There is not
much of a temperature climb to cone 9/10 after the FeS2/FeS3 starts to
break down. so maybe a longer soak at the end would let this reaction
take place more completely and allow the pinholes to heal better. I
was thinking that somehow soaking and putting the kiln in oxidation
around 2100F until just before cone 9 would possibly help the sulfur
compounds to break down and off-gas faster. the question there, is how
deep the oxygen can permeate into the semi vitrified clay body at these
temperatures and help the reaction along. My Dad, who is a chemist,
thought maybe adding something like KnO3 (salt peter) in a small
percentage to the clay body might encourage this breakdown too.
any thoughts on these speculations (especially adding saltpeter in
about 1/2% by weight)?
thanks,
jon
jon byler
3-D Building Technician
Art Department
Auburn University
Auburn, AL 36849
(334) 844-5279
On Jan 22, 2006, at 10:50 PM, Ivor and Olive Lewis wrote:
> Dear Jon Byler,
>
> I support Dave Finkelnburg's suggestion about Iron oxide decomposing
> at temperatures above cone 9. This has been described in work on Oil
> Spot Tenmoku glazes fired in the cone 10 range. Bubbles are due to
> Oxygen which is not mopped up by a reducing atmosphere.
>
> I brought this to the attention of the group in the past when I
> discovered bubbles in glaze tests that were supposed to be clear with
> a high bright surface. The conclusion I drew was that some or the raw
> materials contained sufficient iron to promote this reaction and that
> it was not due to gas coming from the clay body, in my case a white
> smooth porcelain.
>
> One observation that lead me to this conclusion was that although the
> glazes would normally have been thin and transparent the samples I
> fired were about 3 mm thick and showed a very pale green tinge, good
> enough to be considered Celadon.
>
> I suggest you find out the oxide assay compositions of your glaze raw
> materials and look for residual iron oxide, stated as Fe2O3.
>
> I do not think this can be prevented. It may be ameliorated by
> adjusting the firing schedule.
>
> Best regards,
>
> Ivor Lewis.
> Redhill,
> South Australia.
>
>
> _______________________________________________________________________
> _______
> 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.
>
Dave Finkelnburg on tue 24 jan 06
Jonathon,
I agree, from your reduction schedule it does sound like you have effectively reduced the iron in your clay and glaze and oxygen off-gassing from iron oxide should not cause bubbles/blisters/pinholes in your glaze.
The sulfur in your clay as iron sulfide (pyrite) begins to burn off above 400°C and is gone well below your top bisque temperature IF enough oxygen is present in the bisque firing. Vent the bisque well, don't stack it too tight, fire it clean and you will not have sulfur to worry about in the glaze firing. If you still get pinholes, then something else will be the cause.
If you research roasting of sulfide ores you will find the sulfides are all burned off by 975°C.
Regards,
Dave Finkelnburg
jonathan byler wrote:
Until now, I have kept the kiln reducing so that there is a healthy
flame coming from the bottom spy hole from about 1800F until shut down,
The reason I think we are having some kind of Iron Pyrite problem is
that it does not melt until about 2193F.
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Kathi LeSueur on tue 24 jan 06
Pinholing is a problem I fought for years with one glaze. It was a
glaze with titanium as the opacifier and it seems like a number of peope
who've reprorted pinholing have it with a glaze containing titanium.
Most of us were firing cone 10 reduction with a clay that had iron in
it. My problem was further complicated by the fact that it was limited
to vase and cylinder forms.
Many people on the list offered suggestions. I tried them all. Slower
firing. Changes in the reduction cycles. Difference in trimming
technique. .But in the end, I had to use a different clay for the glaze
that was pinholing. Since going with a clay that has no iron in it I've
eliminated the problem.
Kathi
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