Paul Taylor on fri 15 nov 02
Dear Sylvia
Most of these early reduction pots are biscuit fired so the carbon has
been driven out of them previous to the reduction firing and/or great care
is taken to rid the pot of carbon just before the body reduction.
Carbon will burn off if you keep the kiln well oxidized in a soak just
before you reduce. So even early reduction potters still can have a second
bite of the cherry after the biscuit - if they oxidize to 750c and for a
good hour at about 750c. Even so they are sailing pretty close to the wind
and I expect they accept the occasional bloat as a small price to pay for
'the wonder pots'.
The big problems come with early reduction is in an uneven kiln firing and
some times too early reduction will cause the pots ( mostly those with lids
at the bottom of the kiln) to bloat because they have not had time to burn
out carbon. There are other problems in getting rid of carbon .
Some times too late an oxidation will cause the pots near the burners to
bloat because they have started to seal and or the secondary air in a kiln
is not mixing with the flame soon enough to cause local reduction . It is
very difficult to work out what is happening when a pot is bloated by being
too near the bag wall or flame path.
The carbon that you burn out in the biscuit firing can be in lumps . you
can see these partials in the sieve if you use gray clay in a glaze but even
porcelain seems to hold some organic carbon that needs to be released.
However the carbon that you put in the body in a reduction firing is a gas
that is already expanded to its maximum by the kiln temperature. So it will
not expand in the clay to form bloating.
From all this you must realize that a good oxidized biscuit firing solves
all hassle. You can do much what you like after a perfectly oxidized biscuit
firing. But its not a rule that you have a perfectly oxidizes pot before you
fire it a second time because some people rely on the second firing to take
away carbon as well . I have occasionally done this with large lidded shapes
or stack bowls. These,I have suspected, may still have carbon in after the
biscuit. So about 850c I will soak them in oxidation to be sure to be sure.
I do not start reducing until 950c if I was starting reduction at a lower
temperature I would be more obsessive with the biscuit firing.
For carbon trapping and shino you are probably best to start with a well
oxidized pot. but I do not know for certain if the pot has to be oxidized
completely or reduced so that the iron has melted and migrated through out
the body and the carbon has also been oxidized out of it. This is possible
and maybe necessary for some of these effects.
And do not be over confident just because you fire the biscuit in an
electric kiln . Because a fiber kiln can be virtually air tight, there may
not be enough air in the kiln and/or the pots may be pact too economically
to release the carbon.
looking through all this . I can see why pottery books try to be clear
and simplistic. to accommodate all of us they have to be, because if all the
possible permutations and exceptions and warnings were given the advice
would be more confusing than informative.
All the early reduction and carbon trapping is for people who are willing
to forgo the simplicity of following the rules . They expect extra special
results . I am weary the hidden agendas that go with these enthusiasms. One
of them is that somehow it is necessary to do all these fancy techniques to
make something aesthetically worth while.
Another is that these techniques are with out disaster . All that happens
is that the disasters are smashes while the potter gets the balance between
oxidation and reduction right and we all pretend like a conjurer that it
just came out of a hat like magic a testimony to our innate genius . Hard
work and a pile of shards is so 'uncool'.
I think the best potters are not put off by a few disasters as long as
they learn by them and don't end up either avoiding them or worse trying to
pass off bloating and crazing as some sort of intrinsic part of the hand
made process.
--------------------------------
I would like to know exactly what happens to iron in a firing . If you look
at an old fashioned house brick- usually fired with coal and what ever came
to hand - you will see that the brick is red yellow and deep purple
sometimes on the same brick. This is not a matter of simple one part is
reduced and one not because the colors are so many and contradictory. From
this i surmise that iron is very sensitive to atmosphere and its also fixed
when it goes into fusion and yet will still change but not in the same way.
