Ivor and Olive Lewis on wed 29 nov 06
Dear Vince,
I do not disagree with what you have said.
But fluxing activity, that is the creation of a molten phase, cannot =
start until your reaction between Iron oxide and Silica has taken place.
Iron oxides do not melt at low enough temperatures to create the fluid =
environment that leads to vitrification.=20
Check on the relationships between the two black Iron oxides and Iron =
Silicate.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Vince Pitelka on wed 29 nov 06
Ivor Lewis wrote:
"I do not disagree with what you have said. But fluxing activity, that is
the creation of a molten phase, cannot start until your reaction between
Iron oxide and Silica has taken place. Iron oxides do not melt at low
enough temperatures to create the fluid environment that leads to
vitrification. Check on the relationships between the two black Iron oxides
and Iron Silicate."
Ivor -
You seem to be getting this information from scientific texts. I have only
information from ceramics texts, along with almost 40 years of practical
experience in ceramics. Depending on the specific formula, most terracotta
bodies cannot be fired beyond cone 1 or 2 because they will bloat and slump,
due to the powerful fluxing action of the iron. But more to the point here,
even below bisque-fire temperatures, a reducing atmosphere can transform red
iron oxide to black and cause an early reaction between iron and silica, and
at that stage of the firing it can reduce the porosity of the claybody and
promote carbon coring.
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Bruce Girrell on thu 30 nov 06
Vince Pitelka wrote:
> even below bisque-fire temperatures, a reducing atmosphere can
> transform red iron oxide to black and cause an early reaction between iron
and
> silica, and at that stage of the firing it can reduce the porosity of the
claybody and
> promote carbon coring.
I was unable to find a reference that indicated at what temperature a
Fe2O3 --> Fe3O4 or Fe2O3 --> FeO reduction reaction would take place. Do you
have any info on that? Ivor, do you know? Anybody?
Bruce Girrell
Ivor and Olive Lewis on fri 1 dec 06
Dear Vince,
My main source of scientific information relating to Ceramics has, for =
some time, been Kingery, Bowen and Uhlmann, "Introduction to Ceramics" =
2nd Ed. In particular their discussions in Ch 9 "Reactions with and =
between Solids" and Ch 10 "Grain Growth, Sintering and Vitrification" =
give deeper meaning to many of the terms we use in a common sense way. =
Strangely, the only reference to "Flux" relates to the magnetic =
properties of ceramic materials.
I would direct your attention to the following, relating to Viscous =
Flow, which I quote from p. 763.
"Among inorganic oxide materials, the viscosity is often found to be a =
strong function of the composition as well as temperature. In the case =
of silicates, the viscosity is almost invariably found to decrease with =
increasing concentrations of modifying cations. In many cases this =
variation is quite pronounced."
My other sources are published by the ACS. These include the series of =
wall charts of Ceramic Phase Diagrams of the more important systems, =
including FeO-Fe2O3-SiO2 and CaO-FeO-SiO2 and "Phase Diagrams of =
Ceramists" by Levin et al.
As I said earlier, I do not dispute your description of what takes =
place, of what we can observe. If you were to read about the metallurgy =
of Iron you would find that Iron oxides and hydroxides are reactive =
towards Carbon and Carbon monoxide at much lower temperatures than we =
use to fire our work. Iron ores can be reduced to metal at red heat when =
heated in contact with charcoal. This process is enhanced if the ore is =
mixed with Lime.
I have a very simple view of what happens when we fire clay and glaze. =
It is easy to comprehend........ Some ingredients decompose. Some =
ingredients react chemically with others in the solid state. One =
ingredient, whether original or new born, has a lower melting point than =
any other. When that one become a liquid it become an environment in =
which many other ingredients dissolve. We call that a vitreous melt.
