Randy McCall on mon 24 may 10
Can someone tell me how pure red iron oxide affects glazes when compared to=
=3D
the regular red iron oxide? Do you get different effects. Is it like a c=
=3D
omparison of cobalt carbonate and cobalt oxide?
Randy
=3D
Robert Harris on mon 24 may 10
As with much glazing "it all depends". Even the purest RIO isn't as pure as
you might think.
We tracked down the suppliers of a number of different RIOs and they gave
analyses of between 50 and 85% (none were above 85%). In addition the pure
iron oxides were not particularly consistent. This is why everyone goes on
about testing batches. In our glazes, as long as the total amount of iron i=
s
constant, the impurities do not seem to affect the glaze. YMMV depending on
the glaze base you are using.
Personally I have transitioned to using Black Iron Oxide for everything as
this is significantly purer than any of the RIOs. Obviously the formulae ar=
e
different : FeO (black) vs a combination of FeO and Fe2O3 - which is usuall=
y
written as Fe3O4 (RIO) so you need to figure out new amounts of black iron
oxide to use.
The problem with black Iron oxide is that it has a larger granule size than
RIO. Which can lead to speckling. However Iron can actually dissolve in the
melt so if you have a good long soak with a "melty glaze" this can
ameliorate the speckling.
Here is a Tenmoku using black iron oxide fired at cone 10 in an electric
kiln.
http://prometheanpottery.files.wordpress.com/2010/01/03_100ppi.jpg
hope this helps
Robert
http://prometheanpottery.wordpress.com/
http://prometheanpottery.etsy.com/
Snail Scott on mon 24 may 10
On May 24, 2010, at 6:59 AM, Randy McCall wrote:
> Can someone tell me how pure red iron oxide affects glazes when
> compared to the regular red iron oxide? Do you get different
> effects. Is it like a comparison of cobalt carbonate and cobalt
> oxide?
It's more a matter of degree. Cobalt carb has only half
as much cobalt per unit of weight at cobalt ox, and is
much finer in texture, so these are major and minor-
but-significant factors.
It's also unclear on what 'regular' RIO is, for you.
Relative purity of iron oxide is not a matter of type, and
the difference in the amount of iron per weight unit is
fairly trivial. Might need a bit of tweaking, and there may
be a visible difference due to the nature of the impurities,
but nothing on the order of the cobalt-carb and cobalt-ox
distinction. More like the difference between mang-carb
and mang-ox, with the 'impurities' wild card tossed in.
-Snail
John Britt on mon 24 may 10
Try synthetic red iron oxide! You will know the difference because it co=
=3D
sts
between $3.00 and $5.00 a pound. It is at least 95% pure (according to th=
=3D
e
MSDS).
I would try www.USPIGMENT.COM
or two synthetic red irons, 4686 and 4284, that are sold by Laguna
and Georgies Clay. I am sure other companies sell them too. (You will ha=
=3D
ve
to search in your area.)=3D20
They have a very small particle size (I think around 325 mesh).
Try them and you will be amazed!
It matters.
Johnbrittpottery.com
Robert Harris on mon 24 may 10
I looked at the MSDS for "synthetic RIO" (I think it was 4284) that I was
sent from the distributor I use (this was a while back), and it stated that
the purity of this was anywhere between 80-99%. Perhaps things have changed=
.
On the other hand it was such a pain getting the MSDS out of them (and it
looked pretty old) that I'm not sure how trusting I should be.
Anyway for what it's worth I batch tested a whole bunch of RIOs and decided
it really wasn't worth the uncertainty.
Rob
On Mon, May 24, 2010 at 1:24 PM, John Britt wro=
te:
> Try synthetic red iron oxide! You will know the difference because it
> costs
> between $3.00 and $5.00 a pound. It is at least 95% pure (according to th=
e
> MSDS).
>
> I would try www.USPIGMENT.COM
>
> or two synthetic red irons, 4686 and 4284, that are sold by Laguna
> and Georgies Clay. I am sure other companies sell them too. (You will ha=
ve
> to search in your area.)
>
> They have a very small particle size (I think around 325 mesh).
>
> Try them and you will be amazed!
>
> It matters.
>
> Johnbrittpottery.com
>
>
Robert Harris on mon 24 may 10
Just did some online checking. The current MSDS for 4284 states it is
between 40-60% synthetic Iron Oxide and 30-50% natural iron oxide. So there
is still a bit of uncertainty there.
Also I saw a number of other MSDS' online for "Synthetic Iron Oxide" which
quote Fe3O4 as the formula. From a synthetic point of view there is no such
chemical. In this case iron can only take the Valence states II and III.
Fe3O4 is therefore a mixture of FeO and Fe2O3 - and again any such mixture
is going to have varying amounts of iron in per unit mass.
