Ron Roy on sat 27 sep 97
>----------------------------Original message----------------------------
> The local clay is definitely a ^10 clay. Could it be the
>>reduction? Too much?
My understanding is that clay cannot be "over reduced."
Either the Fe2O3 in the clay is not reduced (Fe2O3) or it is all reduced to
FeO or there is a mixture of both.
I realize there is a skin of re-oxidized FeO on the surface which gives the
different shades of grey to brown in reduction fired clay if there is
enough iron present.
I also understand there can be different degrees of reduction within the
body if the reduction is not complete. I think this is a dangerous
situation by the way.
I also understand the reduced clay can be anywhere from light grey to black
depending on the amount of iron in the body to start with.
Quite a preamble to what is really a question. Are there those of you who
think that clay can be over reduced? If so - I would like to understand how
it can happen - which oxide or oxides are over reduced in order for this to
happen.
Ron Roy
93 Pegasus Trail
Scarborough,Canada
M1G 3N8
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm
David Hendley on mon 29 sep 97
Ron,
I don't have a clue as to what is really happening,
but I know from years of experience that the same clay body
when broken apart can be light grey or dark grey.
It's also common to see a "sandwich" effect of:
brown, thin layer (reoxidized)
dark grey, thin-medium layer
light grey, the thickest section
dark grey, and brown again.
Maybe this is what you call incomplete reduction?
Why do you call incomplete reduction a dangerous situation?
I guess my definition of "over reduced" is a body that is dark grey
and has compromised strength as evidenced by small pieces of the
pot left behind on the kiln shelf. I think it is a matter of both
firing practice and claybody formulation.
David Hendley
At 02:50 PM 9/27/97 EDT, you wrote:
>----------------------------Original message----------------------------
>>----------------------------Original message----------------------------
>> The local clay is definitely a ^10 clay. Could it be the
>>>reduction? Too much?
>
>
>
>My understanding is that clay cannot be "over reduced."
>
>Either the Fe2O3 in the clay is not reduced (Fe2O3) or it is all reduced to
>FeO or there is a mixture of both.
>
>I realize there is a skin of re-oxidized FeO on the surface which gives the
>different shades of grey to brown in reduction fired clay if there is
>enough iron present.
>
>I also understand there can be different degrees of reduction within the
>body if the reduction is not complete. I think this is a dangerous
>situation by the way.
>
>I also understand the reduced clay can be anywhere from light grey to black
>depending on the amount of iron in the body to start with.
>
>Quite a preamble to what is really a question. Are there those of you who
>think that clay can be over reduced? If so - I would like to understand how
>it can happen - which oxide or oxides are over reduced in order for this to
>happen.
>
>
>Ron Roy
>93 Pegasus Trail
>Scarborough,Canada
>M1G 3N8
>Evenings, call 416 439 2621
>Fax, 416 438 7849
>Studio: 416-752-7862.
>Email ronroy@astral.magic.ca
>Home page http://digitalfire.com/education/people/ronroy.htm
>
>
David Hendley
Maydelle, Texas
See David Hendley's Pottery Page at
http://www.sosis.com/hendley/david/
paul m wilmoth on thu 2 oct 97
Dear Ron,
Over reduction is called "BLACK CORE" and can severly reduce
the strength of a clay body, I have done this and it makes the work very
brittle.
Regards -- Paul
Ron Roy on thu 2 oct 97
This answer from David was to my question about "over-reduced clay - which
I don't think can happen.
Iron (Fe2O3) in reduced clay and glazes becomes FeO
Reduced Fe2O3 (FeO) is a strong flux in clay and glazes.
The grey or black colour of reduced clay is usually the result of iron.
The more iron the darker the clay.
If the colour in the body varies it means the iron was not fully reduced or
got reoxidized somewhere along the way.
I can remember when I was learning to fire my kiln - if I did not get into
reduction before 1100C (my pyrometer) the clay did not get a proper
reduction. I say get into reduction at the latest by 1050C and stay in till
at least 1150C to make sure you get a complete body reduction - I stay in
reduction any where from 900C to 1050C (depending on the types of glazes)
till the end.
