Kris Griffith on wed 3 dec 97
I was under the impressing that the black coring,(grey to black color of the
clay body in cross section) after reduction firing was simply caused by the
chemical change of red iron oxide (FeO2) which contains oxygen molicules to
black iron oxide (Fe) which does not have oxygen molicules, because they are
used up in the reduction atmosphere. The glaze over the body prevents the =
iron
from reoxydizing.
Just trying to make sence of this all.
Kris
-------
From: Bill Amsterlaw=5BSMTP:wamster=40slic.com=5D
Sent: Saturday, November 29, 1997 9:41 AM
To: Multiple recipients of list CLAYART
Subject: Re: Firing schedule for copper reds.... / Black coring
----------------------------Original message----------------------------
Hi Ron:
Regarding black coring....
Ron Roy wrote:
=3E=3E
Guess you have to define what you mean by black coring - bloating? or
different shades of gray and/or black in the body cross section.
....
What you are referring to (black core) is a problem which happens because
of improper bisque firing. Not enough oxygen to burn off organics between
700C and 900C.
=3C=3C
Ok, so you want to burn off all the carbon before you hit 900C =3D 1652F =3D
cone 010. You achieve this by firing in an oxidizing atmosphere with a
slow rise from 700-900C. This could be done in a bisque-firing or a
single-firing. If you are doing a glaze firing and you have already burnt
off the carbon in a bisque firing, you can start reducing at 800C=3B if you
are single-firing, don't start reduction until you are above 900C. If
there is still carbon in the clay at 900C, it will reduce red iron oxide to
black iron oxide and cause premature vitrification.
Some questions about all this...
So, what's =22body reduction=22? When you reduce at cone 06 or so to
accomplish a =22body reduction=22, aren't you trying to reduce red iron =
oxide
to black iron oxide? That would mean that there is more than one way to
reduce red iron oxide to black: You can do it with the kiln atmosphere,
too. That would mean that even if you had burnt out all the carbonaceous
material in a prior bisque firing, a heavy reduction atmosphere below 900C
could do the same thing to red iron oxide that carbonaceous material could
do.
So, then ... Is the difference between =22black coring=22 and =22body =
reduction=22
a quantitative one? ... or are there unique processes that apply to each
term? Is there a continuous spectrum between =22black coring=22 and =22no =
body
reduction=22? Is =22over-reduction=22 the same thing as =22black coring=22?=
Is it
possible to look at a broken shard and distinguish between =22black =
coring=22
and =22heavy body reduction=22? How much is too much? What's so bad about
black coring, anyway? If you don't get bloating, is the clay ok regardless
of its color?
- Bill Amsterlaw (wamster=40slic.com)
Plattsburgh, NY
Ron Roy on mon 8 dec 97
Hi Kris,
Yes - body reduction reduces red iron to black (a simplified explanation)
- if this is not complete you get different colours in cross section - it
should be the same all the way through. The reason is - black iron is a
heavy duty flux and red iron is not. If you have different colours in cross
section you have different melting in the clay and therefore stresses
because of that. Even reduction would be best.
If the organics are not oxidized between 700C and 900C the result is
overfiring of the body - leading to overfiring of the body - I wish I
understood the chemistry here - but let me speculate. If there is
insufficient oxygen during this stage the result can be overfiring of the
body - Perhaps it's the combination of reduced iron and sulphur I don't
know.
What I do know is that it happens - and I have only seen it with clays that
have iron in them.
Once upon a time - in my capacity as clay watcher and glaze problem solver
- suddenly - we started getting customer problems - pinholing - or more
specifically blistering. The kind of blistering which could not be fixed
with refiring - in fact got worse. This was happening in all types of
firing. Turns out mostly to those who fired faster and were stacking their
kilns tight - and to those using darker clays. We finally got enough
information that we could narrow our suspicions to a few of our basic
clays. We started sieving them to see if we could find something - and did
- little black specks - smaller than the head of a pin - they were hard but
I could crush them between my fingernails - coal - many of our basic clays
have a fair bit of lignite - there is an advantage - the more coal the more
plasticity - but in this case either it was too big to burn out properly or
there was too much. The cure was to fire slow enough to give the oxidation
process time to happen. The blistering was happening because the clay was
overfiring and starting to break down - producing gases which were coming
through the glaze.