Essentially if you want to use RIO - batch test, batch test, batch test!
Robert
On Mon, May 24, 2010 at 2:23 PM, Robert Harris wro=
te:
> I looked at the MSDS for "synthetic RIO" (I think it was 4284) that I was
> sent from the distributor I use (this was a while back), and it stated th=
at
> the purity of this was anywhere between 80-99%. Perhaps things have chang=
ed.
> On the other hand it was such a pain getting the MSDS out of them (and it
> looked pretty old) that I'm not sure how trusting I should be.
>
> Anyway for what it's worth I batch tested a whole bunch of RIOs and decid=
ed
> it really wasn't worth the uncertainty.
>
>
> Rob
>
>
>
>
> On Mon, May 24, 2010 at 1:24 PM, John Britt w=
rote:
>
>> Try synthetic red iron oxide! You will know the difference because it
>> costs
>> between $3.00 and $5.00 a pound. It is at least 95% pure (according to t=
he
>> MSDS).
>>
>> I would try www.USPIGMENT.COM
>>
>> or two synthetic red irons, 4686 and 4284, that are sold by Laguna
>> and Georgies Clay. I am sure other companies sell them too. (You will
>> have
>> to search in your area.)
>>
>> They have a very small particle size (I think around 325 mesh).
>>
>> Try them and you will be amazed!
>>
>> It matters.
>>
>> Johnbrittpottery.com
>>
>>
>
Robert Harris on mon 24 may 10
Oh yeah...and of course Black Iron oxide is by definition synthetic!
And I use black iron oxide in my (fake) Persimmon and Ohata glazes.
See
http://prometheanpottery.files.wordpress.com/2009/12/large-persimmon-canist=
er.jpg
and
http://www.etsy.com/view_transaction.php?transaction_id=3D23422560
Cone 10 electric.
I'm not advocating BIO for everything. But I find it does alleviate the nee=
d
to test the hell out of every batch of RIO I use!
What can I say, I'm a little lazy.
Robert
On Mon, May 24, 2010 at 1:24 PM, John Britt wro=
te:
> Try synthetic red iron oxide! You will know the difference because it
> costs
> between $3.00 and $5.00 a pound. It is at least 95% pure (according to th=
e
> MSDS).
>
> I would try www.USPIGMENT.COM
>
> or two synthetic red irons, 4686 and 4284, that are sold by Laguna
> and Georgies Clay. I am sure other companies sell them too. (You will ha=
ve
> to search in your area.)
>
> They have a very small particle size (I think around 325 mesh).
>
> Try them and you will be amazed!
>
> It matters.
>
> Johnbrittpottery.com
>
>
L TURNER on mon 24 may 10
The best discussion of the differences among the iron oxides normally
encountered by the studio potter is the one by David Hewitt (1925 -
2006). It was first published in Ceramic Review No 186 in 2000 and
is available from an archived copy of his website:
http://web.archive.org/web/20050507172942/www.dhpot.demon.co.uk/Raw+Materia=
ls.htm
,
or more easily in pdf format from:
http://www.goldenarts.com.hk/download/Raw%20Materials%20-%20Do%20you%20know=
%20what%20you%20are%20buying.pdf
A further point that Hewitt doesn't address head on. "Black iron"
doesn't really exist as a pure material at ambient conditions since it
is unstable below about 575 deg C and is a non-stoichiometric
compound. What is sold as "black iron oxide" is actually best
represented as Fe3O4.
The DigitaFire website: www.digitalfire.com also has good information
on the various iron oxides. two of the links are:
Iron Oxide Black
http://digitalfire.com/4sight/material/iron_oxide_black_873.html
Iron Oxide Red http://digitalfire.com/4sight/material/iron_oxide_red_874.h=
tml
use the search feature to find links of yellow, spanish red, and other
iron oxide pigments.
>MSDS for "synthetic RIO" (I think it was 4284) that I was
>sent from the distributor I use (this was a while back)... stated that
>the purity of this was anywhere between 80-99%.
The MSDS is a source of good safety information, but is not the best
source of technical or material quality data. Get the technical
datasheet for this information. If you are buying iron oxide through a
distributor, and in less than full sack quantities, finding the name
of the supplier is often the most difficult step in the task of
getting the technical data. The MSDS can be helpful here as it must
list the name and contact information of the main supplier.
Regards,
Lou Turner,
The Woodlands, TX
Veena Raghavan on mon 24 may 10
Very very nice, Robert. Looks great.
Thanks for sharing.
