Anthony Wolking on wed 27 jan 99
Dear Tech Gurus,
I have been experiencing profound blistering of Laguna B-mix and B-mix with
sand during a few cone 10 salt firings. It was suggested to me that
overreduction may be causing the blistering as a result of black coreing. The
broken pots do not show any black coreing but several appear to have heavy
delamination of the pot wall. What is strange is some come out just fine.
Let me explain my proceedure: throw, dry, bisgue to cone 04, rinse, glaze,
allow to dry for 1-2 days, load, candle for 4 hours to get about 200-400
degrees(my kiln soaks up a lot of moisture in the northern Kentucky humidity,
so I play it safe), both burners on for a regular increase of 200 degrees
until cone 08, half-hour body reduction, leave in slight reduction until I
start to salt around cone 8-9, salt through 10 and shut off. Some reduction
during salting. So in total about a 12-14 hour firing. All ports closed, and
damper in to shut kiln down. I have tried some quick cooling to around 2000
degrees F.
I have had a difficult transition from natural gas to propane and have notice
that the kiln exhibits a reducing flame from all ports after the kiln is shut
off. Could this be a Problem?
Any suggestion would greatly be appreciated. If I need to supply more
information let me know.
Thanks again,
Anthony
agwclay@aol.com
Anthony Wolking on thu 28 jan 99
Andy,
My bisgue firings are in an electric kiln, and usually take anywhere from
12-14 hours due to the age of the kiln. All of my spyhole plugs are closed
from the time I turn it on, could this be causing a less oxidation atmosphere,
on the verge of mild reduction? I think my bisgue is generous in length, but
let me know if it need to be longer.
Thanks for the reply,
Anthony
Andrew Buck on thu 28 jan 99
Anthony,
Do you bisque in the same kiln you glaze fire in? Is there a reduction
atmosphere during the bisque firing? How fast do you fire the bisque
firing? It was suggested recently on the list that, if, during the
initial firing stage, not enough time was taken for burnt vapors from the
organic materials in the clay to escape, then a kind of carbon forms that
later causes the problems you are experiencing. This is more common with
the denser clay bodies like porcelain. Because there is very little iron
in the clay, you will not see a "black core" as such. Try a slower bisque
firing and make sure there is an oxidation or neutral atmosphere in the
kiln that will allow the vapors to escape the walls of the pots. Good
luck and let us know what happens.
Andy Buck
Raincreek Pottery
Port Orchard, Washington
On Wed, 27 Jan 1999, Anthony Wolking wrote:
> ----------------------------Original message----------------------------
> Dear Tech Gurus,
>
> I have been experiencing profound blistering of Laguna B-mix and B-mix with
> sand during a few cone 10 salt firings. It was suggested to me that
> overreduction may be causing the blistering as a result of black coreing. The
> broken pots do not show any black coreing but several appear to have heavy
> delamination of the pot wall. What is strange is some come out just fine.
>
> Let me explain my proceedure: throw, dry, bisgue to cone 04, rinse, glaze,
> allow to dry for 1-2 days, load, candle for 4 hours to get about 200-400
> degrees(my kiln soaks up a lot of moisture in the northern Kentucky humidity,
> so I play it safe), both burners on for a regular increase of 200 degrees
> until cone 08, half-hour body reduction, leave in slight reduction until I
> start to salt around cone 8-9, salt through 10 and shut off. Some reduction
> during salting. So in total about a 12-14 hour firing. All ports closed, and
> damper in to shut kiln down. I have tried some quick cooling to around 2000
> degrees F.
>
> I have had a difficult transition from natural gas to propane and have notice
> that the kiln exhibits a reducing flame from all ports after the kiln is shut
> off. Could this be a Problem?
>
> Any suggestion would greatly be appreciated. If I need to supply more
> information let me know.
>
> Thanks again,
>
> Anthony
> agwclay@aol.com
>
Ron Roy on fri 29 jan 99
Hi Anthony,
No such thing as over reduction - either the Fe2O3 is reduced to FeO or
not. Besides there is hardly any iron in that clay so even if you did not
have a "clean" bisque firing I would not expect that to be the cause of the
problem.
I suspect the clay is simply overfired - were they all made from the same
batch - you can tell which batch if there is a batch number on the boxes.
Did they all have the same glaze on them? Were some made thiner than others
- try to find the differences in the ones that blistered and those that did
not.