I sure would like someone who understands to explain why the clay gets
overfired in one situation (call it bisque reduction?) but not in typical
body reduction.
>----------------------------Original message----------------------------
>I was under the impressing that the black coring,(grey to black color of the
>clay body in cross section) after reduction firing was simply caused by the
>chemical change of red iron oxide (FeO2) which contains oxygen molecules to
>black iron oxide (Fe) which does not have oxygen molecules, because they are
>used up in the reduction atmosphere. The glaze over the body prevents the iron
>from reoxydizing.
>Just trying to make sense of this all.
>Kris
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
Lyla Kaplan on thu 15 oct 98
Hi-
I'm having a problem with black coring on my ^10 orange stoneware. We ease
up on reduction after the body reduction, but not too much so as not to
sacrifice the copper reds (on b-mix), and we're not reducing so much that
we get a smoking train.
So, does anyone know of a moist ^10 claybody one can get in the 48 states
that can withstand a lot of reduction? Standard Ceramics was kind enough
to send me samples of #205 and #259, but those appear to core in spots as
well. Are darker clay bodies more prone to coring?
Thanks,
lyla
durham, north carolina
Jonathan Kaplan on fri 16 oct 98
>----------------------------Original message----------------------------
>Hi-
>
>I'm having a problem with black coring on my ^10 orange stoneware. We ease
>up on reduction after the body reduction, but not too much so as not to
>sacrifice the copper reds (on b-mix), and we're not reducing so much that
>we get a smoking train.
>
>So, does anyone know of a moist ^10 claybody one can get in the 48 states
>that can withstand a lot of reduction? Standard Ceramics was kind enough
>to send me samples of #205 and #259, but those appear to core in spots as
>well. Are darker clay bodies more prone to coring?
>
>Thanks,
>lyla
>durham, north carolina
Yes, of course darker clays are much more prone the black coring and bloating.
I would make sure that my firings are "standardized" both in the bisque as
well as in the glaze. And of course the word "standardization" is anathema
to potters. So lets assume that you have an oxygen probe and you can fire
well into the excess air range to approximately cone 06-05 for you bisque
to properly burn off all the carbonaceous materials. Fine. So then lets
assume that your bisque then is clean.
With a glaze firing, lets again assume that you are using you oxygen probe
and can fire up to a point(and this is again, bordering on excess air)
where you begin a "body reduction," I know those who fire copper reds
begin a reduction earlier than lets say cone 08 or so, and if this
reduction is heavy and your clay body is a dark one indicating the presence
of red clays or iron oxide etc. the body will either black core or bloat.
Your probe could read in the .5 range or higher to about .7 at this point,
which according to AIC is in the moderate to heavy reduction range,. aned
INMO, heavy reduction is not needed and is wasteful. After body reduction
or what ever you choose to call it, the kiln is set for its climb to cone
10 or so in a neutral to moderate reduction until you decide that the
firing is over or youu clear the kiln at the end, or whatever.
The point being that heavy reduction, even in the case of copper reds is
not necessary. Black coring and bloating are the result of improper bisque,
too early a reduction, too heavy a reduction, and a clay body that may have
to high a quantity of iron bearing materials, either clay or oxides.
Now these are my own observations and IMHO, I have been skeptical of B mix
even though Laguna swears by it but there are many potters who have
continuing problems with this body. My own preference in high fire bodies
is one that has virtually no fireclay content, a good blend of kaolins and
a touch of red clays perhaps for the plastic materials, and for the non
plastic side a combination of flint, spar, and pyrophyllite.