Veena
In a message dated 5/24/2010 12:50:01 PM Eastern Daylight Time,
robertgharris@GMAIL.COM writes:
>
> The problem with black Iron oxide is that it has a larger granule size
> than
> RIO. Which can lead to speckling. However Iron can actually dissolve in
> the
> melt so if you have a good long soak with a "melty glaze" this can
> ameliorate the speckling.
>
> Here is a Tenmoku using black iron oxide fired at cone 10 in an electric
> kiln.
>
> http://prometheanpottery.files.wordpress.com/2010/01/03_100ppi.jpg
>
> hope this helps
>
> Robert
VeenaRaghavan@cs.com
Robert Harris on mon 24 may 10
Hmm....apparently all my assumptions are out of whack. I do know that I see=
m
to have less variation when using Black Iron Oxide in my glazes but from th=
e
sound of it that is more likely because the supplier is more reliable than
anything else.... On the other hand it is possible that my supplier makes i=
t
from very pure magnetite deposits which would be precisely Fe3)4 and
theoretically not as prone to decomposition/oxidation.
So to sum up...Synthetic Red Iron oxide is theoretically the most reliable
source of Iron...but watch your supplier and test test test test!
Rob
P.S. Have you actually tried to get the datasheet from a supplier or
distributor? I have (and asked for the actual analysis of the batch I was
using) and despite several emails, follow up phone calls and promises to fa=
x
stuff I NEVER managed to get the proper analysis out of anyone. I either go=
t
fobbed off with an MSDS or what was obviously a generic analysis sheet
(usually from 1979 or sometime similar).
No doubt I was not really important enough to bother with (after all I don'=
t
order 100s of tons!). I even tried waving my Ph.D. in their face to no
effect. Oh well......
On Mon, May 24, 2010 at 6:36 PM, L TURNER wrot=
e:
> The best discussion of the differences among the iron oxides normally
> encountered by the studio potter is the one by David Hewitt (1925 -
> 2006). It was first published in Ceramic Review No 186 in 2000 and
> is available from an archived copy of his website:
>
> http://web.archive.org/web/20050507172942/www.dhpot.demon.co.uk/Raw+Mater=
ials.htm
> ,
>
> or more easily in pdf format from:
>
> http://www.goldenarts.com.hk/download/Raw%20Materials%20-%20Do%20you%20kn=
ow%20what%20you%20are%20buying.pdf
>
> A further point that Hewitt doesn't address head on. "Black iron"
> doesn't really exist as a pure material at ambient conditions since it
> is unstable below about 575 deg C and is a non-stoichiometric
> compound. What is sold as "black iron oxide" is actually best
> represented as Fe3O4.
>
>
> The DigitaFire website: www.digitalfire.com also has good information
> on the various iron oxides. two of the links are:
> Iron Oxide Black
> http://digitalfire.com/4sight/material/iron_oxide_black_873.html
> Iron Oxide Red
> http://digitalfire.com/4sight/material/iron_oxide_red_874.html
> use the search feature to find links of yellow, spanish red, and other
> iron oxide pigments.
>
>
>
> >MSDS for "synthetic RIO" (I think it was 4284) that I was
> >sent from the distributor I use (this was a while back)... stated that
> >the purity of this was anywhere between 80-99%.
>
>
> The MSDS is a source of good safety information, but is not the best
> source of technical or material quality data. Get the technical
> datasheet for this information. If you are buying iron oxide through a
> distributor, and in less than full sack quantities, finding the name
> of the supplier is often the most difficult step in the task of
> getting the technical data. The MSDS can be helpful here as it must
> list the name and contact information of the main supplier.
>
> Regards,
>
> Lou Turner,
> The Woodlands, TX
>
Robert Harris on mon 24 may 10
I realise I am beating this one to death but just out of interest - the
formula weights of theoretical iron oxides are as follows.
Fe2O3 - 159.68
Fe3O4 - 231.54
FeO - 71.845
To get one mole of iron we would need 71.845g of FeO or 77.18g of Fe3O4 or
79.84g of Fe2O3.
Since the difference between the first and last number (and as we've
ascertained FeO is unstable) is only about 10% I would suggest that if
you're seeing major differences in your recipes it's entirely due to
impurities in your Iron oxide and NOT the valence state.
Robert
John Britt on tue 25 may 10
Robert,
I don't think you are betting it to death. I am wondering if Black iron
oxide is 100% pure?=3D20
Also, you forgot to mention how the mesh size effects things. If your red=
=3D
or
yellow iron is 325 mesh, more of it will melt vs the black iron at 80 - 1=
=3D
25
mesh. So the same amount will not have the same effect.
www.johnbrittpottery.com
Neon-Cat on tue 25 may 10
Just to keep you all thinking...