Iron oxide (FeO) Mp 1380 deg C and Silica (Tridymite) Mp 1723 deg C =
react before they melt to form Iron Metasilicate (Fe2SiO3) which has a =
melting point of 1205 Deg C. I suspect that when Lime is present, as it =
is in many surface clays, there are other solid state reactions.Their =
products could bring the final fusion point down to 1100 deg C. Nor =
should we ignore the presence of residual soluble salts which have =
relatively low melting points, some of which will act as solvents when =
they melt.
If you insist on telling me that Black Iron oxide is a Strong Flux I =
wish to know what causes it to change its refractory character so that =
it becomes a fluid two hundred degrees below its published melting =
points (See Levin et al, p 570). You know my story and my sources!! =
What're your's??
Have a good weekend.
Best regards,
Ivor
Tommy Orndorff on fri 1 dec 06
Bruce, you may want to read John Britt's article on Oil-Spot Glazing (since
oil-spotting is the result of Fe2O3 --> FeO in oxidation). Obviously not
your answer for temp in reduction, although I think his book "Complete Guide
to High Fire Glazes" does give the temp in the oil-spot section (I think its
around 900deg F). Cheers.
the site: http://www.ceramicstoday.com/articles/oilspot.htm
> I was unable to find a reference that indicated at what temperature a
> Fe2O3 --> Fe3O4 or Fe2O3 --> FeO reduction reaction would take place. Do >
you
> have any info on that? Ivor, do you know? Anybody?
>
> Bruce Girrell
--
Tommy Orndorff
Appalachian State University
Vince Pitelka on fri 1 dec 06
Ivor Lewis wrote:
"If you insist on telling me that Black Iron oxide is a Strong Flux I wish
to know what causes it to change its refractory character so that it becomes
a fluid two hundred degrees below its published melting points (See Levin
et al, p 570). You know my story and my sources!! What're your's??"
Experience.
I do not mean to be flippant Ivor, and I respect your dedication and
thoroughness in research, but I have only my own personal practical
experience to go on, and I will trust that above all else. I have seen how
early reduction can cause carbon coring in an iron-bearing body well below
normal body-reduction temperatures, and how else do you explain that?
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Ivor and Olive Lewis on fri 1 dec 06
Dear Bruce Girrell,=20
Greenwood and Earnshaw, speaking of the conversion of Iron Ore to Pig =
Iron give a temperature range of 200-700 deg C for the changes to FeO. =
700-1200 for the reduction of FeO to metallic iron by Carbon monoxide.
Publication date 1997
Which would make it impossible for FeO to be a substance that could =
initiate melting in a mixture of silicates, carbonates and what have you =
in the presence of organic stuff in clay.
Unless of course "Flux" means something other than melting. But who =
would ever think that would be possible.
Best regards,
Ivor
Liisa Reid on sat 2 dec 06
I wonder what is the effect of sulfur on black coring in an iron
bearing body
with early reduction. It is many years since I have fired
stoneware, but when
I did I noticed a huge change in how my body behaved, which was
eventually
traced to a higher sulfur content in the stoneware clay. Does
sulfur influence
the melting point of iron?
Liisa
On Dec 1, 2006, at 9:11 PM, Vince Pitelka wrote:
> Ivor Lewis wrote:
> "If you insist on telling me that Black Iron oxide is a Strong Flux
> I wish
> to know what causes it to change its refractory character so that
> it becomes
> a fluid two hundred degrees below its published melting points
> (See Levin
> et al, p 570). You know my story and my sources!! What're your's??"
>
> Experience.
>
> I do not mean to be flippant Ivor, and I respect your dedication and
> thoroughness in research, but I have only my own personal practical
> experience to go on, and I will trust that above all else. I have
> seen how
> early reduction can cause carbon coring in an iron-bearing body
> well below
> normal body-reduction temperatures, and how else do you explain that?
> - Vince
>
> Vince Pitelka
> Appalachian Center for Craft, Tennessee Technological University
> Smithville TN 37166, 615/597-6801 x111
> vpitelka@dtccom.net, wpitelka@tntech.edu
> http://iweb.tntech.edu/wpitelka/
> http://www.tntech.edu/craftcenter/
>
> ______________________________________________________________________
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Ivor and Olive Lewis on sat 2 dec 06
Dear Vince,
Experience?