Another example of this is when I oxidize my kiln all the way to
biscuit the pots come out pink
If I neutral fire or slightly reduce and then clear the carbon at 750c
to 950c the pots are buff. Some times iron spots show like flecks of brown
on the surface and these flecks are increased in the glaze firing.
Since shino glazes are very much dependent on the reoxidization of
previously reduced iron showing on the glaze surface i am wondering if it is
this that the early reduction is encouraging. so if you slightly reduce a
very well oxidized biscuit pot you get this iron migrating to the surface as
red.
Or looking closer ??? does the iron migrating from the body to the glaze
surface showing red, add to the fine iron crystals showing pink to give a
more in depth effect. Is the reason you have to reduce the body iron so
early because it will only migrate to the surface through the body and
glaze layer if it is reduced iron and the layers have not formed into a
glass.
--
Regards from Paul Taylor
http://www.anu.ie/westportpottery
In time the biggest hammer will fail to smash a nut. that is the nature of
'disaster' - but also 'hope'.
> From: Mondloch
> Organization: Silver Creek Pottery
> Reply-To: Mondloch
> Date: Wed, 13 Nov 2002 16:09:34 -0600
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: Re: Shino Reduction
>
>> Many start reduction much earlier to get strong shino color -- 1250-1450
> or
>> so fahrenheit. I think it was Ken Ferguson, who I once read, started
>> reduction at 1250. I'd reduce that early but I also found that at around
> 1450
>> I could still get good color.
>
> Hi,
> Could someone explain to me why early body reduction like this doesn't cause
> black core problems since this temp range is earlier then that recommended
> for complete oxidation in bisque firings.
>
> In my Hamer book under Black Core is says "Black core occurs when the carbon
> inside the body has not been successfully burnt out.... The lack of
> sufficient oxygen between 750C(1382F) and 900C(1652) by an unclean
> atmosphere or by too rapid a firing can lead to breakdown, bloating et at
> higher temperatures...black core is often a fault arising in the bisque
> firing that does not show itself until the glaze firing."
>
> ok. So maybe there's a difference between the carbon naturally in a body
> from that deposited by early reduction? But what about those of you who
> single fire shino glazes if you don't get the clay's carbon all burned out
> before you want to do the early redux?
>
> confused,
> Sylvia Mondloch
>
>
> ---
> Mark & Sylvia Mondloch
> Silver Creek Pottery & Forge
> W6725 Hwy 144
> Random Lake ,Wi 53075
> HotArt@silvercreekpottery.com
> http://www.silvercreekpottery.com
>
Mondloch on sun 17 nov 02
Paul wrote:
> >But its not a rule that you have a perfectly oxidizes pot before you fire
it a second time because some people rely on the second firing to take away
carbon as well .>>
Hi Paul,
I've always wondered about the conventional wisdom of bisquing to a higher
temp-like ^06- to remove all the carbon. It would seem that if you take the
clay that high in the first firing, then it would be too dense for the
second firing to do much good as far as burning out carbon.
>> However the carbon that you put in the body in a reduction firing is a
gas that is already expanded to its maximum by the kiln temperature. So it
will not expand in the clay to form bloating.>>
Interesting. I never heard it put that way. I've gone to single firing and
I have quite a bit of larger work 1/2 to 1" thick out of a coarse stoneware
clay that contains a good amount of sulfur/carbon. I've been firing up slow
and soaking about 1 and 1/2 hr at about 1650F(900C) and then doing a body
redux after that. The shino glaze I've been playing with is redder though
with an earlier redux. Now I'm wondering if there may be an advantage to
dropping the temp back down after the soak to do the redux.
> > Some times too late an oxidation will cause the pots near the burners to
bloat because they have started to seal and or the secondary air in a kiln
is not mixing with the flame soon enough to cause local reduction . It is
very difficult to work out what is happening when a pot is bloated by being
too near the bag wall or flame path.>>
Huh? Can you say that again only slower?
