John Post on wed 2 may 12
> being a powerful flux, will
> make the clay body, ceteris paribus, more vitreous.
I'm thinking ceteris paribus is one of the planets Darth Vader wanted
to blow up in Star Wars, but I could be mistaken.
John Post
Sterling Heights, Michigan
http://www.johnpost.us
Follow me on Twitter
https://twitter.com/UCSArtTeacher
Lili Krakowski on wed 2 may 12
Being a grouchy old stick-in-the mud. Ok.
I know there are books out there on Electric Kilns and Their Firing. I =
=3D
own several. I love Emmanuel Cooper's book, especially that he, like so =
=3D
many other British potters was doing superb work in electric kilns when, =
=3D
in this country, electric firers were pariahs.
Zakin's book, as I gather from the book dealer websites, has been =3D
revised. So it may include, as RonJohn's "Mastering the Art..." does =3D
the business of cooling and holding cycles.
However. Please! Newbies et al. The basic composition of glazes, the =3D
chemical interactions, the effect of heat and so on are the same no =3D
matter how you fire. The difference between fuel burning fire, and =3D
electric fire is about the same as that between yeast and baking =3D
powder/soda baking. =3D20
"A woman's a fool for believing, a fool to break her own heart" the song =
=3D
says. But a newbie potter is a fool for believing that s/he can succeed =
=3D
without a basic knowledge of glaze composition, what the materials do, =3D
glaze faults, curing them...and like that.
So get a good basic book or two. I steadily suggest Harry Fraser's =3D
"Glazes for the Craft Potter" (ISBN 57498-076-9) That is the new =3D
edition. I own both editions. For a newcomer to glaze #2 is better. =3D
And get a glaze program. Will pay for itself in materials and saved =3D
test firing costs!
I am whining not just because Whining is a popular art form these days, =3D
but because there is this recurring hysteria about a particular form of =3D
glaze, or of firing that -somehow is Today's Special and needs a whole =3D
lot of hot-off-the press books...It is Woodburners! It is Raku! It is =3D
Naked Raku! It is Pit-firing! and on and on. No. It is clay, glaze , =3D
fire. THAT is what one needs to know.
The same afflicts cookery. French cuisine! Italian cuisine! Chinese! =3D
Thai! NO. It is food, seasonings, cooking. All the same idea. =3D
Understand what
happens when something is cooked in dry heat, in liquid, in grease...and =
=3D
that is that. But understand it!
And that is all I am going to say about that...at least right now.
Lili Krakowski
Be of good courage
Snail Scott on wed 2 may 12
On May 2, 2012, at 8:55 AM, Lili Krakowski wrote:
> ... The basic composition of glazes, the chemical interactions, the =3D
effect of heat and so on are the same no matter how you fire. The =3D
difference between fuel burning fire, and electric fire is about the =3D
same as that between yeast and baking powder/soda baking...
I'm with Lili on this. Why the assumption that electrically=3D20
fired ceramics differs in any significant way from work=3D20
fired by other methods? Heat + clay =3D3D ceramics. The=3D20
main difference is the availability of reduction atmosphere=3D20
in a fuel-fired kiln, which is problematic (though not=3D20
impossible) in an electric kiln. Beyond that, every kiln=3D20
of every type is different, and contributes its own issues=3D20
and attributes to the process. =3D20
Remember when microwave ovens were new? They all=3D20
came packaged with a little 'Microwave Oven Cookery'=3D20
cookbook of some sort. I doubt that any of them still do,=3D20
because we are long since over the notion that it's that=3D20
different. 'No browning/crisping, etc? OK, got it, now I'm=3D20
moving on to just figure out good food using this tool.'
Most of the attributes that people use to differentiate=3D20
electric kilns from others have nothing to do with the=3D20
fuel, but with the physical configuration of the kiln:=3D20
lightly insulated, not good at very high temps or at=3D20
slow cooling. These are all true of _some_ fuel kilns,=3D20
too. There's more variety among kilns in general than=3D20
between electric and gas as categories.