Dear Vince,
Experience !
Dear Vince,
I recall being told so long ago...
A Man convinced against his will is of the same opinion still.
I will not try to persuade you to think otherwise.
But I do hope others will keep open minds, that they will search and =
sift through our knowledge base.
Believe what you wish.
Best regards,
Ivor
Ps. It is only the lack of pressure that prevents Iron oxide under =
reducing condition from changing into a most noxious gas. But please, do =
not believe me.
Ivor and Olive Lewis on sat 2 dec 06
Dear Tommy Orndorff,
The process John Britt described is the thermal decomposition of Red =
Iron Oxide with the evolution of Oxygen. It has nothing to do with =
reduction nor does it help in any way to resolve the issue of whether or =
not any of the iron oxides are fluxes or not.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Lynne and Bruce Girrell on sat 2 dec 06
Ivor,
Thank you for that information. That saves some work.
But I don't understand what you mean by this statement:
>Which would make it impossible for FeO to be a substance that could
initiate melting
>in a mixture of silicates, carbonates and what have you in the presence of
organic stuff in clay.
I'm not sure what form of iron has such powerful fluxing properties but I do
know that iron is capable of promoting melting of refractory materials to a
phenominal degree. The part that I am unsure of is whether black coring is
due to a particular form of iron resulting from an improper (reducing)
bisque that causes fluxing action during the glaze firing or if it is due to
residual carbon remaining from the reducing bisque that causes a local
reduction of iron in the subsequent firing.
Onward
Bruce Girrell
Timothy Joko-Veltman on sat 2 dec 06
On 12/2/06, Ivor and Olive Lewis wrote:
> Dear Vince,
>
> Experience?
>
> Dear Vince,
>
> Experience !
>
> Dear Vince,
>
> I recall being told so long ago...
Ivor,
You slight experience, yet it is experience that informs all the texts
and authorities in which you place so much stock. Science is merely
formalized experience. Does that invalidate the informal experience
of Vince (and countless others). By no means; formalization does not
make any fact "more true" than if it were left "unexplained". What
goes up, comes down - even without Newton.
If you want to know the details, the exact mechanism, great! (And
"Good luck".) But quit trashing other people's knowledge just because
it's not laid out the way you like it - because THAT is a closed mind
if I've ever seen one. And if you don't believe me or Vince (or
anyone else who says so) that iron oxide is a powerful flux in
reduction - and why should you? - then devise a reproduceable
experiment that disproves it - then we'd all be the better for it.
Finally, don't forget that perhaps you are missing something
(eutectics come to mind). The books and authorities don't have all
the answers, and glaze chemistry is an extremely complex subject.
Regards,
Tim
Ron Roy on sun 3 dec 06
Hi Bruce,
My understanding of the problem is.
Iron in body is reduced in bisque because of lack of oxygen to support
complete combustion of organics in clay.
Fe2O3 is easily reduced to FeO by carbon monoxide due to incomplete
combustion of organics.
See Seger categorization of oxides - Fe2O3 would wind up in column with
other RO3 oxides like Al2O3.
FeO (reduced Fe2O3) will wind up in the RO column with the other fluxes -
Hamer says of reduced iron oxide that it is helping to flux starting about
800C.
Don't be distracted by melting points of oxides by themselves - All will
start fluxing at lower temperatures when in a clay or glaze system. Just
look at meting points of pure oxides and then look at melting points of
combined materials to see the differences.
Bloating is not due to carbon but is overfired clay breaking down and
producing gases. Black coring is something else in my mind - a difference
in colour inside a clay body due to different degrees of reduction and/or
oxidation - which - if it results in bloating can be viewed as something
different from bloating.
This can happen during single firing as well and care should be taken to
fire slow enough and provide oxygen for complete combustion of organics
between 700C and 1050C.