Its very important to fire with a degree of useful information that goes
beyond the anmount of flame at the spy hole to indicate back pressure, the
smell, the visual observations, etc etc. and to have some way, a CO2
analyzer (my last choice) or an oxygen probe (my all time first choice), to
provide some way of standardizing and monitoring the atmosphere in the
kiln.
Understanding the mechanics of reduction is quite important, as well as the
means to achieve this and the means to effectively monitor it.
Jonathan
Jonathan Kaplan, president jonathan@csn.net
http://www.sni.net/ceramicdesign/
Ceramic Design Group Ltd./Production Services
PO Box 775112
Steamboat Springs CO 80477
(USPS deliveries only)
Plant Location
1280 13th Street Unit 13
Steamboat Springs CO 80487
( UPS, courier, and common carrier deliveries)
(970) 879-9139*voice and fax
http://www.sni.net/ceramicdesign/
http://digitalfire.com/education/clay/kaplan1.htm
Edouard Bastarache on fri 16 oct 98
Hello Lyla,
i use Plainsman # H440 made by Tony Hansen, a member of Clayart.
I can reduce a lot without problems.The more reduction, the darker it gets.
Plainsman can certainly ship anywhere in the USA.
His e-mail is thansen@digitalfire.com
Tucker's Pottery Supplies owned by Frank Tucker makes wonderful
clays according to Kathy McDonald,both are members of Clayart;
i am sure Frank can also ship anywhere in the USA, his e-mail is
tuckers@passport.ca
Later,
Edouard Bastarache
edouardb@sorel-tracy.qc.ca
http://www.sorel-tracy.qc.ca/~edouardb/
----------
> De : Lyla Kaplan
> A : CLAYART@LSV.UKY.EDU
> Objet : black coring
> Date : 15 octobre, 1998 09:27
>
> ----------------------------Original message----------------------------
> Hi-
>
> I'm having a problem with black coring on my ^10 orange stoneware. We
ease
> up on reduction after the body reduction, but not too much so as not to
> sacrifice the copper reds (on b-mix), and we're not reducing so much that
> we get a smoking train.
>
> So, does anyone know of a moist ^10 claybody one can get in the 48 states
> that can withstand a lot of reduction? Standard Ceramics was kind enough
> to send me samples of #205 and #259, but those appear to core in spots as
> well. Are darker clay bodies more prone to coring?
>
> Thanks,
> lyla
> durham, north carolina
Ron Roy on fri 16 oct 98
It's not the reduction in the glaze firing - no such thing as over
reduction in our temperature range. The problem is in the bisque firing.
The organics have to burn (oxidize) as they are released from the clay -
make sure you are in oxidation between 700C to 900C and go slow enough to
give the process a chance.
If the organics can't find enough free oxygen they will start looking for
it - if there is iron in your clay they will find it there - Fe2O3 is not
much of a flux at our temperatures - but reduced iron - FeO - is a big time
flux starting at 900C or so.
Black coring is sometimes used to describe bloating which is simply the
overfuxing of the body due to reduced iron.
>----------------------------Original message----------------------------
>Hi-
>
>I'm having a problem with black coring on my ^10 orange stoneware. We ease
>up on reduction after the body reduction, but not too much so as not to
>sacrifice the copper reds (on b-mix), and we're not reducing so much that
>we get a smoking train.
>
>So, does anyone know of a moist ^10 claybody one can get in the 48 states
>that can withstand a lot of reduction? Standard Ceramics was kind enough
>to send me samples of #205 and #259, but those appear to core in spots as
>well. Are darker clay bodies more prone to coring?
>
>Thanks,
>lyla
>durham, north carolina
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
gambaru on fri 16 oct 98
Could your problem be in the bisque firing method that you use ? MB
-----Original Message-----
From: Lyla Kaplan
To: CLAYART@LSV.UKY.EDU
Date: Thursday, October 15, 1998 9:11 AM
Subject: black coring
----------------------------Original message----------------------------
Hi-
I'm having a problem with black coring on my ^10 orange stoneware. We ease
up on reduction after the body reduction, but not too much so as not to
sacrifice the copper reds (on b-mix), and we're not reducing so much that
we get a smoking train.