Saying "How do I fire electrically?' makes as much sense=3D20
(and as little) as " How do I use blue glazes?" There are a=3D20
few specific issues (watch that cobalt with magnesium,=3D20
for example), but it mostly comes down to learning what=3D20
the tool does well, and how to work with it, in a learning=3D20
curve not dissimilar to learning a new glaze's habits, or a=3D20
new clay body's properties, or the ways of any new kiln.=3D20
The power source of the kiln is only one variable among=3D20
many, and one of the less important.
-Snail=3D
Drake Ash on wed 2 may 12
i appreciate this advice, guys! it can be overwhelming for those of us who
are newish to contemplate all these different techniques. i will look into
that book, lili. thanks--drake
On Wed, May 2, 2012 at 2:51 PM, Snail Scott wrot=
e:
> On May 2, 2012, at 8:55 AM, Lili Krakowski wrote:
> > ... The basic composition of glazes, the chemical interactions, the
> effect of heat and so on are the same no matter how you fire. The
> difference between fuel burning fire, and electric fire is about the same
> as that between yeast and baking powder/soda baking...
>
>
> I'm with Lili on this. Why the assumption that electrically
> fired ceramics differs in any significant way from work
> fired by other methods? Heat + clay =3D ceramics. The
> main difference is the availability of reduction atmosphere
> in a fuel-fired kiln, which is problematic (though not
> impossible) in an electric kiln. Beyond that, every kiln
> of every type is different, and contributes its own issues
> and attributes to the process.
>
> Remember when microwave ovens were new? They all
> came packaged with a little 'Microwave Oven Cookery'
> cookbook of some sort. I doubt that any of them still do,
> because we are long since over the notion that it's that
> different. 'No browning/crisping, etc? OK, got it, now I'm
> moving on to just figure out good food using this tool.'
>
> Most of the attributes that people use to differentiate
> electric kilns from others have nothing to do with the
> fuel, but with the physical configuration of the kiln:
> lightly insulated, not good at very high temps or at
> slow cooling. These are all true of _some_ fuel kilns,
> too. There's more variety among kilns in general than
> between electric and gas as categories.
>
> Saying "How do I fire electrically?' makes as much sense
> (and as little) as " How do I use blue glazes?" There are a
> few specific issues (watch that cobalt with magnesium,
> for example), but it mostly comes down to learning what
> the tool does well, and how to work with it, in a learning
> curve not dissimilar to learning a new glaze's habits, or a
> new clay body's properties, or the ways of any new kiln.
> The power source of the kiln is only one variable among
> many, and one of the less important.
>
> -Snail
James Freeman on wed 2 may 12
On Wed, May 2, 2012 at 2:51 PM, Snail Scott wrot=
e:
Why the assumption that electrically
fired ceramics differs in any significant way from work
fired by other methods? Heat + clay =3D ceramics.
Perhaps I am off base, but it seems to me that there is a rather
significant difference between ceramic materials fired in electric versus
fuel kilns, and that difference pertains to iron oxide, whether in the clay
body or in the glaze.
Red iron oxide, Fe2O3, (aka iron (III) oxide, aka ferric oxide) is an
amphoteric. That is, it, like alumina, serves to "stiffen" the glaze,
rendering it less fluid, and aiding in its "sticking" to the pot.
In a reduction atmosphere, or at temperatures something above cone 10 in
any atmosphere, red iron oxide is reduced to black iron oxide, FeO, (aka
iron (II) oxide, aka ferrous oxide). Black iron oxide is a powerful flux.
That is, it serves to hasten and increase the melt and thus the fluidity of
the glaze.