RR
>Thank you for that information. That saves some work.
>
>But I don't understand what you mean by this statement:
>
>>Which would make it impossible for FeO to be a substance that could
>initiate melting
>>in a mixture of silicates, carbonates and what have you in the presence of
>organic stuff in clay.
>
>I'm not sure what form of iron has such powerful fluxing properties but I do
>know that iron is capable of promoting melting of refractory materials to a
>phenominal degree. The part that I am unsure of is whether black coring is
>due to a particular form of iron resulting from an improper (reducing)
>bisque that causes fluxing action during the glaze firing or if it is due to
>residual carbon remaining from the reducing bisque that causes a local
>reduction of iron in the subsequent firing.
>
>Onward
>
>Bruce Girrell
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Ron Roy on mon 4 dec 06
Hi Liisa,
When sulphur burns it takes oxygen from where ever it can - if there is not
enough oxygen in the kiln - because it has been used up burning other stuff
- like carbon - then it takes the oxygen from Fe2O3 - turning it into a
flux - and can lead to overfiring of your clay in a glaze firing.
It is not always a problem by the way - if your clay was not tight enough
at your glaze temperature - the extra melting might be beneficial.
RR
>I wonder what is the effect of sulfur on black coring in an iron
>bearing body
>with early reduction. It is many years since I have fired
>stoneware, but when
>I did I noticed a huge change in how my body behaved, which was
>eventually
>traced to a higher sulfur content in the stoneware clay. Does
>sulfur influence
>the melting point of iron?
>Liisa
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Bruce Girrell on mon 4 dec 06
Ron Roy wrote:
> Iron in body is reduced in bisque because of lack of oxygen to support
> complete combustion of organics in clay.
Good so far. My understanding as well.
But let's say that the bisque is done properly in an oxidizing environment.
The iron doesn't go away; it simply stays as Fe2O3.
Now comes the glaze firing. Body reduction starts at red heat (and in my
firings, I pretty much run a reducing environment right from the start,
especially if I am trying to get some carbon trap shinos). There is plenty
of time for the gases to penetrate and interact with the iron in the body.
Why is the iron no longer reduced, resulting in fluxing problems?
It seems to me that the cause of the problem must have been removed during
the bisque. That tends to make me think that the organics/carbon may be the
real culprit. If not burned away during the bisque, the carbon could produce
a local reduction effect.
But that is just speculation on my part.
Comments?
Bruce Girrell
Ivor and Olive Lewis on mon 4 dec 06
Dear Liisa Reid,
One of the very first experiments we did when I started High School was =
to examine and then react pure Iron with Flowers of Sulphur. This =
investigation allowed us to learn about elements, mixtures and =
compounds. The elements Sulphur and Iron react when heated to generate =
Iron Sulphide. Various separating methods and the effects of treating =
all three substances with dilute acid helped us to learn the important =
basic concepts of Chemical Science.
Now to Clay and Iron Sulphide. This is present in many natural plastic =
earths. It is found as bright golden speckles of a mineral Iron Pyrites =
in some clays. When heated in air it decomposes to give Sulphur Dioxide =
and Iron Oxide. In the absence of air I presume it will not decompose =
immediately but it will melt just under 1200 deg C. I would not know if =
the molten compound will dissolve silicate minerals. If it decomposes to =
Sulphur then this is a strong reducing agent. But there would be no Iron =
oxide, only free Iron. Analysis would be needed to sort out the true =
nature of this chemistry.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Ivor and Olive Lewis on mon 4 dec 06
Dear Bruce Girrell,
I implied that, in the presence of carbonaceous materials which would =
decompose to give free carbon or Carbon monoxide, that above 700 deg =
Celsius the result of reducing any Iron oxide mineral in a clay would be =
free metallic Iron. This element, which melts above 1500 deg C could not =
make things melt. So it is removed from the picture.