Here=3DE2=3D80=3D99s a study (it=3DE2=3D80=3D99s not as bad to read as the =
title might =3D
imply) of
the red-color overglazes and the transparent glazes of the famous
Hizen porcelains of the Kakiemon-style first developed by the
Kakiemon-kiln family in 1650-80's that graced tables of nobility as
far away as Europe and America and are as bright and pretty today as
they were when they were made.
The article addresses techniques and sequence of glaze application,
glaze interactions with the porcelain body, firing practices,
morphology of the crystal structures, and red pigment sources. One
called =3DE2=3D80=3D9CFukiya-style Bengara=3DE2=3D80=3D9D was made by chemi=
cal treatmen=3D
t from
FeSO4=3DE2=3D88=3D997H2O (also called Iron(II) sulfate heptahydrate) to Fe2=
O3 at
about 650 =3DCB=3D9AC in air. How pure might that have been way back when?
There are so many factors that influence color production in ceramics.
This article addresses a group of very specific porcelains created by
similar techniques. What happens with this glaze and overglaze and
clay body may not happen in another ceramic system. My point is that
while generalities are good and useful guides they are not the
end-all.
=3DE2=3D80=3D9CLocal structures and electronic band states of =3DCE=3DB1-Fe=
2O3
polycrystalline particles included in the red-color overglazes and the
transparent glazes of the Kakiemon-style porcelains by means of X-ray
absorption spectra (=3DD0=3D9F)=3DE2=3D80=3D9D
M. Hidaka; H. Horiuchi; K. Ohashi; R. P. Wijesundera; L. S. R. Kumara;
Nark Eon Sung
Cer=3DC3=3DA2mica vol.55 no.335 S=3DC3=3DA3o Paulo July/Sept. 2009
http://www.scielo.br/scielo.php?pid=3D3DS0366-69132009000300001&script=3D3D=
sci_=3D
arttext
PDF Version:
http://www.scielo.br/pdf/ce/v55n335/v55n335a01.pdf
Japanese Pottery Primer:
http://www.e-yakimono.net/guide/index.html
example of Kakiemon piece:
http://www.e-yakimono.net/guide/html/porcelain.html
Iron(II) sulfate heptahydrate:
http://fscimage.fishersci.com/msds/09870.htm
What is of interest to me is when (temperature) and under what
conditions (oxidation/reduction/neutral or a combination;
heating/cooling rates) our iron pigments change from one mineral form
to another and how these various forms become incorporated in the
ceramic body or glaze (in the lattice; glassy matrix; as
=3DE2=3D80=3D9Cstuffed-ions=3DE2=3D80=3D9D, in exsolution processes, etc.) =
and then app=3D
ear to us
as color. In low temperature work (under 800 =3DE2=3D80=3D93 1000 C) one ca=
n read=3D
ily
see a mix of =3DE2=3D80=3D98flashing=3DE2=3D80=3D99 iron colors from a comb=
ination of
lepidocrocite, goethite, maghemite, magnetite, hematite, etc. as the
iron experiences various transformations and runs through intermediate
species due to heat.
As a beginner I don=3DE2=3D80=3D99t have much at all in the way of standard=
studi=3D
o
equipment and my technique is just not up to par to justify the extra
funds for more expensive pigments trying for that professional-look
now. If I=3DE2=3D80=3D99m not able to make up a glaze the way I=3DE2=3D80=
=3D99d like to=3D
, or I=3DE2=3D80=3D99m
having trouble applying glazes, then getting the work fired, and
working consistently on a daily basis, an expensive pigment isn=3DE2=3D80=
=3D99t
going to save the day. Thus choice of colorants might-should-be
realistically geared to one=3DE2=3D80=3D99s personal and professional level=
as on=3D
e
keeps an eye on achieving a better mark down the road. Along the way
it is nice to know that we have plenty of choices and where pigments
might be procured. It is also very, very nice that more experienced
potters share how they have refined their own techniques and
practices, down to what their preferred oxide choice may be, with and
without a dash of this and that.
Marian
Neon-Cat
Robert Harris on thu 27 may 10
I just checked on the Laguna website. They claim that their black Iron Oxid=
e
is >99% Fe3O4. That must be the reason I need a lot less than the
"calculated" amount compared to RIO, and probably why I don;t have the batc=
h
to batch problem I do with other iron oxides.
I wonder if the black iron oxide I get is milled finer than yours - or
perhaps I just don't notice speckling (it certainly has less speckling than
CuO). I also can put it through a 120 mesh sieve with absolutely no problem=
.
I also fire pretty slowly for an electric firing and Emily's glazes are
pretty molten, so maybe it just has more of a chance to dissolve in the
melt? No doubt the old Chinese potters would be laughing in their graves at
our quibbling. At least we don't have to crush up our own rocks! I wonder
whether they did batch tests and how big their "batches" of crushed up rock=
s
were. Or perhaps thir glazes just varied a bit from firing to firing.