In a cone 6 electric firing, red iron oxide in a glaze will remain red iron
oxide, and will cause the glaze to be stiffer. In a fuel kiln, the
reduction, even if minor, will convert the red iron oxide to the black
form, and will cause the glaze to be much runnier than in an electric
kiln. The color of the glaze is also affected when the iron oxide changes
from red to black, which is why fuel-fired glazes tend to be more muted
(like mixing black into your paint), and why temmoku will be black in a
fuel kiln, but muddy brown in an electric kiln, and why celadons are
impossible (in any practical way) in an electric kiln. This is also why
iron saturate reds need to be fired in oxidation (to remain red), or, if
fired in a fuel kiln, why they need an "oxidation clean up" at the end of
the firing, which allows the iron at the very surface of the glaze to
reoxidize to a red state.
Likewise if you use an iron-bearing clay. In an electric firing, the iron
remains red, and remains an amphoteric. In reduction, the red iron in the
clay body is reduced to the black form, which, being a powerful flux, will
make the clay body, ceteris paribus, more vitreous. And again, color is
affected, which is why iron bearing clays tend to fire beige in an electric
kiln, but grey in a fuel kiln.
For what it's worth.
...James
James Freeman
"Talk sense to a fool, and he calls you foolish."
-Euripides
http://www.jamesfreemanstudio.com
http://www.flickr.com/photos/jamesfreemanstudio/
http://www.jamesfreemanstudio.com/resources
Pottery by John on thu 3 may 12
James, or knowledgeable others,
The discussion of the two different states of an oxide of iron made me
wonder, if I use the black iron oxide (FeO) in a glaze formulation, and fir=
e
in an electric oxidizing atmosphere, does the FeO stay a powerful flux, or
does it change to red iron oxide as the temperature increases and take me
back to iron as an amphoteric?
I have noted that John Post, in his experimentation with glazes shared on
his website, uses the black iron oxide and gets good movement in the glazes=
.
Is the black iron oxide FeO the reason?
John Lowes
Sandy Springs, Georgia
http://wynhillpottery.weebly.com/
> Red iron oxide, Fe2O3, (aka iron (III) oxide, aka ferric oxide) is an
> amphoteric. That is, it, like alumina, serves to "stiffen" the glaze,
> rendering it less fluid, and aiding in its "sticking" to the pot.
>
> In a reduction atmosphere, or at temperatures something above cone 10 in
> any atmosphere, red iron oxide is reduced to black iron oxide, FeO, (aka
> iron (II) oxide, aka ferrous oxide). Black iron oxide is a powerful flux=
.
> That is, it serves to hasten and increase the melt and thus the fluidity
> of
> the glaze.
James Freeman on thu 3 may 12
That's an excellent question, John. I'm not a chemist, so can only guess.
I know there are several real-life chemists on the list, so perhaps they
can provide a definitive answer. One must consider, however, that when we
fire our glazes, we are very far from controlled, laboratory conditions, so
the theoretically correct "textbook" answer may be quite different from
what we will actually see.
Iron oxide can take on a whole bunch of forms, but only three of them are
relatively common, red (Fe2O3), black (FeO), and magnetic (Fe3O4, which can
be thought of as FeO + Fe2O3). Of the three, the highest form, red, is by
far the most common. Think rust, which we see everywhere. The lowest
form, FeO, or black iron oxide, is the rarest, and from what I understand
the least stable, always wanting to further oxidize to one of the higher
forms.
My uneducated guess is that when fired in what we call oxidation (open air,
neutral atmosphere), the FeO will convert to metallic iron (which will then
also oxidize) plus Fe3O4 (magnetite), and that the magnetite will then
convert to Fe2O3, or common red iron oxide.
We could easily test this by placing a thin dusting of black iron oxide,
magnetic iron oxide, and red iron oxide on a tile, then firing them to
various temperatures in an electric kiln. We could also mix up a test
glaze with red iron and black iron in the same molar amounts (not the same
weight), and observe the results. If the glaze with the black iron comes
out the same red, yellow, or brown color as the sample containing red iron,
then the hypothesis is confirmed (as I am guessing it will be).