Clay that is over fired has many small blisters - if there are air pockets
in the clay you will get larger blisters where the air is.
RR
>----------------------------Original message----------------------------
>Dear Tech Gurus,
>
>I have been experiencing profound blistering of Laguna B-mix and B-mix with
>sand during a few cone 10 salt firings. It was suggested to me that
>overreduction may be causing the blistering as a result of black coreing. The
>broken pots do not show any black coreing but several appear to have heavy
>delamination of the pot wall. What is strange is some come out just fine.
>
>Let me explain my proceedure: throw, dry, bisgue to cone 04, rinse, glaze,
>allow to dry for 1-2 days, load, candle for 4 hours to get about 200-400
>degrees(my kiln soaks up a lot of moisture in the northern Kentucky humidity,
>so I play it safe), both burners on for a regular increase of 200 degrees
>until cone 08, half-hour body reduction, leave in slight reduction until I
>start to salt around cone 8-9, salt through 10 and shut off. Some reduction
>during salting. So in total about a 12-14 hour firing. All ports closed, and
>damper in to shut kiln down. I have tried some quick cooling to around 2000
>degrees F.
>
>I have had a difficult transition from natural gas to propane and have notice
>that the kiln exhibits a reducing flame from all ports after the kiln is shut
>off. Could this be a Problem?
>
>Any suggestion would greatly be appreciated. If I need to supply more
>information let me know.
>
>Thanks again,
>
>Anthony
>agwclay@aol.com
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Andrew Buck on sun 31 jan 99
Anthony,
Ron Roy has much more experience behind his reply to this question than I
do and I expect (gladly) to be told I'm as full of it as the Christmas
goose when I'm off base. However, I was not talking about the over
reduction of iron in the clay body which is called black coring. What I
was commenting on is something that was mentioned in one of the posts on
black coring that showed up in a large discussion about three months ago.
You might search archives on that subject to see what was actually said.
Anyway, it was posted that if some dense clay bodies are fired too quickly
in the bisque or initial phases of once-fired pot (and I now think this is
NOT the case in your situation) or if the atmosphere in the kiln, during
this time, is in too much reduction, then the organic materials in the
clay body turn into a form of carbon that is less able to burn out. This
carbon is then remains in the body to cause problems at the higher firing
temperatures. This, if I am understanding what I read on the list before,
could be a factor in your problem. Not the length of your bisque firing
but the atmosphere in the kiln. My electric kiln came with instruction to
prop the lid open during the initial stages of the firing (actually came
with a soft brick wedge to do this with) and then fire the whole length of
the firing with the top peep hole unplugged. Get dem nasty gasses ot'ta
dare. Enviro-vents work good for this also, I've heard.
If the person that posted about this originally would comment, it sounds
to be a bigger problem than people generally know. And if you think my
ideas are a crock, I'd like to hear that also. |*)
Andy Buck
Raincreek Pottery
Port Orchard, Washington
Beer is for putting into the potter, not the clay.
On Thu, 28 Jan 1999, Anthony Wolking wrote:
> ----------------------------Original message----------------------------
> Andy,
>
> My bisgue firings are in an electric kiln, and usually take anywhere from
> 12-14 hours due to the age of the kiln. All of my spyhole plugs are closed
> from the time I turn it on, could this be causing a less oxidation atmosphere,
> on the verge of mild reduction? I think my bisgue is generous in length, but
> let me know if it need to be longer.
Ron Roy on tue 2 feb 99
Hi Andy,
I have no experience nor can I find any reference to carbon in the body
being responsible for a problem such as bloating - unless there is iron
reduced to FeO which then over fluxes a body. I am not saying it doesn't
happen or cause a problem - I have never seen it - thats all.
At any rate the length of this firing tells me it is not the problem here.
Your sentence "then the organic materials in the clay body turn into a form
of carbon that is less able to burn out" needs some comment I think. There
is carbon in many clays - left over from roots, leaves etc. and deposited
with the clay from where it is mined. Mainly clays which have been
transported to the mine site over time by water usually. Some clays have
more carbon than others and the carbon content in specific clay will vary
as well. This carbon is there in the clay - not created - and should be
given the oxygen and time it needs to burn out before the clay seals over.
It is also possible to deposit carbon in clay during incomplete combustion
I suppose but this would be difficult to do and get it all the way in where
it could be a problem. If we fire in reduction during the crucial time we
simply prevent the oxygen supply to the carbon trying to turn into CO2. It
then (C or CO) takes the Oxygen from any Fe2O3 it can find - turning it
(Fe2O3) into FeO - a strong flux.
