Iron Glazes and Achieving Red Color in Oxidation

Summary

Red iron oxide Fe2O3 decomposes to iron monoxide FeO above °C even in oxidising environments. Iron red is produced by a surface growth of columnar crystals of Fe2O3. Best reds are obtained by a fast cool through the temperature range above °C, where black FeO tends to form crystals, to about 950°C where growth of red Fe2O3 crystals is optimised. Oxygen is required for this phase. Holding the temperature at 950°C for about an hour produces the maximum coverage of iron crystals and best red colour. The colour deteriorates with holds much longer than that, tending towards rust brown.

Following is an extract from John Sankey's Website with detailed instructions on achieving iron reds.Red iron oxide Fedecomposes to iron monoxide FeO above °C even in oxidising environments. Iron red is produced by a surface growth of columnar crystals of Fe. Best reds are obtained by a fast cool through the temperature range above °C, where black FeO tends to form crystals, to about 950°C where growth of red Fecrystals is optimised. Oxygen is required for this phase. Holding the temperature at 950°C for about an hour produces the maximum coverage of iron crystals and best red colour. The colour deteriorates with holds much longer than that, tending towards rust brown.

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The more calcium there is in an iron glaze, the more difficult it is to obtain a good red, however some calcium is required to get red from iron. To get a tomato red also requires phosphorus; the red becomes more orange when magnesium is added to these two. The best iron reds in oxidation firing are obtained with about 0.10 molar CaO, 0.03-0.06 MgO, 0.01-0.02 P2O5 and 0.08-0.12 FeO.

Literature

Colours reported
Black, tomato to rust red to muddy brown with green, honey, rust, rarely blue nuances are reported in the literature. I have seen them all in my tests. I have only seen blues as crust-like surface components; if they are crystalline, the crystals are to small to see at 100x. I have never seen yellows or greens as a surface component, only as body. Reds seem to appear solely as a crystalline surface layer.

Black, tomato to rust red to muddy brown with green, honey, rust, rarely blue nuances are reported in the literature. I have seen them all in my tests. I have only seen blues as crust-like surface components; if they are crystalline, the crystals are to small to see at 100x. I have never seen yellows or greens as a surface component, only as body. Reds seem to appear solely as a crystalline surface layer.

Thickness of application
This is often mentioned as a major determining factor in colour of red iron glazes. In my own work, I have found that a minumum thickness of about 50 µm is required.

Cooling temperature
Cooling schedule is known to be important. Refiring to cone 04 is often recommended to improve the red. Murrow (Ceramics Monthly Sept ) found that shino glazes turned red only below 982°C. Marians (Ceramics Monthly June/July ) found that temperature holds between 980°C and 870°C during the cooling cycle were critical to the development of red in her iron glaze. She saw three components, one believed to be iron-rich and two silica-rich, at low magnification. My own work confirms these findings, that holds at 950°C for an hour during cooling produce the best reds.

Colour analysis
Murrow&Vandiver (noted in Ceramics Monthly Sept ) found that the red colour of shino glazes came from a surface layer of ferric microcrystals about 20 µm thick. Under this layer, the glaze was white. My collaboration with John Stirling found that in iron glazes the surface is iron sesquioxide (Fe2O3) while deeper iron is iron monoxide (FeO). Park&Lee (J.Ceram.Soc.Japan 113():161-165 ) found that in high magnesia glazes the red colour is magnesioferrite MgO.Fe2O3.

Calcia
Cardew (Pioneer Pottery) states that alkali earths must be minimised for an iron red, that even 0.2 calcia will turn an iron glaze brown, and proposes a special frit to this end. My work confirms this for calcia.

Magnesia
Park&Lee (op.cit.) found by X-ray diffraction that in their glazes magnesia forms a red colour as magnesioferrite MgO.Fe2O3, and that magnesioferrite crystal formation is closely related to the presence of whitlockite-type crystals Ca.9(Mg,Fe).(PO4).6(PO3OH). Phosphorus seems to crystallize as whitlockite at -°C, magnesioferrite at 900-950°C. Edouard Bastarache (unpublished) considers that the presence of dolomite (Ca,Mg)CO3 in iron red glazes makes better reds.

