mel jacobson on wed 13 oct 04
a question that has been on my mind.
does clay contain gas?
often it is said that we burn off gasses that
are in the clay during bisque firing.
i do not think clay has gas.
organic material is burned in the kiln during
bisque firing..it will turn to gas at some point.
this is what escapes. and, we want that out of the
clay body. if we do not burn all the crap during
bisque, it will burn during the glaze firing and
cause pinholes and other such trouble.
ron/ivor please elaborate.
mine is just a guess.
mel
From:
Minnetonka, Minnesota, U.S.A.
web site: my.pclink.com/~melpots
or try: http://www.pclink.com/melpots
new/ http://www.rid-a-tick.com
Anne Webb on wed 13 oct 04
as per hamer,, organic matter and sulphur
>From: mel jacobson
>Reply-To: Clayart
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: question/bisque/gasses
>Date: Wed, 13 Oct 2004 06:56:37 -0500
>
>a question that has been on my mind.
>
>does clay contain gas?
>
>often it is said that we burn off gasses that
>are in the clay during bisque firing.
>
>i do not think clay has gas.
>
>organic material is burned in the kiln during
>bisque firing..it will turn to gas at some point.
>this is what escapes. and, we want that out of the
>clay body. if we do not burn all the crap during
>bisque, it will burn during the glaze firing and
>cause pinholes and other such trouble.
>
>ron/ivor please elaborate.
>mine is just a guess.
>mel
>From:
>Minnetonka, Minnesota, U.S.A.
>web site: my.pclink.com/~melpots
>or try: http://www.pclink.com/melpots
>new/ http://www.rid-a-tick.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.
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Anne Webb on wed 13 oct 04
just curious, lee.. how long do your glaze firings take? if you fire slow
this may erradicate some of the problems typically caused by bisquing too
low..? ..anne
>From: Lee Love
>Reply-To: Clayart
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: Re: question/bisque/gasses
>Date: Thu, 14 Oct 2004 00:57:14 +0900
>
>mel jacobson wrote:
>
>>ron/ivor please elaborate.
>>mine is just a guess.
>
> I have not experienced any problems with organics or
>"gassing" at cone 012 bisque temps, not in my own work or in my
>teacher's work or Toshi's at NCC (Toshi introduced me to 012 bisque.)
> And I can assure you that my clay has a higher percentage of
>organics in it than the industrial clays we use in the States.
>(When you torch the shigaraki, you can see the organics glow on the
>surface of the clay, as they burn up.) You can also notice the
>organics by the aroma coming off of my throwing water bowl. I am
>guessing that red heat takes care of problems organics may create in
>the clay.
>
> I single fire some of my work. If there are going to be
>any problems with organics or gasses venting, you should see them in
>single firing. I found that shinos cause problems if you put them on
>greenware (you get bloating), but are less likely to cause problems on
>leatherhard.
>
>--
>in Mashiko, Japan http://mashiko.org
>http://www.livejournal.com/users/togeika/ WEB LOG
>http://public.fotki.com/togeika/ Photos!
>
>______________________________________________________________________________
>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.
_________________________________________________________________
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Ivor and Olive Lewis on thu 14 oct 04
Dear Mel,
You are correct, I do not think clay has gas either. But then, it all
depends on what you mean...........
I hope other people read your question carefully!!
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Lee Love on thu 14 oct 04
mel jacobson wrote:
> ron/ivor please elaborate.
> mine is just a guess.
I have not experienced any problems with organics or
"gassing" at cone 012 bisque temps, not in my own work or in my
teacher's work or Toshi's at NCC (Toshi introduced me to 012 bisque.)
And I can assure you that my clay has a higher percentage of
organics in it than the industrial clays we use in the States.
(When you torch the shigaraki, you can see the organics glow on the
surface of the clay, as they burn up.) You can also notice the
organics by the aroma coming off of my throwing water bowl. I am
guessing that red heat takes care of problems organics may create in
the clay.
I single fire some of my work. If there are going to be
any problems with organics or gasses venting, you should see them in
single firing. I found that shinos cause problems if you put them on
greenware (you get bloating), but are less likely to cause problems on
leatherhard.
--
in Mashiko, Japan http://mashiko.org
http://www.livejournal.com/users/togeika/ WEB LOG
http://public.fotki.com/togeika/ Photos!