So if you break a pot open and you have different "layers" of colour it
means you have fluxed each layer a different amount. This means you have a
different expansion rate for each layer - not the best strategy for making
durable ware.
I am not talking about the re-oxidized outer layer which gives reduction
fired clay it's distinctive colouring.
The more iron oxide in the clay the more fluxing from it when it is
reduced. If the body is over fluxed then it will glassify - as any over
fluxed body will do. The fault is not in the "over" reduction but in the
over fluxing of the body. The remedy would be to lower the iron or raise
the refractories.
There is another ominous part to functional reduced bodies fluxed with
iron. Iron does not do the same job of limiting cristobalite production as
KNaO. If part of your body flux is iron then you cannot have as much KNaO.
I can generalize at this point and say I have yet to see a high iron clay
body which does not have cristobalite. This does not mean other types of
bodies do not have cristobalite - that depends on other factors which
include amount of KNaO and fine silica.
I have never seen any cristobalite in porcelain.
I do plan some experiments to find some sort of correlation between
cristobalite production in relation to iron in the future.
Any over fluxed body will leave pieces on a kiln shelf - porcelain
certainly does unless steps are taken to prevent it - one of the best ways
of preventing this is to use kiln wash without silica - I use alumina in
wax on the bottom of my foot rings as well.
I was hoping for other contributions (based on experience) on this subject.
It seems to me there is significant misunderstanding on the subject which
could use some light. Thanks to David for his contribution - I am sure his
experience is more common than not. We have had many conversations and I
find his candid and open communications refreshing and inspiring. I too
hope he will bring " a trip to the dump" video to NCECA.
>----------------------------Original message----------------------------
>Ron,
>I don't have a clue as to what is really happening,
>but I know from years of experience that the same clay body
>when broken apart can be light grey or dark grey.
>It's also common to see a "sandwich" effect of:
>brown, thin layer (reoxidized)
>dark grey, thin-medium layer
>light grey, the thickest section
>dark grey, and brown again.
>Maybe this is what you call incomplete reduction?
>Why do you call incomplete reduction a dangerous situation?
>
>I guess my definition of "over reduced" is a body that is dark grey
>and has compromised strength as evidenced by small pieces of the
>pot left behind on the kiln shelf. I think it is a matter of both
>firing practice and claybody formulation.
>
>David Hendley
Ron Roy
93 Pegasus Trail
Scarborough,Canada
M1G 3N8
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm
Kathy on fri 3 oct 97
At 07:49 AM 10/2/97 EDT, you wrote:
>----------------------------Original message----------------------------
>Dear Ron,
>
> Over reduction is called "BLACK CORE" and can severly reduce
>the strength of a clay body, I have done this and it makes the work very
>brittle.
>
>Regards -- Paul
>
I have had this happen with laguna Death Valley from time to time
and it quite seriously impacts on the fired strength of the clay.
Some clays can handle heavy reduction much better than others
I don't know what the factors are that contribute to this
but the amount of iron in the clay must definitely be one of
the important ones.
so...imho...yes clays can be overreduced...especially during the body reduction
phase of firing.
Kathy
_________________________________________________________
mcdonaldk@docker.com
http://www.docker.com/~mcdonaldk
{enter at your own risk construction underway}
Ron Roy on sat 4 oct 97
Dear Paul,
I think there are many who think this is so and was part of my reason for
my post. As Vince pointed out - black core is something else and can happen
with even partially reduced ware. It is overfluxed clay due to improper
firing (not enough oxygen and/or too fast) during the period when carbonous
material burns out of clay (700C to 900C.)
This happens when there is enough iron in the clay to overflux it.
When there are iron specks in the body (from fire clay and/or grog and
such) they can be reduced at this same low temperature and lead to
blistering. The tip off for this problem - when you refire the blisters get
worse.
I would be interested to hear if anyone has seen black coring in porcelain
- or any white body for that matter.
>----------------------------Original message----------------------------
>Dear Ron,
>
> Over reduction is called "BLACK CORE" and can severly reduce
>the strength of a clay body, I have done this and it makes the work very
>brittle.