Soda
Hamer&Hamer (The Potter's Dictionary) mention that soda combined with small amounts of iron produce blue, and that soda encourages red with large amounts of iron, but give no details.

Phosphorus
Bone ash is reported by many to make more reliable reds; some use ferric phosphate instead of red iron oxide and the bone ash (which contains a lot of calcia).

Boron
Hamer&Hamer (The Potter's Dictionary) mention that boron combined with small amounts of iron produce blue, but give no details. Rhodes (Glazes for the Potter) mentions blue from iron and boron; again no details. Obstler (Out of the Earth, Into the Fire) mentions that boron increases green in iron celadons; no details given. Hesselberth&Roy's Waterfall Brown (Mastering Cone 6 Glazes) obtains green from iron; it has 50% more boron than usual formulations, also more soda. True boron iron greens seem to require the near-absence of calcia, which can only be achieved with frits since natural borates are half calcia.

Titania
Weyl (Coloured Glasses) notes that iron is green when it is a network-modifier (equivalent of interstitial atoms in crystals), and that titania moves it to brown by shifting it to a network former (the equivalent of taking part in a crystal lattice). 4% titania achieved this in my experiments with the calcium iron glaze below.

Migration of iron
There are two unpublished reports of sub-surface iron migrating to the surface of glazes. Ron Roy has noticed this under strong reduction, but not in oxidation under otherwise identical conditions. Hank Murrow believes that fluorine in a glaze assists migration of iron to the surface and uses a percent or less cryolite in his glazes to achieve this.

Sankey Iron Red
Custer feldspar 44g
silica 16.5g
bone ash 14g
red iron oxide 11g
EPK 10.5g
talc 10g
lithium carbonate 3
Bentonite 2g
COE: 6.8x10-6/K
calcia: 10% molar
Stoneware (Tucker Smooth White)
Source: Kevin Baldwin, adapted to local clays
Painted on bisque, fired cone 6 electric, one hour hold at 950°C. Vase is 7 cm high. A very even colour as long as it is thicker than 50 µm, red crystals on a black ground with visual depth. It tends to black where it is thin. By far the most reliable of the iron reds I've tried, and the one I've chosen for my own dinner set. The expansion is high, but it works perfectly on my clay, which is fairly low expansion (6.64x10-6/K), probably due to the high potash content, which increases both elasticity and tensile strength.

Microphotos are about 60x. X-ray analysis shows that all the crystals are pure iron oxide; Fe2O3 is the red, FeO the black. Red crystals were analysed at three depths. The data shows that the result of the 950°C soak is primarily to oxidize the iron crystals on the surface to red Fe2O3 while the buried iron remains black FeO. This may be done by movement of oxygen to the surface under chemical-strength forces or by surface oxidation. The glaze is far too viscous at 950°C to permit physical sorting of crystals.

one hour hold at 950°C

no hold

scanning electron microscope photo of Fe2O3 crystals Borate Iron Red:
Gerstley borate 32g
silica 30g
Custer feldspar 20g
red iron oxide 15g
talc 14g
EPK 5g
bone ash 6g
Bentonite 2g
COE: 5.6x10-6/K
calcia: 14% molar
Stoneware (Tucker Smooth White), thrown and trimmed.
Source: published many times under many names
Dipped on bisque, prefire thickness 0.47 mm. Fired cone 6 electric, two hour rise to maximum temperature (°C), held there for 10 min, kiln off until soak temperature reached (typically 30 min.), held there for a soak period, kiln off (5 hr to reach 200°C).