The best strategy for us as studio potters - should be - assume the worst
and fire for that situation. This is why firing slower rather than faster
is safer.
I do think your post is correct on this subject - and I also remember that
other post - I also seem to remember there was no reference to any studies
or literature on the subject - I would be interested to understand more
about this and if it is something we should know about.
I think these new vents are something we should all be thinking about -
keeping the gases out of our studios and homes is an excellent was to go -
and as a side benefit we get cleaner firings - it is also necessary to know
when to turn them off so they don't wreck our pots and glazes.
RR
>----------------------------Original message----------------------------
>Ron Roy has much more experience behind his reply to this question than I
>do and I expect (gladly) to be told I'm as full of it as the Christmas
>goose when I'm off base. However, I was not talking about the over
>reduction of iron in the clay body which is called black coring. What I
>was commenting on is something that was mentioned in one of the posts on
>black coring that showed up in a large discussion about three months ago.
>You might search archives on that subject to see what was actually said.
>Anyway, it was posted that if some dense clay bodies are fired too quickly
>in the bisque or initial phases of once-fired pot (and I now think this is
>NOT the case in your situation) or if the atmosphere in the kiln, during
>this time, is in too much reduction, then the organic materials in the
>clay body turn into a form of carbon that is less able to burn out. This
>carbon is then remains in the body to cause problems at the higher firing
>temperatures. This, if I am understanding what I read on the list before,
>could be a factor in your problem. Not the length of your bisque firing
>but the atmosphere in the kiln. My electric kiln came with instruction to
>prop the lid open during the initial stages of the firing (actually came
>with a soft brick wedge to do this with) and then fire the whole length of
>the firing with the top peep hole unplugged. Get dem nasty gasses ot'ta
>dare. Enviro-vents work good for this also, I've heard.
>
>If the person that posted about this originally would comment, it sounds
>to be a bigger problem than people generally know. And if you think my
>ideas are a crock, I'd like to hear that also. |*)
>
>Andy Buck
>Raincreek Pottery
>Port Orchard, Washington
>Beer is for putting into the potter, not the clay.
>
>On Thu, 28 Jan 1999, Anthony Wolking wrote:
>
>> ----------------------------Original message----------------------------
>> Andy,
>>
>> My bisgue firings are in an electric kiln, and usually take anywhere from
>> 12-14 hours due to the age of the kiln. All of my spyhole plugs are closed
>> from the time I turn it on, could this be causing a less oxidation
>>atmosphere,
>> on the verge of mild reduction? I think my bisgue is generous in length, but
>> let me know if it need to be longer.
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
=?iso-8859-1?Q?Ren=E9?= Sprattling on thu 4 feb 99
Anthony, It may not be your fault. Have talked to several potters here who
are having the same problems with blistering of B-mix. Maybe a bad clay
batch? A slight change in clay formula or consistency with ingredients? I'd
call Laguana.
Rene, CA
----------------------------Original message----------------------------
>Dear Tech Gurus,
>
>I have been experiencing profound blistering of Laguna B-mix and B-mix with
>sand during a few cone 10 salt firings. It was suggested to me that
>overreduction may be causing the blistering as a result of black coreing. The
>broken pots do not show any black coreing but several appear to have heavy
>delamination of the pot wall. What is strange is some come out just fine.
Muddy
Donn Buchfinck on fri 5 feb 99
I think a couple of the problems with b-mix is the clay they use in the clay
body
I might be wrong but I think that they use a kaolin that is used primarily for
the paper industry that has been milled a certain way.
the other thing about laguna is that they buy materials in such large
quantities that if the product changes it is hard to determine the cause
the problem with the b-mix bloating in the salt kiln deals with I think that
the salt seals the clay body at lower temperatures before the carbon has had a
chance to burn out.
something that might be causing this is if a potter is using wax on the pot.
wax does not burn off right away, it gets hot and soaks into the clay body,
and this acts as a reduction material once the kiln gets hot enough for the
salt to start vaporizing and sealing the clay.
to remedy this you have heat the kiln in the beginning slowly
I know it is to a lot of potters best interest to just turn the kiln on and go
full guns but firing too fast in the beginning is what is causing the problem
I would love to hear other people's opinion
Donn Buchfinck
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