Lee Love on fri 15 oct 04
Anne Webb wrote:
> just curious, lee.. how long do your glaze firings take? if you fire slow
> this may erradicate some of the problems typically caused by bisquing too
> low..? ..anne
My glaze firings run from 14 to 16 hours (about the same as
when I gas fired.) In gas, I have fired 012 bisque in as little as 12
hours. A lot of that time in the woodklin is at low temp, in the
begining (below 700*C.) I have to start the stoking below the
grates, to keep the heat from raising too quickly. (When I start stoking
above the grate, I hit cone 012 in less than an hour.) Part of the
reason for going slow in the begining is because I fire some unglazed
shigaraki and bizen without bisquing and I also fire some once fired
(since I read David's article on single-firing.)
--
Lee in Mashiko, Japan http://mashiko.org
http://www.livejournal.com/users/togeika/ WEB LOG
http://public.fotki.com/togeika/ Photos!
Ron Roy on sat 16 oct 04
>does clay contain gas?
>often it is said that we burn off gasses that
>are in the clay during bisque firing.
>i do not think clay has gas.
>organic material is burned in the kiln during
>bisque firing..it will turn to gas at some point.
>this is what escapes. and, we want that out of the
>clay body. if we do not burn all the crap during
>bisque, it will burn during the glaze firing and
>cause pinholes and other such trouble.
>ron/ivor please elaborate.
Hi Mel,
I have not thought about that - perhaps some - from decaying organic matter
but it would leave during early stages of a firing.
Many potters have gas - and hot air too.
Any gases in glaze that gets into the ware - probably some but I would not
think it would be a problem.
If a glaze has a flourine compound in it - and is trapped in the body
during firing that can be a problem - cryolite does that.
The main thing we are talking about though - is if the carbonous material
in a clay body is not burned out properly - and there is iron in the body
that gets reduced during the bisque firing (it can happen in insingle fire
as well) - that reduced iron becomes a strong flux - and can overflux the
body. When that happens the clay starts to break down and produces gas -
which will try to get through the now molten glaze. This all happens at the
very end of the firing.
The result is pin holes and blisters - and sometime bloating of the clay.
If you refire to get rid of the blisters and/or pin holes the problem
persits - and can even get worse.
When you refire anything you are actually reaching a higher parometric cone
eqivelent - because of the extra heat work. Just put a used cone in your
next firing and see what happens.
I digress - so if refiring results in the same problem or worse - it's the
bisque firing that is the problem - because the clay is being overfired -
again.
If the probem is cured by refiring - This is because the glaze - on
refiring - got more heat work. That glaze needs to be adjusted to fire a
bit lower - or fired higher - or soaked longer - or fired down a bit - so
the pin holes have a chance to heal over.
Well I don't know if everyone understands this - I've tried to explain it
so many times now that I am thinking it's my fault there are still
questions.
Anyone - please - edit this so everyone gets it - I'm beginning to think
I'm talking a language many don't understand.
RR
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Kathi LeSueur on sat 16 oct 04
ronroy@CA.INTER.NET wrote
>>The main thing we are talking about though - is if the carbonous material in a clay body is not burned out properly - and there is iron in the bodythat gets reduced during the bisque firing (it can happen in insingle fire
>>as well) - that reduced iron becomes a strong flux - and can overflux the
>>body. When that happens the clay starts to break down and produces gas -
>>which will try to get through the now molten glaze. This all happens at the
>>very end of the firing.
>>
>>The result is pin holes and blisters - and sometime bloating of the clay.>>>>>
>>
Ron,
If I bisque my iron bearing clay to cone 05 (or 04) in a well-vented
electric kiln, then fire in my gas kiln with a mild body reduction
starting when cone 010 drops and ending when 05 drops, am I likely to
experience the above problem. And, if I am likely, at what cone would
you suggest starting a body reduction?
Thanks so much,
Kathi
>>
>>
Mark & Sylvia Mondloch on sun 17 oct 04
RR wrote:>
> The main thing we are talking about though - is if the carbonous material
> in a clay body is not burned out properly - and there is iron in the body
> that gets reduced during the bisque firing (it can happen in insingle fire
> as well) - that reduced iron becomes a strong flux - and can overflux the
> body. When that happens the clay starts to break down and produces gas -
> which will try to get through the now molten glaze. This all happens at
> the
> very end of the firing.