I did a series of runs with the same bowl, changing only the soak temperatures. The first involved a long soak to fully develop all crystals that might be formed. The microphoto (about 50x) shows that there are two types of components to this glaze. One group forms very small crystals or crusts on the surface; it forms a rust colour with long soak times. The other component oozes out to the surface without crystalizing and is often yellow, sometimes bright. It is probably ferrosilite (see below). Under certain temperature regimes the ferrosilate is coloured a dull red by the iron. Bright colours always seem associated with surface components; the two other microphotos shown are typical of the variety of colours seen. X-ray analyses show that the surface crusts are thinner than 4 µm, the effective penetration depth of the 20 kV electrons used; reliable analyses could not be obtained.

Black gradually took over the bowl with repeated firings (the 980°C at right was the eighth in the series). So, if you don't get the colour you wish with this glaze, try refirings, but not too many. I obtained the most interesting colours with moderate-length soaks in the 900-980°C range.

X-ray analysis of the final glaze (the 980°C photo) showed considerable fine and medium-scale differentiation in the surface. There were large patches of nearly pure silica (87%). Other patches had double the concentration of iron as the total glaze formulation; all these were low in calcia. There was also evidence of calcium silicate. It was not possible to match the X-ray image to an optically-visible feature of the glaze.

In contrast to some reports, I found this to be a very well-mannered glaze as I mixed it, not at all runny, or even droopy when laid on thickly, as you can see from the lack of problems with the sharp horizontal edges of the 8 cm diameter sugar bowl.

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5 hr hold at 870°C
 
30 min hold at 940°C
 
30 min hold at 980°C
 
glaze painted thickly on the outside, thinly on the inside, 1 hr hold at 920°C
 
scanning electron microscope photo of a 0.7 mm square portion of the surface, showing a typical crust-like differentiation. Calcium Iron:
Wollastonite 28g
EPK 28g
Fusion F2 frit 23g
silica 17g
red iron oxide 7g
nepheline syenite 4g
Bentonite 2g
cobalt carbonate 2g
COE: 5.5x10-6/K
calcia: 17% molar
Porcelain (Tucker 6-50), thrown and trimmed.
Dipped on bisque, fired cone 6 electric. This began as an attempt at a matte lustre black, but turned out to be a microcrystalline glaze. With the same bowl, a fast cool of this glaze gives a glossy black, then a refire followed by a slow cool converts it to a mottled black and green semi-matte, and vice versa. Green crystal size is dependent upon cooling rate. Cooling from °C at 50C/hr to 800°C produced crystals typically 2 mm in diameter. At 80C/hr the crystals were about 1 mm, at 100C/hr, ½ mm. 150C/hr is required to get a smooth surface, but then it's close to glossy. Unless fired glossy, iron in excess of 7% comes out of solution to form a metallic layer between the green crystals. Bowl is 10 cm diameter.

Besides the individual oxides, possible mineral compositions include Andalusite Al2O3.SiO2, Anorthite CaO.Al2O3.2(SiO2), Wollastonite CaO.SiO2, Fayalite 2(FeO).SiO2, Hedenbergite CaFe.2(SiO3) and Ferrosilite FeO.SiO2. [mineral photos]

The right half of the bowl is the result of a 10 hr soak at °C after firing to °C. X-ray analysis shows that the crystals are FeO; as shown in the microphoto (about 100x), they are mostly long rhombs. The surface is rough to the touch.

The left half of the bowl had a 10 hr soak at 870°C after firing to °C. This produces a surface layer of greenish-yellow crystal needles that grow in six rays from a central point. The colour could match either Hedenbergite or Ferrosilite, but the crystal form is most consistent with Hedenbergite. The crystals are so thin that their X-ray output was mixed with background material output, but it also indicates Hedenbergite.

Not an attractive or useful glaze.

I thank John Stirling of Natural Resources Canada's Geological Survey for making the scanning electron microscope guided X-ray analyses.