I do not dispute the universal observation that clay rich in ferrous or =
ferric minerals will melt catastrophically at a relatively low =
temperature.
Your question about "Black Coring" would be settled by analysis. The =
answer might already exist in the records of the American Ceramic =
Society or the British Ceramic Research Association. The archives of =
neither institution is nearby so I cannot do the literature search.
Thanks for your contribution.
Best regards,
Ivor
John Hesselberth on mon 4 dec 06
On Dec 4, 2006, at 3:22 PM, Bruce Girrell wrote:
> Now comes the glaze firing. Body reduction starts at red heat (and
> in my
> firings, I pretty much run a reducing environment right from the
> start,
> especially if I am trying to get some carbon trap shinos). There is
> plenty
> of time for the gases to penetrate and interact with the iron in
> the body.
> Why is the iron no longer reduced, resulting in fluxing problems?
Hi Bruce,
But when does the glaze seal over? When it does it will dramatically
decrease the penetration of gases into the body. One could test that
aspect by placing an unglazed piece next to a glazed piece during
glaze firing.
Regards,
John
Bruce Girrell on tue 5 dec 06
John Hesselberth wrote:
> But when does the glaze seal over? When it does it will dramatically
> decrease the penetration of gases into the body.
Tichane and others have shown that after the glaze seals over little else
happens other than surface effects (which, of course can be dramatic, but
don't affect this discussion).
But my point was that if some reduction effect happens involving Fe2O3 in
the body of the ware during a bisque firing, then that same effect could
also occur during a glaze firing precisely because the glaze has _not_
sealed over. I then speculated that the primary culprit must be the carbon
that did not get oxidized during the bisque, not the presence of a
particular form of iron that was formed during the bisque, since this same
form of iron could be created during the glaze firing in a reducing
environment.
Bruce Girrell
Bruce Girrell on tue 5 dec 06
Ivor Lewis wrote:
> ...above 700 deg Celsius the result of reducing any Iron oxide
> mineral in a clay would be free metallic Iron. This element,
> which melts above 1500 deg C could not make things melt. So it is
> removed from the picture.
OK, So if I am understanding you properly, what you said is that the problem
is not due to FeO because FeO would reduce to Fe at temperatures within the
range of a normal bisque. Please correct me if I am mistaken.
But why do you conclude that the metallic iron has no effect in the melt
simply because its melting point is 1500 deg C? Tin melts at 232 degrees.
Lead melts at 327 degrees. 63/37 tin/lead solder melts at 185 degrees, lower
than the melting point of either constituent.
Fluxing and eutectic mixtures are very non-intuitive in their behavior. I
don't see that the melting point of iron eliminates it from performing a
pivotal role in the creation of a silica melt.
Bruce Girrell
Ivor and Olive Lewis on wed 6 dec 06
Dear Ron Roy,
I am not sure what you are explaining here
"When sulphur burns it takes oxygen from where ever it can - if there is =
not enough oxygen in the kiln because it has been used up burning other =
stuff - like carbon - then it takes the oxygen from Fe2O3 - turning it =
into a flux - and can lead to overfiring of your clay in a glaze =
firing."
Are you making an assumption that some clay minerals contain free =
elemental sulphur?
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Ivor and Olive Lewis on thu 7 dec 06
Dear Bruce Girrell,
<temperatures within the
range of a normal bisque. Please correct me if I am mistaken.>>
In reducing conditions that is what I mean ! In neutral or oxidising =
conditions Iron oxide (Fe2O3) remains unchanged if there is nothing for =
it to react with in the solid state.
<solder melts at 185 degrees>>
Good point! That Eutectic alloy is prepared by dissolving Lead in molten =
Tin. Perhaps what ceramists would have to do is to simulate that =
reaction is dissolve free Silica (SiO2) in molten Iron Oxide (Fe2O3) in =
an oxygen bearing atmosphere to get the same effects. This will give =
compounds that can contribute to a eutectic of about 1140 deg C. Quite =
counter intuitive .
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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