>
> The result is pin holes and blisters - and sometime bloating of the clay.
>
Hi,
I try to relate this to what it means to me as a single-firer.I understand
that- given enough time - carbon will burn out by 900 C (1652F) so I fire
very slow in full oxidation up to that point (just like a slow bisque)
expecting that the glazes are not yet melting and won't seal in the escaping
gasses. So far so good.
But I know my claybody contains significant sulfur. According to Hamer and
Hamer under 'Breakdown' , sulfur is just as effective as the carbon in
reducing iron and the sulfur isn't oxidized until between 900C (1652F) and
1150C( 2102 F) which by that time my glazes are starting to melt. So just
how significant is the sulfur out-gassing to causing glaze faults? And how
does one deal with it if you want to do reduction before 1150C(2100F)? I'd
assume that eliminating iron bearing and/or sulfur bearing clays from the
claybody is one solution, but I'm wondering if there's any firing strategies
one can use. It seems that I read somewhere (can't remember where) that
reduction will actually curtail the sulfur out-gassing... but that isn't
what I'm getting by reading Hamer.
> Well I don't know if everyone understands this - I've tried to explain it
> so many times now that I am thinking it's my fault there are still
> questions
I don't think that this is something that most of us "get" all at once, but
rather a little bit at a time. That's not your fault. ;)
Sylvia
-
Mark & Sylvia Mondloch
Silver Creek Pottery & Forge
W6725 Hwy 144
Random Lake ,Wi 53075
HotArt@silvercreekpottery.com
http://www.silvercreekpottery.com
Ron Roy on sun 17 oct 04
Hi Kathi,
I have never had a problem and I start reduction at different times for
different clay bodies and glazes - and I sometimes reduce heavily starting
at cone 08 even with a high iron body.
You are the first person to bring this question up - I have often wondered
about that.
If the iron is reduced in the bisque and caused a problem - why not when
it's reduced in the glazes firing?
All I can come up with is heat work. Remember - the iron - if reduced
during the bisque firing gets to melt on the way up and on the way down -
reduced iron is a flux from about 800C up. It again gets to start melting
again starting at 800C in the glaze firing.
If it's not reduced in the bisque - then it does not start to help the melt
till it's reduced during the glaze firing. Also the fact that it has been
formulated to be properly vitrified during the reducing glazes period - if
the person who designed it knew what they were doing of course.
Tuckers tests all the cone 10 bodies at cone 10 oxidation and reduction by
the way - and I see some differences in the absorption numbers between the
two - but not as much as I would guess most of the time.
Firings in my old kiln were very slow towards the end and the oxidation
firings were faster so it's not a fair comparison. May new gas kiln fires
faster so the data will be more comparable. The new data will be more
comparable.
RR
>>>The main thing we are talking about though - is if the carbonous
>>>material in a clay body is not burned out properly - and there is iron
>>>in the bodythat gets reduced during the bisque firing (it can happen in
>>>insingle fire
>>>as well) - that reduced iron becomes a strong flux - and can overflux the
>>>body. When that happens the clay starts to break down and produces gas -
>>>which will try to get through the now molten glaze. This all happens at the
>>>very end of the firing.
>>>
>>>The result is pin holes and blisters - and sometime bloating of the
>>>clay.>>>>>
>>>
>
>Ron,
>
>If I bisque my iron bearing clay to cone 05 (or 04) in a well-vented
>electric kiln, then fire in my gas kiln with a mild body reduction
> starting when cone 010 drops and ending when 05 drops, am I likely to
>experience the above problem. And, if I am likely, at what cone would
>you suggest starting a body reduction?
>
>Thanks so much,
>
>Kathi
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on sun 17 oct 04
Hello Sylvia,
Good post - and you bring up a couple of questions I can only guess at the
answers.
Stephen Hill warns of the problems for single fire by the way - I wonder if
he learned the hard way? I do think you are wise to fire slowly - and I do
think you are right about the higher burning temp of sulphur - I can
remember when I started to smell it in the old days BV (before venting.)
I seem to remember something about reduction and sulphur - the reduction
lowering the ignition point of the sulphur.
Local reduction will be happening and will probably help to liberate any
sulphur but you could give the load a shot of reduction when it is safe to
do so - like at 800C or so.
You can always leave test bars around in the kiln by the way - absorbency
tests will tell you what is happening.