Anyway, I think this is definitely one of those cases where you "pays your
money and takes your pick."
Robert
----------------------------------------------------------
PrometheanPottery.wordpress.com
PrometheanPottery.etsy.com
Neon-Cat on thu 27 may 10
Robert =3D96 quick, simple note:
4 Fe3O4(s) + O2(g) ---> 6 Fe2O3(s)
Given the above equation and if it goes to completion, let=3D92s say we
have a glaze batch with 90 grams of black iron oxide, we=3D92ll have 0.389
mol Fe3O4.
(90g/231.36g =3D3D 0.389)
0.389 mol Fe3O4 X (6 Fe2O3/4Fe3O4) =3D3D 0.5835 mol Fe2O3 (a-hematite)
produced or we could say that 0.5835 mol Fe2O3 X 159.57 =3D3D 93.1 gram
equivalent Fe2O3 by transformation in oxidizing conditions.
On a mol-to-mol basis you are getting 1.5 times (one and a half times)
as much hematite out of firing as there is magnetite (Fe3O4).
In general, during the beginning of firing, the oxidation of the
magnetite occurs in two stages. The first stage is between 200 -- 350
C (392 - 662 F), sometimes lasting into the 500 C/932 F temperature
range, where a low-temperature oxidation to maghemite (gamma-Fe2O3)
takes place. The second stage of oxidation starts around 400 C (752 F)
and leads to totally oxidized (or not) pigment grains around 900 to
1100 C (1652 =3D96 2012 F), depending on the elemental analysis of your
Fe3O4, the impurity content (trace elements, binders, granulating
agents, etc.), particle size and distribution, particle shape,
particle surface area, density, agglomeration, crystal structure
(morphology), rheological properties (deformation and flow of the
dispersed under heat stress), oxygen partial pressure, firing
schedule, etc.
In simple terms magnetite is often written as FeO=3DB7Fe2O3 (with Fe3O4
used to denote a mix of above). In fancier versions magnetite is known
to have an inverse spinel structure, where Fe2+ ions occupy octahedral
sites while Fe3+ ions are distributed in both octahedral and
tetrahedral positions. This structure can be written as
(Fe3+)A(Fe3+Fe2+)BO4. Magnetite crystals belong to the cubic space
group Fd3m.
Oxidation of magnetite yields the intermediate phase maghemite
(gamma-Fe2O3) without alteration of the inverse spinel structure. One
Fe2+ cation can then be desorbed into the glaze melt solution and two
other Fe2+ ions go on to be oxidized. The vacancies created at
octahedral sites of maghemite determine the basic unit as (Fe3+)A(Fe3+
5/3V1/3)BO4.
That=3D92s enough typing =3D96 back to clay for me...
Have fun!
Marian
Neon-Cat
Neon-Cat on fri 28 may 10
Robert, you wrote previously:
=3D93if you're seeing major differences in your recipes it's entirely due
to impurities in your Iron oxide and NOT the valence state.=3D94
and
=3D93I just checked on the Laguna website. They claim that their black
Iron Oxide is >99% Fe3O4. That must be the reason I need a lot less
than the "calculated" amount compared to RIO...=3D93
Then you wrote me: =3D93I'm not quite sure where you are going with this...=
=3D
=3D94
Oh, I was just presenting another way of looking at iron as pigment in
our work, one that is a little more in tune with how the rest of the
world is looking at fired clay and glazes these days. And putting out
some other factors that influence oxide use that had not been
mentioned. It is not all about impurities or the specific amount of
iron atoms. For some things I=3D92d rather have a good colorant with a few
impurities than one I had to worry about getting through different
stages before it could be useful. And, to give you a clue as to why
you may be having success using less of your BIO. And, so others might
think a bit before they leap the fence after that 99% pure Fe3O4 to
meet all their needs.
Your way, with the Fe3O4 you may get 1.5 times the amount of hematite
others might get using the same amount of Fe2O3. You will also have
the 1.167 mol of iron atoms verses the 1.128 mol of iron atoms other
get with their equal amount of Fe2O3. That=3D92s about 2.35 times 10 to
the 22nd bonus iron atoms. But I like to deal with just the hematite
as reactant or product, not split hairs over individual iron atoms
like you do. Your way you run the risk of incomplete reactions that
may or may not be important to some glazes by creating a glaze with
some magnetite, some maghemite, and some hematite not to mention the
silicates and other compounds that will form or try to form. A glaze
made with BIO could be well oxidized at the surface but have residual
un-oxidized and unreacted species further in or near the glaze-clay
body interface. Would this be bad or good? It all speaks to color or
possible uneven color or mottling, hue, depth, crystal effects, etc.