>> The big problems come with early reduction is in an uneven kiln firing
and some times too early reduction will cause the pots ( mostly those with
lids at the bottom of the kiln) to bloat because they have not had time to
burn out carbon. There are other problems in getting rid of carbon .> >
Yes. The floor in my kiln lags way behind early in the firing, so I think I
really need the bisque temp soak I've been doing- thinking I should even
increase it.
I find it hard to get the heat to the floor while keeping the damper open to
keep it fully oxidized.
thanks for the great post,
Sylvia
---
> Mark & Sylvia Mondloch
> Silver Creek Pottery & Forge
> W6725 Hwy 144
> Random Lake ,Wi 53075
> HotArt@silvercreekpottery.com
> http://www.silvercreekpottery.com
>
>
> > Dear Sylvia
> >
> > Most of these early reduction pots are biscuit fired so the carbon
has
> > been driven out of them previous to the reduction firing and/or great
care
> > is taken to rid the pot of carbon just before the body reduction.
> >
> > Carbon will burn off if you keep the kiln well oxidized in a soak
just
> > before you reduce. So even early reduction potters still can have a
second
> > bite of the cherry after the biscuit - if they oxidize to 750c and for a
> > good hour at about 750c. Even so they are sailing pretty close to the
wind
> > and I expect they accept the occasional bloat as a small price to pay
for
> > 'the wonder pots'.
> >
> > The big problems come with early reduction is in an uneven kiln firing
> and
> > some times too early reduction will cause the pots ( mostly those with
> lids
> > at the bottom of the kiln) to bloat because they have not had time to
burn
> > out carbon. There are other problems in getting rid of carbon .
> >
> > Some times too late an oxidation will cause the pots near the
burners
> to
> > bloat because they have started to seal and or the secondary air in a
kiln
> > is not mixing with the flame soon enough to cause local reduction . It
is
> > very difficult to work out what is happening when a pot is bloated by
> being
> > too near the bag wall or flame path.
> >
> > The carbon that you burn out in the biscuit firing can be in lumps .
> you
> > can see these partials in the sieve if you use gray clay in a glaze but
> even
> > porcelain seems to hold some organic carbon that needs to be released.
> >
> > However the carbon that you put in the body in a reduction firing is a
> gas
> > that is already expanded to its maximum by the kiln temperature. So it
> will
> > not expand in the clay to form bloating.
> >
> > From all this you must realize that a good oxidized biscuit firing
> solves
> > all hassle. You can do much what you like after a perfectly oxidized
> biscuit
> > firing. But its not a rule that you have a perfectly oxidizes pot before
> you
> > fire it a second time because some people rely on the second firing to
> take
> > away carbon as well . I have occasionally done this with large lidded
> shapes
> > or stack bowls. These,I have suspected, may still have carbon in after
the
> > biscuit. So about 850c I will soak them in oxidation to be sure to be
> sure.
> > I do not start reducing until 950c if I was starting reduction at a
lower
> > temperature I would be more obsessive with the biscuit firing.
> >
> > For carbon trapping and shino you are probably best to start with a
well
> > oxidized pot. but I do not know for certain if the pot has to be
oxidized
> > completely or reduced so that the iron has melted and migrated through
out
> > the body and the carbon has also been oxidized out of it. This is
possible
> > and maybe necessary for some of these effects.
> >
> > And do not be over confident just because you fire the biscuit in an
> > electric kiln . Because a fiber kiln can be virtually air tight, there
may
> > not be enough air in the kiln and/or the pots may be pact too
economically
> > to release the carbon.
> >
> > looking through all this . I can see why pottery books try to be
> clear
> > and simplistic. to accommodate all of us they have to be, because if all
> the
> > possible permutations and exceptions and warnings were given the advice
> > would be more confusing than informative.
> >
> > All the early reduction and carbon trapping is for people who are
> willing
> > to forgo the simplicity of following the rules . They expect extra
> special
> > results . I am weary the hidden agendas that go with these enthusiasms.