Finding clays without carbon and sulphur is going to be tricky - The miners
know they can be a problem and will say they are low - I have data from KT
for instance - on all their clays - the earlier one had carbon listed - in
the more recent one it is left out. Besides - the amounts of carbon and
sulfur will vary from lot to lot.
I think the better way is to assume you are going to get some and tailor
your techniques to deal with it.
The one problem I am concerned about is glazes with a lot of boron in them
- if they seal over the body before the gases escape there will be problems
- does anyone have any problems with high iron clays when firing in
reduction at cone 6?
RR
>Hi,
>I try to relate this to what it means to me as a single-firer.I understand
>that- given enough time - carbon will burn out by 900 C (1652F) so I fire
>very slow in full oxidation up to that point (just like a slow bisque)
>expecting that the glazes are not yet melting and won't seal in the escaping
>gasses. So far so good.
>
>But I know my claybody contains significant sulfur. According to Hamer and
>Hamer under 'Breakdown' , sulfur is just as effective as the carbon in
>reducing iron and the sulfur isn't oxidized until between 900C (1652F) and
>1150C( 2102 F) which by that time my glazes are starting to melt. So just
>how significant is the sulfur out-gassing to causing glaze faults? And how
>does one deal with it if you want to do reduction before 1150C(2100F)? I'd
>assume that eliminating iron bearing and/or sulfur bearing clays from the
>claybody is one solution, but I'm wondering if there's any firing strategies
>one can use. It seems that I read somewhere (can't remember where) that
>reduction will actually curtail the sulfur out-gassing... but that isn't
>what I'm getting by reading Hamer.
>
>
>> Well I don't know if everyone understands this - I've tried to explain it
>> so many times now that I am thinking it's my fault there are still
>> questions
>
>I don't think that this is something that most of us "get" all at once, but
>rather a little bit at a time. That's not your fault. ;)
>
>Sylvia
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Louis Katz on sun 17 oct 04
Sometime in 1993 (I think), I was working at the Bray clay business and
we had cutomer with a bloating problem on a white claybody. Several
possiblilities were discussed as reasons. Poor burnout of carbonaceous
materials was one consideration. The customer was quite able with a
kiln and said that the kiln oxidized from red heat to 08 and that the
bisques showed the pink color of reduction not the pale color of
reduction (mildly iron bearing body).
We also considered chunks of some undermixed material but broken bloats
did not show typical glassy margins.
I wrote clayart. Richard Burkett said that according to his father many
sulphur compounds are hard to volatilize under 2000 F(? might have been
a different number). in oxidizing conditions and that it would be
easier to make them dissappear if the were reduced first. On the
customers remaining bisque he tried a short reduction at 1800 F(? can't
remember, might have been 1500F) in the bisque and the problem went
away. I don't know if the solution is what actually solved the problem,
but it seems like it.
I may be reading my very old Handbook of Chemistry and Physics (1949)
wrong, but it looks like Barium Carbonate does not decompose until
1450C. So when this substance goes into solution maybe it give off its
CO2.
MaGnesium Carbonate looses its CO2 at 900 C
Iron Pyrite melts at 1171 C and decomposes above that.
Seems to me that there is lots of stuff that can give off bubbles after
carbon burns and that oxidation will not help much of it. Maybe the
oxidation is playing some other role.
I think the processes involved in this problem are far more complex
than normally assumed and that the rule of thumb, "oxidize until you
need reduction" is just that a rule of thumb that may not always work.
Louis
On Oct 17, 2004, at 10:54 AM, Mark & Sylvia Mondloch wrote:
> RR wrote:>
>> The main thing we are talking about though - is if the carbonous
>> material
>> in a clay body is not burned out properly - and there is iron in the
>> body
>> that gets reduced during the bisque firing (it can happen in insingle
>> fire
>> as well) - that reduced iron becomes a strong flux - and can overflux
>> the
>> body. When that happens the clay starts to break down and produces
>> gas -
>> which will try to get through the now molten glaze. This all happens
>> at
>> the
>> very end of the firing.
>>
>> The result is pin holes and blisters - and sometime bloating of the
>> clay.
>>
>
> Hi,
> I try to relate this to what it means to me as a single-firer.I
> understand
> that- given enough time - carbon will burn out by 900 C (1652F) so I
> fire
> very slow in full oxidation up to that point (just like a slow bisque)
> expecting that the glazes are not yet melting and won't seal in the
> escaping
> gasses. So far so good.