Some recipes do specify oxides, and for a reason; others don=3D92t. As
single entities magnetite and maghemite are both dark (brown to black)
and hematite is our traditional red colorant. If I were going for a
red glaze I=3D92d just as soon not begin with a dark colorant.
There are hundreds of studies conducted at various firing schedules to
varying peak temperatures. I think I read one using a 150 C
degree/hour schedule, not sure and my computer is bogged down (I need
more memory or something or less stuff) with too much right now to run
a search easily for that file or a reference for it. Folks have had
the ability for some time now to follow and identify the creation of
minerals and phases and monitor transformations throughout the course
of an entire firing, up and back. Each system is different of course,
but up to about the temperatures I mentioned in the previous post
that=3D92s pretty much what happens in oxidation firing. After that, it
depends on what you=3D92re making in your glaze and/or clay body and how
(temps, atmosphere, etc.).
There=3D92s a wealth of information out there and testing methods and
equipment are quite good now if you have the time and patience to
search and explore beyond what we have in our standard texts. To me,
clay science has to be more than subbing numbers and =3D93equivalents=3D94
using calculation software. Each of our materials is unique and adds
its own special magic to a glaze or clay body, pigments included. Not
to rock the boat but ... how they do that is interesting to me.
Isn=3D92t it nice there are those tricky questions and things to ponder?
I=3D92ll leave you and the others to wonder about the other things you
mentioned, I=3D92m back into a making mode (hematite all the way!).
Marian
Neon-Cat
Robert Harris on fri 28 may 10
I'm not quite sure where you are going with this...
In summary 0.389 mols of Fe3O4 black iron oxide (let's call it magnetite -
although since the stuff they sell to us in bags is synthetic I'm not sure
what the crystal structure looks like, or whether it bears any resemblance
to the mineral.) gets altered during the first phase of the firing into
0.5835 mols of Fe2O3. Of course in both cases we still have 1.167mols of
iron itself.
As you indicate the magnetite will (may?) oxidise during the early part of
the firing into a form of Fe2O3. My chemistry focus was always a bit more
organic that inorganic so I'm not going to even wonder what the crystal
structure might be. I wonder if the studies that have been done have used
150C/hr temperature increases (about my firing schedule) and whether there
is time for thorough oxidation - and or crystal structure changes.
In addition one thing that I have read in ceramic/glazing textbooks is that
Fe2O3 with thermally decompose into FeO (or possibly Fe3O4) at higher
temperatures (e.g. Cone 10 1275C). I cannot find a scientific reference for
this online however. I suspect that this is a reaction that occurs within
the melt. I know that this is the perceived basis for Oil Spots (John B.
perhaps you could chime in here!), where decomposition of Fe2O3 yields
bubbles of O2 that cause the spotting.
I wonder what the proportion of Fe2+ to Fe3+ there is in say a Tenmoku or
oil spots (which look pretty black to me). Of course I don't have any
information about wavelength absorption of Fe2+ vs Fe3+ when it is
presumably in the form of some sort of silicate rather than pure oxides.
Equally I wonder if tomato reds have more Fe3+ or if the phosphorous alters
the crystal structure so that it absorbs different wavelengths.
Not really sure where I'm going with this post either....
Robert
On Thu, May 27, 2010 at 5:51 PM, Neon-Cat wrote:
> Robert =3D96 quick, simple note:
>
> 4 Fe3O4(s) + O2(g) ---> 6 Fe2O3(s)
>
> Given the above equation and if it goes to completion, let=3D92s say we
> have a glaze batch with 90 grams of black iron oxide, we=3D92ll have 0.38=
9
> mol Fe3O4.
> (90g/231.36g =3D3D 0.389)
>
> 0.389 mol Fe3O4 X (6 Fe2O3/4Fe3O4) =3D3D 0.5835 mol Fe2O3 (a-hematite)
> produced or we could say that 0.5835 mol Fe2O3 X 159.57 =3D3D 93.1 gram
> equivalent Fe2O3 by transformation in oxidizing conditions.
>
> On a mol-to-mol basis you are getting 1.5 times (one and a half times)
> as much hematite out of firing as there is magnetite (Fe3O4).
>
> In general, during the beginning of firing, the oxidation of the
> magnetite occurs in two stages. The first stage is between 200 -- 350
> C (392 - 662 F), sometimes lasting into the 500 C/932 F temperature
> range, where a low-temperature oxidation to maghemite (gamma-Fe2O3)
> takes place. The second stage of oxidation starts around 400 C (752 F)
> and leads to totally oxidized (or not) pigment grains around 900 to
> 1100 C (1652 =3D96 2012 F), depending on the elemental analysis of your
> Fe3O4, the impurity content (trace elements, binders, granulating
> agents, etc.), particle size and distribution, particle shape,
> particle surface area, density, agglomeration, crystal structure
> (morphology), rheological properties (deformation and flow of the
> dispersed under heat stress), oxygen partial pressure, firing
> schedule, etc.