I am not doing much in the studio at the moment (no exhibitions scheduled
until September), so am not running many kilns. I might do a bisque this
weekend, and if so, will fire samples of various forms of iron. I may do a
small cone 6 firing next week, and if so, will test both black and red iron
in a glaze or two. If anyone else wants to run the tests, that would be
great! I'd be very curious to know the results. It will be interesting
also to compare what happens to the plain iron samples versus those same
chemicals when part of a glaze.
All the best.
...James
James Freeman
"Talk sense to a fool, and he calls you foolish."
-Euripides
http://www.jamesfreemanstudio.com
http://www.flickr.com/photos/jamesfreemanstudio/
http://www.jamesfreemanstudio.com/resources
On Thu, May 3, 2012 at 8:58 AM, Pottery by John t
> wrote:
> James, or knowledgeable others,
>
> The discussion of the two different states of an oxide of iron made me
> wonder, if I use the black iron oxide (FeO) in a glaze formulation, and
> fire
> in an electric oxidizing atmosphere, does the FeO stay a powerful flux, o=
r
> does it change to red iron oxide as the temperature increases and take me
> back to iron as an amphoteric?
>
> I have noted that John Post, in his experimentation with glazes shared on
> his website, uses the black iron oxide and gets good movement in the
> glazes.
> Is the black iron oxide FeO the reason?
>
> John Lowes
> Sandy Springs, Georgia
> http://wynhillpottery.weebly.**com/
>
>
> Red iron oxide, Fe2O3, (aka iron (III) oxide, aka ferric oxide) is an
>> amphoteric. That is, it, like alumina, serves to "stiffen" the glaze,
>> rendering it less fluid, and aiding in its "sticking" to the pot.
>>
>> In a reduction atmosphere, or at temperatures something above cone 10 in
>> any atmosphere, red iron oxide is reduced to black iron oxide, FeO, (aka
>> iron (II) oxide, aka ferrous oxide). Black iron oxide is a powerful flu=
x.
>> That is, it serves to hasten and increase the melt and thus the fluidity
>> of
>> the glaze.
>>
>
Snail Scott on thu 3 may 12
On May 2, 2012, at 5:58 PM, James Freeman wrote:
> On Wed, May 2, 2012 at 2:51 PM, Snail Scott =3D
wrote:
>> Why the assumption that electrically
>> fired ceramics differs in any significant way from work
>> fired by other methods? Heat + clay =3D3D ceramics.
>=3D20
>=3D20
> Perhaps I am off base, but it seems to me that there is a rather
> significant difference between ceramic materials fired in electric =3D
versus
> fuel kilns, and that difference pertains to iron oxide, whether in the =
=3D
clay
> body or in the glaze...
Hey, remember, I DID make a specific exception=3D20
in mentioning reduction issues, right in the first=3D20
paragraph just after the sentence you quote here.=3D20
I just didn't belabor the issue, as it's been covered in=3D20
detail by others.=3D20
In fact, the whole reason I currently work at ^6 is=3D20
because my favorite homemade clay body, a high-
iron body, worked just right for me at my old temp,=3D20
^4, in reduction. I had to up the temp and lower=3D20
the iron to make it work in oxidation, which is how=3D20
I wound up at ^6 about 20 years ago.
My broad intent was to point up that reduction is=3D20
a minor factor in the broad scope of ceramics=3D20
practice. It's far from irrelevant, but it seems to=3D20
get obsessive attention at times. Design, function,=3D20
clay-handling techniques, surface choices, scale,=3D20
temp and length of firings, ergonomics, meaning,=3D20
context, etc. - these don't change with the firing=3D20
atmosphere. (And let's don't forget that fuel-burning=3D20
kilns can fire in oxidation/neutral atmosphere,=3D20
too.) The difference between an electric kiln and,=3D20
say, a gas-fired kiln, isn't even as great as the=3D20
difference (as in my previous analogy) between=3D20
an oven and a microwave. No one should feel=3D20
as though they are traveling to Mars without a=3D20
map just because they are switching from gas to=3D20
electric or vice-versa. Some stuff differs, sure, but=3D20
most stuff - the most important stuff - remains the=3D20
same.