>
> But I know my claybody contains significant sulfur. According to Hamer
> and
> Hamer under 'Breakdown' , sulfur is just as effective as the carbon in
> reducing iron and the sulfur isn't oxidized until between 900C (1652F)
> and
> 1150C( 2102 F) which by that time my glazes are starting to melt. So
> just
> how significant is the sulfur out-gassing to causing glaze faults? And
> how
> does one deal with it if you want to do reduction before 1150C(2100F)?
> I'd
> assume that eliminating iron bearing and/or sulfur bearing clays from
> the
> claybody is one solution, but I'm wondering if there's any firing
> strategies
> one can use. It seems that I read somewhere (can't remember where) that
> reduction will actually curtail the sulfur out-gassing... but that
> isn't
> what I'm getting by reading Hamer.
>
>
>> Well I don't know if everyone understands this - I've tried to
>> explain it
>> so many times now that I am thinking it's my fault there are still
>> questions
>
> I don't think that this is something that most of us "get" all at
> once, but
> rather a little bit at a time. That's not your fault. ;)
>
> Sylvia
>
> -
> Mark & Sylvia Mondloch
> Silver Creek Pottery & Forge
> W6725 Hwy 144
> Random Lake ,Wi 53075
> HotArt@silvercreekpottery.com
> http://www.silvercreekpottery.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.
>
Bob Santerre on mon 18 oct 04
Ron,
Perhaps you'd offer some comments on a problem I've had with refirings
to eliminate glaze blisters. My clay body is Highwater Clay Moonwhite
(a white stone ware, bisque firing to cone 06 ox,). The glazes are a
Tenmoku base glaze with an accent/overglaze of a Rutile formulation
(that fluxes quite a bit at cone 10-11 reduction). The problem has been
blisters occurring mainly in regions of large vases and bowls where only
the Tenmoku glaze has been applied (small forms don't seem to have
blisters at all). I grind out the blisters and overglaze them with the
Rutile and re-fire. The results are consistently the following: the
blisters I repaired are nicely filled in by the grinding and
over-glazing, but unfortunately (maddeningly) new blisters have
appeared, sometimes more new ones form than were repaired initially. If
I now repeat the process and do a second re-firing, the same results are
obtained ... old blisters repaired, but new ones appear. Seems like an
endless process and the blisters are winning!
My question is ... after three firings one would think that whatever
gaseous evolution is causing the blisters, it should be dissipating
with repeated firings and you should eventually produce a saleable pot.
Obviously this is not happening ... re-firing to repair blisters has
been a total waste of my time and gas. What's your best theory to
explain these observations? Any and all comments from the List are
welcomed. Does anyone know how to fix glaze blisters? So far my
experience suggests a large hammer's the best solution.
Bob
/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
Ron Roy wrote:
>Hi Kathi,
>
>I have never had a problem and I start reduction at different times for
>different clay bodies and glazes - and I sometimes reduce heavily starting
>at cone 08 even with a high iron body.
>
>You are the first person to bring this question up - I have often wondered
>about that.
>
>If the iron is reduced in the bisque and caused a problem - why not when
>it's reduced in the glazes firing?
>
>All I can come up with is heat work. Remember - the iron - if reduced
>during the bisque firing gets to melt on the way up and on the way down -
>reduced iron is a flux from about 800C up. It again gets to start melting
>again starting at 800C in the glaze firing.
>
>If it's not reduced in the bisque - then it does not start to help the melt
>till it's reduced during the glaze firing. Also the fact that it has been
>formulated to be properly vitrified during the reducing glazes period - if
>the person who designed it knew what they were doing of course.
>
>Tuckers tests all the cone 10 bodies at cone 10 oxidation and reduction by
>the way - and I see some differences in the absorption numbers between the
>two - but not as much as I would guess most of the time.
>
>Firings in my old kiln were very slow towards the end and the oxidation
>firings were faster so it's not a fair comparison. May new gas kiln fires
>faster so the data will be more comparable. The new data will be more
>comparable.
>
>RR
>
John Hesselberth on mon 18 oct 04
On Monday, October 18, 2004, at 11:20 AM, Bob Santerre wrote:
> My question is ... after three firings one would think that whatever
> gaseous evolution is causing the blisters, it should be dissipating
> with repeated firings and you should eventually produce a saleable pot.