> One
> > of them is that somehow it is necessary to do all these fancy techniques
> to
> > make something aesthetically worth while.
> >
> > Another is that these techniques are with out disaster . All that
> happens
> > is that the disasters are smashes while the potter gets the balance
> between
> > oxidation and reduction right and we all pretend like a conjurer that it
> > just came out of a hat like magic a testimony to our innate genius .
Hard
> > work and a pile of shards is so 'uncool'.
> >
> > I think the best potters are not put off by a few disasters as long as
> > they learn by them and don't end up either avoiding them or worse trying
> to
> > pass off bloating and crazing as some sort of intrinsic part of the hand
> > made process.
> >
> > --------------------------------
> >
> > I would like to know exactly what happens to iron in a firing . If you
> look
> > at an old fashioned house brick- usually fired with coal and what ever
> came
> > to hand - you will see that the brick is red yellow and deep purple
> > sometimes on the same brick. This is not a matter of simple one part is
> > reduced and one not because the colors are so many and contradictory.
From
> > this i surmise that iron is very sensitive to atmosphere and its also
> fixed
> > when it goes into fusion and yet will still change but not in the same
> way.
> >
> > Another example of this is when I oxidize my kiln all the way to
> > biscuit the pots come out pink
> >
> > If I neutral fire or slightly reduce and then clear the carbon at
750c
> > to 950c the pots are buff. Some times iron spots show like flecks of
> brown
> > on the surface and these flecks are increased in the glaze firing.
> >
> > Since shino glazes are very much dependent on the reoxidization of
> > previously reduced iron showing on the glaze surface i am wondering if
it
> is
> > this that the early reduction is encouraging. so if you slightly reduce
a
> > very well oxidized biscuit pot you get this iron migrating to the
surface
> as
> > red.
> >
> > Or looking closer ??? does the iron migrating from the body to the
> glaze
> > surface showing red, add to the fine iron crystals showing pink to give
a
> > more in depth effect. Is the reason you have to reduce the body iron so
> > early because it will only migrate to the surface through the body and
> > glaze layer if it is reduced iron and the layers have not formed into a
> > glass.
> >
> >
> >
> > --
> > Regards from Paul Taylor
> >
> > http://www.anu.ie/westportpottery
> >
> > In time the biggest hammer will fail to smash a nut. that is the nature
of
> > 'disaster' - but also 'hope'.
>
>
>
Ron Roy on fri 22 nov 02
There is a misunderstanding in what follows - it is not the carbon in a
clay body that causes black coring and bloating - it is the iron - which,
when reduced, becomes a strong flux and causes overfiring of the clay. That
overfiring causes gases to be released and hence the bloating. Many times
the bloating is localized - it means the iron in that part of the pot was
reduced and winds up over fired.
It is true that carbon, sulphur and other organics can create the reducing
conditions that are necessary to reduce the iron - to fast firing and/or
too little oxygen are the problems that are taken care of when ware is
fired slow enough and with enough oxygen - to keep the combustion process
from looking for more oxygen. Fe2O3 gives up it's O2 easily - so if there
is CO around it takes the oxygen from the Fe2O3.
Once the iron is reduced in a clay body it is almost impossible to
reoxidize it - so relying on that to happen in a second firing is simply
not going to happen.
RR
> Paul wrote:
>> >But its not a rule that you have a perfectly oxidizes pot before you fire
>it a second time because some people rely on the second firing to take away
>carbon as well .>>
>
>Hi Paul,
> I've always wondered about the conventional wisdom of bisquing to a higher
>temp-like ^06- to remove all the carbon. It would seem that if you take the
>clay that high in the first firing, then it would be too dense for the
>second firing to do much good as far as burning out carbon.