>
> In simple terms magnetite is often written as FeO=3DB7Fe2O3 (with Fe3O4
> used to denote a mix of above). In fancier versions magnetite is known
> to have an inverse spinel structure, where Fe2+ ions occupy octahedral
> sites while Fe3+ ions are distributed in both octahedral and
> tetrahedral positions. This structure can be written as
> (Fe3+)A(Fe3+Fe2+)BO4. Magnetite crystals belong to the cubic space
> group Fd3m.
>
> Oxidation of magnetite yields the intermediate phase maghemite
> (gamma-Fe2O3) without alteration of the inverse spinel structure. One
> Fe2+ cation can then be desorbed into the glaze melt solution and two
> other Fe2+ ions go on to be oxidized. The vacancies created at
> octahedral sites of maghemite determine the basic unit as (Fe3+)A(Fe3+
> 5/3V1/3)BO4.
>
> That=3D92s enough typing =3D96 back to clay for me...
> Have fun!
>
> Marian
> Neon-Cat
>
--=3D20
----------------------------------------------------------
PrometheanPottery.wordpress.com
PrometheanPottery.etsy.com
Robert Harris on fri 28 may 10
Marian - Hmmm definitely something to think about. BUT I question whether
there is any hematite or magnetite left in a Cone 10 (electric) glaze (let
alone in a reduction atmosphere).
From my reading on the subject you're going to produce various iron
silicates etc. Which is why I focused on iron atoms not on the various iron
oxide minerals. We have a very nice iron green glaze
http://prometheanpottery.files.wordpress.com/2010/04/promethean-pottery-mos=
=3D
sy-mahogany-canisters-watermarked.jpg
and iron blue glaze
http://www.etsy.com/listing/45963028/stoneware-goblets-or-chalices-in
Both of these get their colouration from iron - but obviously nothing to do
with hematite or magnetite in the glaze. There may be 'some' hematite or
magnetite crystals still in the glaze, but these are certainly not importan=
=3D
t
for coloration.
This is why I wrote
>>>I wonder what the proportion of Fe2+ to Fe3+ there is in say a Tenmoku o=
=3D
r
oil spots (which look pretty black to me). Of course I don't have any
information about wavelength absorption of Fe2+ vs Fe3+ when it is
presumably in the form of some sort of silicate rather than pure oxides.
Equally I wonder if tomato reds have more Fe3+ or if the phosphorous alters
the crystal structure so that it absorbs different wavelengths.<<<
this previously.
I would also guess that the reason that some recipes call for particular
oxides is more to do with particle size and ability to dissolve in the melt
(especially for celadons) than due to crystal structure. Since you obviousl=
=3D
y
have an inorganic chemistry or geology background I would be interested to
know if you had a hypothesis as to how small percentages of TiO2 turn
celadons from blue to green.
Please don't feel I'm trying to be argumentative, I'm not - I'm trying to
learn something by discussing it!
Robert
On Fri, May 28, 2010 at 7:29 AM, Neon-Cat wrote:
> Robert, you wrote previously:
>
> =3D93if you're seeing major differences in your recipes it's entirely due
> to impurities in your Iron oxide and NOT the valence state.=3D94
>
> and
>
> =3D93I just checked on the Laguna website. They claim that their black
> Iron Oxide is >99% Fe3O4. That must be the reason I need a lot less
> than the "calculated" amount compared to RIO...=3D93
>
> Then you wrote me: =3D93I'm not quite sure where you are going with this.=
..=3D
=3D94
>
> Oh, I was just presenting another way of looking at iron as pigment in
> our work, one that is a little more in tune with how the rest of the
> world is looking at fired clay and glazes these days. And putting out
> some other factors that influence oxide use that had not been
> mentioned. It is not all about impurities or the specific amount of
> iron atoms. For some things I=3D92d rather have a good colorant with a fe=
w
> impurities than one I had to worry about getting through different
> stages before it could be useful. And, to give you a clue as to why
> you may be having success using less of your BIO. And, so others might
> think a bit before they leap the fence after that 99% pure Fe3O4 to
> meet all their needs.