Hi bob,
I hope John Britt jumps in here too as he is probably the world's
expert in this area. This is very likely thermal reduction of iron
oxide that is giving off the gas--and in a tenmoku you have a lot of
iron to go around. Read John's writings (in his book or his recent
NCECA presentation or his CM article) on oil spot glazes to get some
ideas on how to let the blisters clear (or keep them from forming).
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Ron Roy on wed 20 oct 04
Hi Bob,
It sounds like the clay is being over fired - do some absorption tests on
it and that should tell you something - if you need instructions let me
know.
When a clay with iron in it is not fired clean in the bisque firing - and
the iron is reduced and becomes a flux - and overfires the clay it's the
same thing as having a clay that will be overfired at top temperature.
Both situations are overfired clay - with the white body it's because it
has too much flux to start with.
The clue is usually - when you fire it again - which means more heat work -
the clay decomposes a bit more - and produces more gas.
Does it happen with any other glazes?
Or maybe - because the tenmoku's tend to seal over faster and trap gas left
over from an unclean bisque - keeping in mind - all that iron in a tenmoku
glaze helping to seal it over before other glazes.
Better tell us about your bisque firing - as I said - I always go to 04 for
bisque.
Are the blisters round? Do they happen where the clay is thicker?
Still sounds like overfing of the clay to me - more firing more gas - more
blisters.
RR
>Perhaps you'd offer some comments on a problem I've had with refirings
>to eliminate glaze blisters. My clay body is Highwater Clay Moonwhite
>(a white stone ware, bisque firing to cone 06 ox,). The glazes are a
>Tenmoku base glaze with an accent/overglaze of a Rutile formulation
>(that fluxes quite a bit at cone 10-11 reduction). The problem has been
>blisters occurring mainly in regions of large vases and bowls where only
>the Tenmoku glaze has been applied (small forms don't seem to have
>blisters at all). I grind out the blisters and overglaze them with the
>Rutile and re-fire. The results are consistently the following: the
>blisters I repaired are nicely filled in by the grinding and
>over-glazing, but unfortunately (maddeningly) new blisters have
>appeared, sometimes more new ones form than were repaired initially. If
>I now repeat the process and do a second re-firing, the same results are
>obtained ... old blisters repaired, but new ones appear. Seems like an
>endless process and the blisters are winning!
>
>My question is ... after three firings one would think that whatever
>gaseous evolution is causing the blisters, it should be dissipating
>with repeated firings and you should eventually produce a saleable pot.
>Obviously this is not happening ... re-firing to repair blisters has
>been a total waste of my time and gas. What's your best theory to
>explain these observations? Any and all comments from the List are
>welcomed. Does anyone know how to fix glaze blisters? So far my
>experience suggests a large hammer's the best solution.
>
>Bob
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Bob Santerre on wed 20 oct 04
Ron,
Appreciate your comments. It sounds like your advice is focussed on
getting the bisque firing right, ie., going to cone 04 with good
oxidation. I've tried bisquing at cone 08 and cone 06 ... really saw no
change in the amount of blistering. Guess I'll give cone 04 a try.
Frankly I've avoided that high a bisque temp because (without any
personal experience) I've believed rumor/teaching that glaze uptake
would be decreased - again it sounds like your advice would be to
thicken the glaze if that becomes a problem.
I have 2 electric kilns I use for bisque firing, one a very old (it has
many cracks) Crusader (3.5 cu ft) and the other a fairly new Bailey (12
cu ft fortress, VERY tight kiln). My larger pots fired (cone 06, 100
degrees/hr) in the Bailey kiln have distinctly worse blistering problems
than pots similarly fired in the Crusader. From your comments, it may
be that the more open Crusader kiln is providing a better oxidizing
atmosphere and thus giving better firing out of organics and leaving
less reduced iron in the bisqueware. Moving up to cone 04 in the
Crusader kiln may solve my problems with blisters on larger pieces ... I
hope so.
Thickness of the piece doesn't seem to be the issue, in fact just the
opposite may be true. Blisters seem to be most prominent on the upper
third of the pots and especially on or near the rims which frequently
are a little thinner than lower sections of the pieces.
The Tenmoku glaze is the only one that gives this problem. Others,
e.g., Celadons, Shaner's Yellow, Leach Clear, etc., do not develop blisters.