>
>
> >> However the carbon that you put in the body in a reduction firing is a
>gas that is already expanded to its maximum by the kiln temperature. So it
>will not expand in the clay to form bloating.>>
>
>
> Interesting. I never heard it put that way. I've gone to single firing and
>I have quite a bit of larger work 1/2 to 1" thick out of a coarse stoneware
>clay that contains a good amount of sulfur/carbon. I've been firing up slow
>and soaking about 1 and 1/2 hr at about 1650F(900C) and then doing a body
>redux after that. The shino glaze I've been playing with is redder though
>with an earlier redux. Now I'm wondering if there may be an advantage to
>dropping the temp back down after the soak to do the redux.
>
>
>> > Some times too late an oxidation will cause the pots near the burners to
>bloat because they have started to seal and or the secondary air in a kiln
>is not mixing with the flame soon enough to cause local reduction . It is
>very difficult to work out what is happening when a pot is bloated by being
>too near the bag wall or flame path.>>
>
>
> Huh? Can you say that again only slower?
>
>
>>> The big problems come with early reduction is in an uneven kiln firing
>and some times too early reduction will cause the pots ( mostly those with
>lids at the bottom of the kiln) to bloat because they have not had time to
>burn out carbon. There are other problems in getting rid of carbon .> >
>
>
> Yes. The floor in my kiln lags way behind early in the firing, so I think I
>really need the bisque temp soak I've been doing- thinking I should even
>increase it.
>I find it hard to get the heat to the floor while keeping the damper open to
>keep it fully oxidized.
>
> thanks for the great post,
>Sylvia
>
> ---
>> Mark & Sylvia Mondloch
>> Silver Creek Pottery & Forge
>> W6725 Hwy 144
>> Random Lake ,Wi 53075
>> HotArt@silvercreekpottery.com
>> http://www.silvercreekpottery.com
>>
>>
>> > Dear Sylvia
>> >
>> > Most of these early reduction pots are biscuit fired so the carbon
>has
>> > been driven out of them previous to the reduction firing and/or great
>care
>> > is taken to rid the pot of carbon just before the body reduction.
>> >
>> > Carbon will burn off if you keep the kiln well oxidized in a soak
>just
>> > before you reduce. So even early reduction potters still can have a
>second
>> > bite of the cherry after the biscuit - if they oxidize to 750c and for a
>> > good hour at about 750c. Even so they are sailing pretty close to the
>wind
>> > and I expect they accept the occasional bloat as a small price to pay
>for
>> > 'the wonder pots'.
>> >
>> > The big problems come with early reduction is in an uneven kiln firing
>> and
>> > some times too early reduction will cause the pots ( mostly those with
>> lids
>> > at the bottom of the kiln) to bloat because they have not had time to
>burn
>> > out carbon. There are other problems in getting rid of carbon .
>> >
>> > Some times too late an oxidation will cause the pots near the
>burners
>> to
>> > bloat because they have started to seal and or the secondary air in a
>kiln
>> > is not mixing with the flame soon enough to cause local reduction . It
>is
>> > very difficult to work out what is happening when a pot is bloated by
>> being
>> > too near the bag wall or flame path.
>> >
>> > The carbon that you burn out in the biscuit firing can be in lumps .
>> you
>> > can see these partials in the sieve if you use gray clay in a glaze but
>> even
>> > porcelain seems to hold some organic carbon that needs to be released.
>> >
>> > However the carbon that you put in the body in a reduction firing is a
>> gas
>> > that is already expanded to its maximum by the kiln temperature. So it
>> will
>> > not expand in the clay to form bloating.
>> >
>> > From all this you must realize that a good oxidized biscuit firing
>> solves
>> > all hassle. You can do much what you like after a perfectly oxidized
>> biscuit
>> > firing. But its not a rule that you have a perfectly oxidizes pot before
>> you
>> > fire it a second time because some people rely on the second firing to
>> take
>> > away carbon as well . I have occasionally done this with large lidded
>> shapes
>> > or stack bowls. These,I have suspected, may still have carbon in after
>the
>> > biscuit. So about 850c I will soak them in oxidation to be sure to be
>> sure.