-Snail =3D20
Robert Harris on thu 3 may 12
This is all off the top of my head, so take it for what you paid for
it (nothing).
As James said Ferrous, or Iron (II) Oxide, FeO is the rarest oxide of
iron and rarely occurs in nature. Its natural inclination is to
disproportionate in Fe (metal) and Fe2O3. I would be hesitant to call
it black iron oxide, as the stuff we buy as "black iron oxide" is
Fe3O4, which occurs naturally as the mineral magnetite (as James
said).
Fe3O4 technically has both Fe2+ ions, and Fe3+ ions in its structure
and is therefore sometimes written more accurately as FeO.Fe3O4 (in
this compound the FeO is stabilised, so no disproportionation occurs.
Again semantics would say that because the stuff we buy from our
suppliers is generally synthesized artificially, it is technically not
precisely magnetite as this is generally restricted to the naturally
occurring mineral. None of this really matters to the potter - except
that fuzzy communication sometimes causes problems.
Unfortunately I cannot wholly answer your question as I don't know
what the thermodynamics of these compounds are. But the basic answer
to you question is "It all depends on temperature".
1. At what temperature does FeO enter the melt, and therefore become a
flux. In the case of Fe2O3, this must be after it gives up the oxygen.
2. At what temperature can the FeO contained within Fe3O4 enter the
melt. Is this earlier than 1?
3. At what temperature does the fluxing activity of FeO actually begin.
Without a doubt Red Iron Oxide, gives up oxygen to form FeO. Black
Iron oxide (Fe3O4), might as well - but I don't know how stable its
structure is, or at what temperature it would occur. I would note that
it may be possible that Fe3O4 is not the powerful flux that FeO is.
Because the FeO that exists in black iron oxide is locked up in a
structure, it may not enter the melt early enough for us to see it's
fluxing capabilites at lower cones. Again I just have no idea.
Upon cooling FeO will definitely for Fe2O3, but this obviously
requires oxygen, and a minimum temperature. For example it may be that
Black Seto (where the pot is drawn out and cooled quickly), freezes
FeO, while slow cooling allows some FeO to transition back to Fe2O3.
Also within a glaze matrix, if the FeO is trapped by early sealing of
a glaze (during cooling) it will remain dissolved in the silica melt
as FeO.
Anyway - I think that's enough rambling, but I hope you can see the
direction of the questions that need to be answered in order to give a
clue about your practical question. I don't know if we actually have
any ideas about the actual answers to the chemistry questions. It may
be a case of ... do it and see.
Robert
On Thu, May 3, 2012 at 9:50 AM, James Freeman
wrote:
> That's an excellent question, John. =3DA0I'm not a chemist, so can only g=
ue=3D
ss.
> I know there are several real-life chemists on the list, so perhaps they
> can provide a definitive answer. =3DA0One must consider, however, that wh=
en=3D
we
> fire our glazes, we are very far from controlled, laboratory conditions, =
=3D
so
> the theoretically correct "textbook" answer may be quite different from
> what we will actually see.
>
> Iron oxide can take on a whole bunch of forms, but only three of them are
> relatively common, red (Fe2O3), black (FeO), and magnetic (Fe3O4, which c=
=3D
an
> be thought of as FeO + Fe2O3). =3DA0Of the three, the highest form, red, =
is=3D
by
> far the most common. =3DA0Think rust, which we see everywhere. =3DA0The l=
owes=3D
t
> form, FeO, or black iron oxide, is the rarest, and from what I understand
> the least stable, always wanting to further oxidize to one of the higher
> forms.
>
> My uneducated guess is that when fired in what we call oxidation (open ai=
=3D
r,
> neutral atmosphere), the FeO will convert to metallic iron (which will th=
=3D
en
> also oxidize) plus Fe3O4 (magnetite), and that the magnetite will then
> convert to Fe2O3, or common red iron oxide.