Ron, thanks again for your thoughtful comments.
Bob
///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
Ron Roy wrote:
>Hi Bob,
>
>It sounds like the clay is being over fired - do some absorption tests on
>it and that should tell you something - if you need instructions let me
>know.
>
>When a clay with iron in it is not fired clean in the bisque firing - and
>the iron is reduced and becomes a flux - and overfires the clay it's the
>same thing as having a clay that will be overfired at top temperature.
>
>Both situations are overfired clay - with the white body it's because it
>has too much flux to start with.
>
>The clue is usually - when you fire it again - which means more heat work -
>the clay decomposes a bit more - and produces more gas.
>
>Does it happen with any other glazes?
>
>Or maybe - because the tenmoku's tend to seal over faster and trap gas left
>over from an unclean bisque - keeping in mind - all that iron in a tenmoku
>glaze helping to seal it over before other glazes.
>
>Better tell us about your bisque firing - as I said - I always go to 04 for
>bisque.
>
>Are the blisters round? Do they happen where the clay is thicker?
>
>Still sounds like overfing of the clay to me - more firing more gas - more
>blisters.
>
>RR
>
>
>
>
>>Perhaps you'd offer some comments on a problem I've had with refirings
>>to eliminate glaze blisters. My clay body is Highwater Clay Moonwhite
>>(a white stone ware, bisque firing to cone 06 ox,). The glazes are a
>>Tenmoku base glaze with an accent/overglaze of a Rutile formulation
>>(that fluxes quite a bit at cone 10-11 reduction). The problem has been
>>blisters occurring mainly in regions of large vases and bowls where only
>>the Tenmoku glaze has been applied (small forms don't seem to have
>>blisters at all). I grind out the blisters and overglaze them with the
>>Rutile and re-fire. The results are consistently the following: the
>>blisters I repaired are nicely filled in by the grinding and
>>over-glazing, but unfortunately (maddeningly) new blisters have
>>appeared, sometimes more new ones form than were repaired initially. If
>>I now repeat the process and do a second re-firing, the same results are
>>obtained ... old blisters repaired, but new ones appear. Seems like an
>>endless process and the blisters are winning!
>>
>>My question is ... after three firings one would think that whatever
>>gaseous evolution is causing the blisters, it should be dissipating
>>with repeated firings and you should eventually produce a saleable pot.
>>Obviously this is not happening ... re-firing to repair blisters has
>>been a total waste of my time and gas. What's your best theory to
>>explain these observations? Any and all comments from the List are
>>welcomed. Does anyone know how to fix glaze blisters? So far my
>>experience suggests a large hammer's the best solution.
>>
>>Bob
>>
>>
>
>Ron Roy
>RR#4
>15084 Little Lake Road
>Brighton, Ontario
>Canada
>K0K 1H0
>Phone: 613-475-9544
>Fax: 613-475-3513
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
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>
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>
>
>
>
>
Ivor and Olive Lewis on thu 21 oct 04
Dear Ron Roy,
Surely one clue to an unreduced firing would be the perception of pure
white to pale pink surface on bisqued white clay. If there is even a
small proportion of Iron oxide in a body white will darken and pink
will be decolourised, changing to grey in the presence of a reducing
atmosphere.
To avoid the problem with dark firing clays, would you agree extending
the time between dull red heat, say 600=BA C to 850=BA is needed. During
this period there should be sufficient primary air to burn the fuel
completely with a copious flow of secondary air and an open damper.
This should prevent decomposition of Ferric oxide.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Ron Roy on fri 22 oct 04
Hi Ivor,
I have always said 700C to 900 - give enough time and make sure there is
extra oxygen to facilitate burning.
Much depends on the type of impurities, thickness of the clay and the
amount of impurities.
=46or instance - it seems that firing to a soft bisque - say 08 - sometimes
will not do the job - and when the temperature is raised to 04 the job gets
done.
That would indicate that there is still ignition going on above even 900C -
so I just got in the habit of slow firing all the way to the end - the
bonus being more even firings.
Some clay mixtures give less concern then others and applying that regime
to all situations would not be necessary in all situations - but learning
the hard way can be painful with large kiln loads of hand made ware.
It is true that you can tell by the colour if ware has been reduced or
oxidized - for porcelain it is cream coloured when oxidized and grey white
when reduced.