>> > I do not start reducing until 950c if I was starting reduction at a
>lower
>> > temperature I would be more obsessive with the biscuit firing.
>> >
>> > For carbon trapping and shino you are probably best to start with a
>well
>> > oxidized pot. but I do not know for certain if the pot has to be
>oxidized
>> > completely or reduced so that the iron has melted and migrated through
>out
>> > the body and the carbon has also been oxidized out of it. This is
>possible
>> > and maybe necessary for some of these effects.
>> >
>> > And do not be over confident just because you fire the biscuit in an
>> > electric kiln . Because a fiber kiln can be virtually air tight, there
>may
>> > not be enough air in the kiln and/or the pots may be pact too
>economically
>> > to release the carbon.
>> >
>> > looking through all this . I can see why pottery books try to be
>> clear
>> > and simplistic. to accommodate all of us they have to be, because if all
>> the
>> > possible permutations and exceptions and warnings were given the advice
>> > would be more confusing than informative.
>> >
>> > All the early reduction and carbon trapping is for people who are
>> willing
>> > to forgo the simplicity of following the rules . They expect extra
>> special
>> > results . I am weary the hidden agendas that go with these enthusiasms.
>> One
>> > of them is that somehow it is necessary to do all these fancy techniques
>> to
>> > make something aesthetically worth while.
>> >
>> > Another is that these techniques are with out disaster . All that
>> happens
>> > is that the disasters are smashes while the potter gets the balance
>> between
>> > oxidation and reduction right and we all pretend like a conjurer that it
>> > just came out of a hat like magic a testimony to our innate genius .
>Hard
>> > work and a pile of shards is so 'uncool'.
>> >
>> > I think the best potters are not put off by a few disasters as long as
>> > they learn by them and don't end up either avoiding them or worse trying
>> to
>> > pass off bloating and crazing as some sort of intrinsic part of the hand
>> > made process.
>> >
>> > --------------------------------
>> >
>> > I would like to know exactly what happens to iron in a firing . If you
>> look
>> > at an old fashioned house brick- usually fired with coal and what ever
>> came
>> > to hand - you will see that the brick is red yellow and deep purple
>> > sometimes on the same brick. This is not a matter of simple one part is
>> > reduced and one not because the colors are so many and contradictory.
>From
>> > this i surmise that iron is very sensitive to atmosphere and its also
>> fixed
>> > when it goes into fusion and yet will still change but not in the same
>> way.
>> >
>> > Another example of this is when I oxidize my kiln all the way to
>> > biscuit the pots come out pink
>> >
>> > If I neutral fire or slightly reduce and then clear the carbon at
>750c
>> > to 950c the pots are buff. Some times iron spots show like flecks of
>> brown
>> > on the surface and these flecks are increased in the glaze firing.
>> >
>> > Since shino glazes are very much dependent on the reoxidization of
>> > previously reduced iron showing on the glaze surface i am wondering if
>it
>> is
>> > this that the early reduction is encouraging. so if you slightly reduce
>a
>> > very well oxidized biscuit pot you get this iron migrating to the
>surface
>> as
>> > red.
>> >
>> > Or looking closer ??? does the iron migrating from the body to the
>> glaze
>> > surface showing red, add to the fine iron crystals showing pink to give
>a
>> > more in depth effect. Is the reason you have to reduce the body iron so
>> > early because it will only migrate to the surface through the body and
>> > glaze layer if it is reduced iron and the layers have not formed into a
>> > glass.
>> >
>> >
>> >
>> > --
>> > Regards from Paul Taylor
>> >
>> > http://www.anu.ie/westportpottery
>> >
>> > In time the biggest hammer will fail to smash a nut. that is the nature
>of
>> > 'disaster' - but also 'hope'.
>>
>>
>>
>
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>
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>
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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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