>
> We could easily test this by placing a thin dusting of black iron oxide,
> magnetic iron oxide, and red iron oxide on a tile, then firing them to
> various temperatures in an electric kiln. =3DA0We could also mix up a tes=
t
> glaze with red iron and black iron in the same molar amounts (not the sam=
=3D
e
> weight), and observe the results. =3DA0If the glaze with the black iron c=
om=3D
es
> out the same red, yellow, or brown color as the sample containing red iro=
=3D
n,
> then the hypothesis is confirmed (as I am guessing it will be).
>
> I am not doing much in the studio at the moment (no exhibitions scheduled
> until September), so am not running many kilns. =3DA0I might do a bisque =
th=3D
is
> weekend, and if so, will fire samples of various forms of iron. =3DA0I ma=
y =3D
do a
> small cone 6 firing next week, and if so, will test both black and red ir=
=3D
on
> in a glaze or two. =3DA0If anyone else wants to run the tests, that would=
b=3D
e
> great! =3DA0I'd be very curious to know the results. =3DA0It will be inte=
rest=3D
ing
> also to compare what happens to the plain iron samples versus those same
> chemicals when part of a glaze.
>
> All the best.
>
> ...James
>
> James Freeman
>
> "Talk sense to a fool, and he calls you foolish."
> -Euripides
>
> http://www.jamesfreemanstudio.com
> http://www.flickr.com/photos/jamesfreemanstudio/
> http://www.jamesfreemanstudio.com/resources
>
>
>
> On Thu, May 3, 2012 at 8:58 AM, Pottery by John =3D
net
>> wrote:
>
>> James, or knowledgeable others,
>>
>> The discussion of the two different states of an oxide of iron made me
>> wonder, if I use the black iron oxide (FeO) in a glaze formulation, and
>> fire
>> in an electric oxidizing atmosphere, does the FeO stay a powerful flux, =
=3D
or
>> does it change to red iron oxide as the temperature increases and take m=
=3D
e
>> back to iron as an amphoteric?
>>
>> I have noted that John Post, in his experimentation with glazes shared o=
=3D
n
>> his website, uses the black iron oxide and gets good movement in the
>> glazes.
>> Is the black iron oxide FeO the reason?
>>
>> John Lowes
>> Sandy Springs, Georgia
>> http://wynhillpottery.weebly.**com/
>>
>>
>> =3DA0Red iron oxide, Fe2O3, (aka iron (III) oxide, aka ferric oxide) is =
an
>>> amphoteric. =3DA0That is, it, like alumina, serves to "stiffen" the gla=
ze=3D
,
>>> rendering it less fluid, and aiding in its "sticking" to the pot.
>>>
>>> In a reduction atmosphere, or at temperatures something above cone 10 i=
=3D
n
>>> any atmosphere, red iron oxide is reduced to black iron oxide, FeO, (ak=
=3D
a
>>> iron (II) oxide, aka ferrous oxide). =3DA0Black iron oxide is a powerfu=
l =3D
flux.
>>> That is, it serves to hasten and increase the melt and thus the fluidit=
=3D
y
>>> of
>>> the glaze.
>>>
>>
--=3D20
----------------------------------------------------------
John Hesselberth on thu 3 may 12
Hi Robert/James,
On May 3, 2012, at 12:03 PM, Robert Harris wrote:
> Unfortunately I cannot wholly answer your question as I don't know
> what the thermodynamics of these compounds are. But the basic answer
> to you question is "It all depends on temperature".