It is not a good indicator however - if you break a piece - then you can be
sure it is oxidized all the way through. It is a case of working blind
because breaking ware to see if it's fired right is not very productive.
Very often it is the inside clay that is not oxidized properly - the
outside getting enough oxygen during the early stages of the crucial buring
out period.
It is also mentioned in the literature that - once the iron oxide is
reduced - it is very difficult to reverse the situation - so that even
refiring will not reoxidize it again. I have not done any experiments to
explor that aspect.
The best approach - to my way of thinking - is assume the worst and tailor
your techniques to deal with any eventuality - simply because our raw
materials do vary.
RR
>Dear Ron Roy,
>Surely one clue to an unreduced firing would be the perception of pure
>white to pale pink surface on bisqued white clay. If there is even a
>small proportion of Iron oxide in a body white will darken and pink
>will be decolourised, changing to grey in the presence of a reducing
>atmosphere.
>To avoid the problem with dark firing clays, would you agree extending
>the time between dull red heat, say 600=BA C to 850=BA is needed. During
>this period there should be sufficient primary air to burn the fuel
>completely with a copious flow of secondary air and an open damper.
>This should prevent decomposition of Ferric oxide.
>Best regards,
>Ivor Lewis.
>Redhill,
>S. Australia.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
=46ax: 613-475-3513=20
wjskw@BELLSOUTH.NET on sat 23 oct 04
Ron:
E-Z solution...toss a test tile or two in the load.
And make sure you have a hammer handy afterward.
Even if the pots are racers, you can still see what's going on
inside the body.
Best,
Wayne Seidl
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Ron Roy
Sent: Friday, October 22, 2004 10:17 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: question/bisque/gasses
It is not a good indicator however - if you break a piece - then you
can be
sure it is oxidized all the way through. It is a case of working
blind
because breaking ware to see if it's fired right is not very
productive.
Very often it is the inside clay that is not oxidized properly - the
outside getting enough oxygen during the early stages of the crucial
buring
out period.
Edouard Bastarache Inc. on sat 23 oct 04
Hello all,
I have been making pots for 36 years and never bisque- fired
above c/08.
When I began, my teacher who graduated from Alfred, told us to
bisque in the vicinity of 1600-1650 to harden the pots in order
to glaze them without breakage.
Over the years I have upped the temperature to near 1750 without
knowing the exact temperature, I do not use any measuring device.
It is only recently while doing my first c/04 firing that I observed
at what temp. I was bisque-firing because it was the first time I was
using a thermocouple/electronic readout on my electric kiln;
reaching the same level of odor, brightness in the kiln and time,
I knew it was my bisque temp. I read 1750 on my readout.
I have always bisque-fired by the nose/eye/clock (time).
Funny thing I have never had the problems actually discussed on
this thread.
Later,
"Ils sont fous ces quebecois"
Edouard Bastarache
Irreductible Quebecois
Indomitable Quebeker
Sorel-Tracy
Quebec
edouardb@sorel-tracy.qc.ca
www.sorel-tracy.qc.ca/~edouardb/Welcome.html
http://perso.wanadoo.fr/smart2000/index.htm
http://www.digitalfire.com/education/toxicity/
Ron Roy on tue 26 oct 04
Hi Wayne,
Thanks for the tip but - there are situations that would not be covered by
just a test tile.
Sometimes the clay is thick and that can be a factor - so the tile would
have to mimic the ware in the kiln. Stacked ware would be more difficult to
get fired clean as well - and pots with lids on them.
Stacking a number of tiles on top of each other may be a better way - or
maybe even putting them in some sort of sagger for instance.
Good to see you are thinking - RR
>Ron:
>E-Z solution...toss a test tile or two in the load.
>And make sure you have a hammer handy afterward.
>Even if the pots are racers, you can still see what's going on
>inside the body.
>
>Best,
>Wayne Seidl
>
>-----Original Message-----
>From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Ron Roy
>Sent: Friday, October 22, 2004 10:17 PM
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: Re: question/bisque/gasses
>
>
>
>It is not a good indicator however - if you break a piece - then you
>can be
>sure it is oxidized all the way through. It is a case of working
>blind
>because breaking ware to see if it's fired right is not very
>productive.
>Very often it is the inside clay that is not oxidized properly - the
>outside getting enough oxygen during the early stages of the crucial
>buring
>out period.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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