>
> Your way, with the Fe3O4 you may get 1.5 times the amount of hematite
> others might get using the same amount of Fe2O3. You will also have
> the 1.167 mol of iron atoms verses the 1.128 mol of iron atoms other
> get with their equal amount of Fe2O3. That=3D92s about 2.35 times 10 to
> the 22nd bonus iron atoms. But I like to deal with just the hematite
> as reactant or product, not split hairs over individual iron atoms
> like you do. Your way you run the risk of incomplete reactions that
> may or may not be important to some glazes by creating a glaze with
> some magnetite, some maghemite, and some hematite not to mention the
> silicates and other compounds that will form or try to form. A glaze
> made with BIO could be well oxidized at the surface but have residual
> un-oxidized and unreacted species further in or near the glaze-clay
> body interface. Would this be bad or good? It all speaks to color or
> possible uneven color or mottling, hue, depth, crystal effects, etc.
> Some recipes do specify oxides, and for a reason; others don=3D92t. As
> single entities magnetite and maghemite are both dark (brown to black)
> and hematite is our traditional red colorant. If I were going for a
> red glaze I=3D92d just as soon not begin with a dark colorant.
>
> There are hundreds of studies conducted at various firing schedules to
> varying peak temperatures. I think I read one using a 150 C
> degree/hour schedule, not sure and my computer is bogged down (I need
> more memory or something or less stuff) with too much right now to run
> a search easily for that file or a reference for it. Folks have had
> the ability for some time now to follow and identify the creation of
> minerals and phases and monitor transformations throughout the course
> of an entire firing, up and back. Each system is different of course,
> but up to about the temperatures I mentioned in the previous post
> that=3D92s pretty much what happens in oxidation firing. After that, it
> depends on what you=3D92re making in your glaze and/or clay body and how
> (temps, atmosphere, etc.).
>
> There=3D92s a wealth of information out there and testing methods and
> equipment are quite good now if you have the time and patience to
> search and explore beyond what we have in our standard texts. To me,
> clay science has to be more than subbing numbers and =3D93equivalents=3D9=
4
> using calculation software. Each of our materials is unique and adds
> its own special magic to a glaze or clay body, pigments included. Not
> to rock the boat but ... how they do that is interesting to me.
>
> Isn=3D92t it nice there are those tricky questions and things to ponder?
> I=3D92ll leave you and the others to wonder about the other things you
> mentioned, I=3D92m back into a making mode (hematite all the way!).
>
> Marian
> Neon-Cat
>
--=3D20
----------------------------------------------------------
PrometheanPottery.wordpress.com
PrometheanPottery.etsy.com
Neon-Cat on fri 28 may 10
Oh, forgot to include a reference for you, Robert =3D96 the thermal
decomposition of red iron oxide (alpha-Fe2O3) at temperatures in
excess of 1200 C (oxidation) results in a grayish silvery black
magnetite. One oft-quoted reference is
=3D93Identification of the pigment in painted pottery from the Xishan
site by Raman microscopy=3D94, J. Zuo, C. Xu, C. Wang, and Z. Yushi,
Journal of Raman Spectroscopy, 30 (1999), 1053-1055.
This transformation is pretty common knowledge now and is mentioned in
many papers. I got some thermal conversion of hematite to the silvery
version of magnetite in my recent cone 10 firings (but not as
decoratively as some do). I also have a nice dusting of red hematite
crystal throughout my experimental glaze versions using the glauconite
(greensand). Unfortunately with my ancient camera and poor photography
techniques they are not visible in photos I took.
I need to move away from the computer and get back to my work. There
will be plenty of time to discuss a myriad of topics as we all go
along. You guys and gals can keep at it.
My science background is unremarkable and included more organic than
inorganic chemistry. Science now is a small interesting personal
glitch post lyme disease. It snuck in with the clay obsession and gets
better and better. Life survival skills might have been a better
gift...science and clay get me in trouble. But who am I to say? Time
will tell.
And no, you don't seem argumentative, just enthusiastic and coming at
it all from a different direction. Whatever works...
Marian
Neon-Cat
ivor & olive lewis on sat 29 may 10
Dear Robert Harris,
In a recent post I suggested using Google to track down the Eltringham
diagram. There is a site which allows you to choose elements and then
provides thermodynamic information. Otherwise Kingery and his collaborators
have a the diagram, P 394 in "Introduction to Ceramics".
For Physical Information about colour in silicate systems the classic text
is W. A. Weyl, "Coloured Glass". ISBN 0-900682-06-X. Pp 89-120 covers the
colours of Iron in glass. This is still "In Print"
There are also several Equilibrium Phase Diagrams that might give some
enlightenment on the relationships between the various oxides of Iron in
alumino-silicate environments.
In spite of access to all of this information I would be hard pressed to
explain blue crystals that grow out of a slip that is loaded with Red Iron
Oxide and Yellow ochre.
Regards,
Ivor Lewis,
Redhill,
South Australia
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