>
>Regards -- Paul
Ron Roy
93 Pegasus Trail
Scarborough,Canada
M1G 3N8
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm
Ric Swenson on mon 6 oct 97
CARBON CORING aka "Black coring" is, I believe ALWAYS a product of a
body with some iron content. At least, in my thirty years of potting, I
have not yet seen it in a porcelaneous body.
R
At 10:45 AM -0400 10/4/97, Ron Roy wrote:
>----------------------------Original message----------------------------
>Dear Paul,
>
>I think there are many who think this is so and was part of my reason for
>my post. As Vince pointed out - black core is something else and can happen
>with even partially reduced ware. It is overfluxed clay due to improper
>firing (not enough oxygen and/or too fast) during the period when carbonous
>material burns out of clay (700C to 900C.)
>
>This happens when there is enough iron in the clay to overflux it.
>
>When there are iron specks in the body (from fire clay and/or grog and
>such) they can be reduced at this same low temperature and lead to
>blistering. The tip off for this problem - when you refire the blisters get
>worse.
>
>I would be interested to hear if anyone has seen black coring in porcelain
>- or any white body for that matter.
>
>>----------------------------Original message----------------------------
>>Dear Ron,
>>
>> Over reduction is called "BLACK CORE" and can severly reduce
>>the strength of a clay body, I have done this and it makes the work very
>>brittle.
>>
>>Regards -- Paul
>
>Ron Roy
>93 Pegasus Trail
>Scarborough,Canada
>M1G 3N8
>Evenings, call 416 439 2621
>Fax, 416 438 7849
>Studio: 416-752-7862.
>Email ronroy@astral.magic.ca
>Home page http://digitalfire.com/education/people/ronroy.htm
R i c S w e n s o n,
P.O. Box 4 9 4
North Bennington, VT U. S. A.
0 5 2 5 7 - 0 4 9 4
Days ( 8 0 2 ) 4 4 2 - 5 4 0 1 ext. 4 6 2 1
or fax ( 8 0 2 ) 4 4 2 - 6 1 6 4
email: rswenson@bennington.edu
"If you are going to teach a bear to dance....
...... be prepared to dance until the bear gets tired."
.....( old Russian
proverb.)
Ron Roy on mon 6 oct 97
Hi Kathy - I don't know if you have read the other posts on this subject
but my comments follow - in the interest of understanding what is really
going on.
Black coring is not over reduction - it is over fluxing of the sealed clay
and is not due the amount of reduction.
I am not sure what is happening to the Laguna clay - if it is black coring
the problem is in the bisque firing - or if once - fired too fast and/or in
reduction during the crucial stage when carbon is released from the clay
body.
If the Laguna Clay is over fluxed in reduction then the problem is an over
fluxed body for the temperature recommended. The signs of over fluxed clay
are - looks glassy when broken, brittle, warping and slumping, - and as the
clay is further over fired - it starts to breakdown - the first sigh of
this is small bubbles apparent on the surface of the clay - as breakdown
continues these bubbles become larger and are called bloating.
If a high iron clay is vitrified at a certain cone in oxidation it will
certainly be over fluxed at that temperature if reduced - because FeO
(reduced Fe2O3) is a strong alkaline flux and Fe2O3 is not.
Can I assume that "heavy reduction" means full reduction? As a clay body
designer with 30 years of experience - I would not formulate a clay body
made for reduction that did not function under full reduction. I would have
to eat most of it.
The Laguna catalogue description of Death Valley Red says it is
orange-brown in oxidation and dark brown in reduction - this indicates a
lot of iron - special care in firing this type of clay must be taken -
getting the carbon out properly would be wise. The target absorption for
this clay is stated as 1.5% at cone ten - if tested there would be quite a
difference if it was fired in oxidation and reduction.
>I have had this happen with Laguna Death Valley from time to time
>and it quite seriously impacts on the fired strength of the clay.
>
>Some clays can handle heavy reduction much better than others
>I don't know what the factors are that contribute to this
>but the amount of iron in the clay must definitely be one of
>the important ones.
>
>
>so...imho...yes clays can be overreduced...especially during the body reduction
>phase of firing.
>Kathy
Ron Roy
93 Pegasus Trail
Scarborough,Canada
M1G 3N8
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm
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