So, does anyone know of a moist ^10 claybody one can get in the 48 states
that can withstand a lot of reduction? Standard Ceramics was kind enough
to send me samples of #205 and #259, but those appear to core in spots as
well. Are darker clay bodies more prone to coring?
Thanks,
lyla
durham, north carolina
Dana Henson on sat 17 oct 98
----------------------------Original message----------------------------
It's not the reduction in the glaze firing - no such thing as over
reduction in our temperature range. The problem is in the bisque firing.
The organics have to burn (oxidize) as they are released from the clay -
make sure you are in oxidation between 700C to 900C and go slow enough to
give the process a chance.
If the organics can't find enough free oxygen they will start looking for
it - if there is iron in your clay they will find it there - Fe2O3 is not
much of a flux at our temperatures - but reduced iron - FeO - is a big time
If the organics can't find enough free oxygen they will start looking for
it - if there is iron in your clay they will find it there - Fe2O3 is not
much of a flux at our temperatures - but reduced iron - FeO - is a big time
flux starting at 900C or so.
Black coring is sometimes used to describe bloating which is simply the
overfuxing of the body due to reduced iron.
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Ron Roy and anyone else willing to offer information:
What can you do if you don't have access to an oxygen probe to determine the
level of oxygen present in the kiln during bisque firing? I would like to
achieve better firings in the bisque to reduce the possibility of black coring
during Cone 10 reduction firings. Suggestions anyone? Many thanks in advance.
Black coring and needing help---
Dana Henson
---Since this is in a school situation, all different types of clay bodies
are in every firing.
Bill Amsterlaw on sat 17 oct 98
Hi Lyla:
You wrote:
<< I'm having a problem with black coring on my ^10 orange stoneware. We
ease up on reduction after the body reduction, but not too much so as not
to sacrifice the copper reds (on b-mix), and we're not reducing so much
that we get a smoking train. >>
Black coring has been discussed on Clayart before. You can find a good
discussion in Hamer's text. The following is my understanding of black
coring (I have been interested in this topic, but I ain't no expert)....
The problem can be simplified like this: Almost all clay contains some
amount of carbon, sulfur, and iron. Above 1650 F, sulfur and carbon in the
clay reduce iron to black iron oxide. Black iron oxide is a potent flux
which immediately begins liquefying the clay body, becoming incorporated
into the melt so that the iron cannot be re-oxidized. (Once you have black
iron oxide in the clay, it remains a potent flux and you can't get rid of
it.)
A dark body would be expected to be more prone to black coring because of
its higher iron content. Black coring is not a problem with white clay.
The solution can be simplified like this: Burn out all the carbon and
sulfur below the temperature at which black iron oxide forms (1650 F).
(You don't need to change clay bodies.)
The sulfur and carbon can be burned out between 1300 F (cone 018) and 1650
F (cone 09). In order to burn these materials out, you need an oxidizing
atmosphere and a slow temperature rise in this temperature range (might
need to be as slow as 50 degrees per hour for some clays).
If you bisque fire in an electric kiln, you need good ventilation in the
kiln to get all the carbon and sulfur out of the clay. If you bisque fire
too fast or have poor ventilation, when you get above 1650 F, black iron
oxide will form, which can then cause black coring and bloating in the
glaze firing. (In other words, you will still have the problem if you do a
too-fast bisque and then try to make up for it with a slow glaze firing:
Black iron oxide already formed in the bisque. Likewise, going too fast in
the bisque between 1300 F and 1650 F and then slowing down toward the end
of the bisque firing will not help; you have to burn out the carbon and
sulfur below 1650 F to prevent black iron oxide from forming.)
If you single fire, it is important to maintain an oxidizing atmosphere
until you get above 1650 F (cone 010).
Hamer says: "Black core is often a fault arising in the biscuit firing that
does not show itself until the glaze firing."