I have quoted a short segment of Robert's answer so I can go on from =3D
there. I would change it to " it all depends on temperature and the =3D
environment in which the iron finds itself." You have both accurately =3D
described what probably happens when the iron in surrounded by gas =3D
(whether air or a 'reduction' gas) and hinted at what might happen in =3D
the melt.=3D20
But what does happen when the iron is incorporated into the melt? There, =
=3D
if it is in a state where it would like to gobble up some more oxygen it =
=3D
has to steal it from somewhere. You think silica is just going to offer =3D
it one of its oxygen atoms on a silver platter? Or calcium? Or whatever? =
=3D
Not likely without a fight. And, of course, iron voluntarily gives up =3D
oxygen when when it reaches a certain temperature--at least it does that =
=3D
in air. But in a silica melt at the same temperature--who knows? Not I. =
=3D
Other elements might be giving off CO2 or desperately hunting for an =3D
oxygen themselves--or who knows what else. I guess my main point is =3D
that a silica melt is an entirely different beast--a very complex and =3D
fascinating one where all sort of chemistry "fights" and "love affairs" =3D
are probably going on. And I haven't even mentioned the idea that some =3D
of the materials in the melt might act as catalysts to speed some =3D
reactions and slow others.
My working hypothesis is that the reduction atmosphere available in a =3D
fuel fired kiln has the biggest effect before the glaze seals over, i.e =3D
when it is still a fluffy powder with lots of gas surrounding each =3D
particle. Once it seals over I think the only difference a reduction =3D
atmosphere makes is on the very surface of the glaze--maybe a handful of =
=3D
molecules deep into the surface. But the bulk of the glaze doesn't even =
=3D
know there is a gas there. It is only concerned with the temperature and =
=3D
those molecules in the immediate vicinity that want to give or take some =
=3D
of what it has. And that depends not only on the thermodynamics, but =3D
also the chemical kinetics and the heat, mass, and momentum transfer =3D
that is going on--- all of which would be extremely complex to try to =3D
describe.
With iron having so many possible physical and chemical states it is one =
=3D
of our most fascinating ingredients. As Robert said, "do it and see".
Regards,
John
John Hesselberth
http://www.masteringglazes.com
http://www.frogpondpottery.com
Robert Harris on thu 3 may 12
Couldn't agree more with John's comments. Just as an aside, since I've
been reading rather a lot on Copper reds, there are definitely those
that are red all the way through, and those that are red just on the
surface few microns. Mostly due to reduction during cooling, or
striking, but possibly also due to late reduction. A read of the
clayart archives shows that good consistent copper reds require early
reduction. A good proof of John's comments about molten glazes
sealing away oxides from the atmosphere.
Robert
On Thu, May 3, 2012 at 2:39 PM, John Hesselberth
wrote:
> Hi Robert/James,
>
> On May 3, 2012, at 12:03 PM, Robert Harris wrote:
>
> Unfortunately I cannot wholly answer your question as I don't know
> what the thermodynamics of these compounds are. But the basic answer
> to you question is "It all depends on temperature".
>
>
> I have quoted a short segment of Robert's answer so I can go on from ther=
=3D
e.
> I would change it to " it all depends on temperature and the environment =
=3D
in
> which the iron finds itself." =3DA0You have both accurately described wha=
t
> probably happens when the iron in surrounded by gas (whether air or a
> 'reduction' gas) and hinted at what might happen in the melt.
>
> But what does happen when the iron is incorporated into the melt? There, =
=3D
if
> it is in a state where it would like to gobble up some more oxygen it has=
=3D
to
> steal it from somewhere. You think silica is just going to offer it one o=
=3D
f
> its oxygen atoms on a silver platter? Or calcium? Or whatever? Not likely
> without a fight. And, of course, iron voluntarily gives up oxygen when wh=
=3D
en
> it reaches a certain temperature--at least it does that in air. =3DA0But =
in=3D
a
> silica melt at the same temperature--who knows? Not I.=3DA0Other elements=
m=3D
ight
> be giving off CO2 or desperately hunting for an oxygen themselves--or who
> knows what else. =3DA0I guess my main point is that a silica melt is an e=
nt=3D
irely
> different beast--a very complex and fascinating one where all sort of
> chemistry "fights" and "love affairs" are probably going on. And I haven'=
=3D
t
> even mentioned the idea that some of the materials in the melt might act =
=3D
as
> catalysts to speed some reactions and slow others.