- Bill Amsterlaw (wamster@slic.com)
Plattsburgh, NY
Cameron Harman on mon 19 oct 98
Dana Henson raised a good point.
The facts are these (by the way this is not my opinion or the
results of some tests I did once. This data has been confirmed
since long before I was born.)
All organic matter will fully oxidize before 900 degrees F, most
of it by 700 degrees. If it goes higher in temperature before
fully oxidizing the organic it will go through a process called
"cracking" which results in the formation of elemental carbon. You
can get elemental carbon out above read heat, but not easily,
sometimes not at all.
This is a question that has been faced in industry over and over
and over again. It is something that every first or second year
ceramic engineer was taught and shown by experiment. It may be
that today's engineers are not all exposed to this, but I suspect
that they are.
The answer is really rather simple. You must hold the temperature
below 900 degrees long enough to burn the organic material
completely, so you run a few test fires at various times between
400 and 900 degrees. Then break the pieces and look at the center,
when its clean you have what you need. But, be aware that a
thicker piece will take longer time.
I sometimes end up with three firing schedules each one longer
than the other for ware having various thicknesses. Once
established I choose what appears to be the appropriate curve and
fire. You get so that you can judge pretty well every time.
Remember, black coring is a separate problem from those
encountered with reduction firing.
Good luck,
Cameron
--
**********************************************************
Cameron G. Harman, Jr. 215-245-4040 fax 215-638-1812
e-mail kilns@kilnman.com
Ceramic Services, Inc 1060 Park Ave. Bensalem, PA 19020
get your free ezine: http://www.kilnman.com/ezine/ezine.html
THE place for total kiln and drier support
**********************************************************
Cameron Harman on mon 19 oct 98
Dana Henson raised a good point.
The facts are these (by the way this is not my opinion or the
results of some tests I did once. This data has been confirmed
since long before I was born.)
All organic matter will fully oxidize before 900 degrees F, most
of it by 700 degrees. If it goes higher in temperature before
fully oxidizing the organic it will go through a process called
"cracking" which results in the formation of elemental carbon. You
can get elemental carbon out above read heat, but not easily,
sometimes not at all.
This is a question that has been faced in industry over and over
and over again. The answer is really rather simple. You must hold
the temperature below 900 degrees long enough to burn the organic
material completely, so you run a few test fires at various times
between 400 and 900 degrees. Then break the pieces and look at the
center, when its clean you have what you need. But, be aware that
a thicker piece will take longer time.
I sometimes end up with three firing schedules each one longer
than the other for ware having various thicknesses. Once
established I choose what appears to be the appropriate curve and
fire. You get so that you can judge pretty well every time.
Remember, black coring is a separate problem from those
encountered with reduction firing.
Good luck,
Cameron
--
**********************************************************
Cameron G. Harman, Jr. 215-245-4040 fax 215-638-1812
e-mail kilns@kilnman.com
Ceramic Services, Inc 1060 Park Ave. Bensalem, PA 19020
get your free ezine: http://www.kilnman.com/ezine/ezine.html
THE place for total kiln and drier support
**********************************************************
Ron Roy on mon 19 oct 98
To find out if there is free oxygen in the kiln - insert a sliver of wood
or cardboard (something combustible) in the spy above red heat - make sure
there is back pressure - if it burns inside the kiln there is excess oxygen
- if the smoke produced does not burn till it gets outside the kiln then
you know you are reducing. Don't get to close - if you do the smell of
burning hair will be your first warning of more damage to come.
It should not be difficult to determine what damper and burner setting will
give reduction at say at 700C - it's then simply a matter of cutting back
the gas, opening the damper or both to get enough O in to completely
combust the gas and have extra O around in case the weather changes or your
latest batch of clay has more combustibles in it than last time.
Low fire cones (the red ones made with red art) will darken and
consequently register a lower temperature (FeO is a strong flux) if there
is not suffient oxygen.
Make sure there is no flame at the damper.
Keep your burners on low or half till you are over 1000C - gas kilns are
usually colder at the bottom so you need some extra time to catch up there.
Keep all primary air open full - especially with atmospheric systems.
Go slow from 700C to 900C - aim for 50C per hour - thats not so easy to do
in some kilns. Just remember - too much Oxygen is not a problem.
If your ware is thick you need to give more time for the burning off - and
so to with a tight stacking.
This can be a problem with electric kilns as well - the vent systems which
pull in fresh air are a great help in this area.
If you put some small pieces of low fire red clay around in your kiln
during a bisque - like straight red art - you can tell when you have either
reduction or oxidation going on. Don't leave em in there for a high firing
- brown puddles and drips for sure.
RR
>Ron Roy and anyone else willing to offer information:
> What can you do if you don't have access to an oxygen probe to determine the
>level of oxygen present in the kiln during bisque firing? I would like to
>achieve better firings in the bisque to reduce the possibility of black coring
>during Cone 10 reduction firings. Suggestions anyone? Many thanks in advance.
> Black coring and needing help---
> Dana Henson
> ---Since this is in a school situation, all different types of clay bodies
>are in every firing.
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
Edouard Bastarache on fri 9 dec 11
According to Robin Hopper :
=3D20
=3DAB A black core occurs when the carbon in the body is not successfully =
=3D
burnt out in the bisque fring. All clays contain some carbonaceous =3D
matter,which must be completely eliminated before the completion of the =3D
bisque firing, In the process of burning out, local reduction takes =3D
place, turning the iron oxide black, possibly with some carbon. Black =3D
coring can be attributed to a lack of sufficient oxygen between 750 and =3D
900 C , through either an unclear atmosphere or through too rapid a =3D
firing cycle. The remedy for the problem is to make certain that the =3D
atmosphere is oxidizing , and too slow the firing slightly. =3DBB
(The Ceramic Spectrum)
Gis,
Edouard Bastarache=3D20
Spertesperantisto=3D20
Sorel-Tracy
Quebec
http://www.flickr.com/photos/30058682@N00/
http://edouardbastarache.blogspot.com/
http://edouardbastaracheblogs2.blogspot.com/
http://www.facebook.com/edouard.bastarache
mel jacobson on sun 11 dec 11
as a person that uses a great deal of iron bearing
clay, and always have, it does give me pause as why
i do not get `black core`...
i just went out and broke some pots...they are pure gray
all the way through. just as they are suppose to be.
hit them with a tool and they ring like bells.
i have no `scientific answer`...i don't do `chemical
symbols`...but, i know i have taken good advice over
the years about clean firing. long bisque with good
clear kilns, well vented.
i fire hotter than many potters, often near
cone 13, and always down fire
in some manner. i get brilliant color when i need it.
and i know i never `over reduce`.
i fire with clear blue flame. fast fire in the early stages
and reduce only with my damper. oxygen is my best
friend as i have very low gas pressure from the house.
many have told me that it is impossible to reach my temps
with house gas pressure. i do.
i am strongly in the camp of clean, clear firing. i do not
like over reduced pots. i love that toast color of clay body.
and, without question, i love the full melt of glaze. i even
love that little roll at the bottom of the glaze line that lets
me know i have max'd out the glaze. full melt.
if i visit another potter and see black carbon all over the
kiln, and a soot ceiling...i know they don't have a clue about
firing. all the soot means dirty flame and unburned fuel.
(extreme fuel wasting too.)
my kilns never smoke. and, just a tiny soot line where the
flame escapes around the door. like three inches..max.
and that comes from burning wood during my down fire.
(and, that is controlled)
so, the conversation is enlightening. i love to read and
pull ideas into my head. it is worthwhile.
i have been influenced by ron roy, and hank murrow over
that last 15 years. that influence has made my work
better. and, there are many other clayart friends that
make working this list a joy. (that list would be in the hundreds.)
mel
from: minnetonka, mn
website: http://www.visi.com/~melpots/
clayart link: http://www.visi.com/~melpots/clayart.html
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