>
> My working hypothesis is that the reduction atmosphere available in a fue=
=3D
l
> fired kiln has the biggest effect before the glaze seals over, i.e when i=
=3D
t
> is still a fluffy powder with lots of gas surrounding each particle. Once=
=3D
it
> seals over I think the only difference a reduction atmosphere makes is on
> the very surface of the glaze--maybe a handful of molecules deep into the
> surface. =3DA0But the bulk of the glaze doesn't even know there is a gas =
th=3D
ere.
> It is only concerned with the temperature and those molecules in the
> immediate vicinity that want to give or take some of what it has. And tha=
=3D
t
> depends not only on the thermodynamics, but also the chemical kinetics an=
=3D
d
> the heat, mass, and momentum transfer that is going on--- all of which wo=
=3D
uld
> be extremely complex to try to describe.
>
> With iron having so many possible physical and chemical states it is one =
=3D
of
> our most fascinating ingredients. As Robert said, "do it and see".
>
> Regards,
>
> John
>
> John Hesselberth
> http://www.masteringglazes.com
> http://www.frogpondpottery.com
>
>
>
--=3D20
----------------------------------------------------------
ivor and olive lewis on fri 4 may 12
Would some one please provide evidence to explain the way in which black
iron oxide becomes a strong flux.?
Reference to Kingery et al, "Introduction to Ceramics" p 394 may help. This
diagram gives an equation of 6FeO + O2 =3D> 2Fe3O4. To get this Oil Spot
effect this must be reversed, which gives a black residue and free oxygen..=
\
I would like to think that there is a reaction between Silicon dioxide with
whatever Iron oxide is used in the batch recipe forming Iron Silicate
(Fe2SiO4) The Phase diagram SiO2-FeO-Fe2O3 shows that this iron silicate
has a melting point of 1205 deg C. How convenient if it is there!
Regards,
Ivor Lewis,
REDHILL,
South Australia
James Freeman on fri 4 may 12
On Fri, May 4, 2012 at 2:53 AM, ivor and olive lewis
wrote:
Would some one please provide evidence to explain the way in which black
iron oxide becomes a strong flux.?
Ivor...
I certainly can't explain the chemistry, and as John H. stated, the actual
chemistry going on in a glaze melt is so utterly complex that very few of
us would ever be able to explain it completely, and any simple textbook
explanation is likely to be incorrect. It is a pretty easy phenomenon to
demonstrate though:
I used to fire clay with steel inclusions, tacks, nails, screws, and
whatnot, in an electric kiln. No problems ever ensued, except that the
surface layers of steel would be converted to what I presumed to be mostly
magnetite (based on the fact that it was magnetic, and based on the
documented chemistry of the composition of mill scale which forms
spontaneously on hot-rolled steel).
A student at the local college tried to duplicate my trick. She stuck some
steel wires, cactus spine-like, into one of her large pots. It came
through the electric bisque just fine, with the expected coat of magnetite
on the steel wires. She then glazed the piece and fired it in the gas
kiln. When it emerged from the kiln, every steel spine was gone, leaving
huge, dark streaks below where they had been, and the pot was fused to the
shelf by a massive pool of a previously tried-and-true glaze which had slid
off the pot, across the shelf, and down onto everything below.
I just consulted Hamer & Hamer, which contains a rather complete section on
iron oxide. About black iron oxide, they state on page 189, "Black iron
oxide is always a flux... ...Black iron oxide becomes a glaze flux at
temperatures over 900C (1652 F). It's inclusion in a glaze recipe should
ideally be as a substitute for part of another flux." Further, on page
192: "Black iron oxide... .. The simplest compound of iron and oxygen, it
is stable at room temperatures but becomes excited at red heat. This
excitement is a loosening of the molecular bonds thereby allowing the black
iron oxide to combine with silicates. It will therefore act as an alkaline
flux in bodies and glazes."
All the best.
...James
James Freeman
"Talk sense to a fool, and he calls you foolish."
-Euripides
http://www.jamesfreemanstudio.com
http://www.flickr.com/photos/jamesfreemanstudio/
http://www.jamesfreemanstudio.com/resources
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