Iron Glaze Chemistry

To investigate the interaction of calcia, magnesia and phosphorus with iron in oxidising fired glazes, a series of mixes were made. A base glaze contained none of any of the three oxides under study, and one glaze each was made up similar to the composition of the base glaze, but with a large quantity of each of the oxides in turn. The target analysis for each was 3.5 Seger SiO2, 0.4 Al2O3 and 0.25 B2O3 (B2O3 and FeO omitted from Seger ratios).

mix recipe molar Seger oxide base 28 silica 28 potassium carbonate 24 kaolin,EPK 10 iron oxide,red 10 frit,Fusion 367 0.602  3.392  SiO2 0.166 0.935 K2O 0.102 0.575 FeO 0.010 0.059 Na2O 0.072 0.404 Al2O3 0.045 0.253 B2O3 magnesia 30 silica 25 magnesium sulphate 25 kaolin,EPK 10 iron oxide,red 10 frit,Fusion 367 0.611  3.497  SiO2 0.163 0.935 MgO 0.098 0.562 FeO 0.072 0.411 Al2O3 0.043 0.248 B2O3 0.010 0.058 Na2O calcia 32 silica 26 kaolin,EPK 22 calcium carbonate 10 iron oxide,red 10 frit,Fusion 367 0.615  3.485  SiO2 0.166 0.939 CaO 0.094 0.533 FeO 0.071 0.405 Al2O3 0.041 0.234 B2O3 0.010 0.055 Na2O phosphorus 27 silica 26 potassium carbonate 23 kaolin,EPK 15 iron phosphate 9 frit,Fusion 367 0.587  3.494  SiO2 0.157 0.936 K2O 0.100 0.596 FeO 0.070 0.418 Al2O3 0.041 0.246 B2O3 0.033 0.197 P2O5 0.010 0.057 Na2O

Since the potassium and magnesium salts are hygroscopic (magnesium sulphate particularly so), each of these was baked at 220°C until anhydrous before weighing. Each glaze mix was then ground in a ball mill until uniformly fine. An attempt was made to use a 200 mesh seive on the dry mixes, but the hygroscopic-powered clumping of the soluble salts made that impractical - lumps had to be taken out after mixing with the oil.

Each tile glaze was made with four 1/4 tsp portions (1 tsp=5 ml) of dry material, using a precision stainless steel measure, then mixed with 3/4 tsp corn oil. (Water could not be used due to the soluble salts required to separate the oxides.) They were fired to °C (cone 10), cooled to 950°C, then held there for 1 hr.

Since the best results were obtained with the maximum amount of phosphorus available from these mixes, and calcium was shown to be required, a supplementary mix was made up using bone ash (which contains calcium) to obtain larger amounts of phosphorus. Finally, the ratio that gave the best colour response was tested with 0-25% added red iron oxide. Some of the 54 test tiles are shown at right. Although not exactly decorative, they show several useful results for oxidation firing above °C:

1. Although excess calcium is known to turn iron glazes brown, some calcium is required to get red from iron.
2. To get an intense red also requires phosphorus, but too much phosphorus turns the glaze brown.
3. The red becomes more orange when magnesium is added to these two, but too much magnesium turns the glaze grey.
4. The best iron colour responses in this series were obtained with 0.10 molar CaO, 0.03-0.06 MgO, 0.01-0.02 P2O5.
5. Using a base with these proportions fired to cone 6, the best reds were obtained with 9-14% added red iron oxide.
6. 1% cryolite (fluorine) may help to brighten the red on the surface a bit, but gives foaming to spitting problems during firing if used even slightly in excess or if insufficiently finely divided.

I have located only one phosphorus-iron mineral that is red: Simferite Li(Mg,Fe+++,Mn+++)2(PO4), which is not a player since none of my test series contained lithia. Park&Lee identified a magnesium-iron compound as their red. However, they seem to have concluded that the role of phosphorus is to tie up calcia so it doesn't interfere with the formation of magnesioferrite. It can't be as simple as that, since I got zero red from any glaze that did not contain calcia, and only a trace of very dark red on glazes that contained calcia but no phosphorus.

My current working hypothesis is that a calcium-phorphorus compound acts as a promotor (perhaps catalyst) for the oxidation of FeO to Fe2O3. It is possible that the two instead tie up something else that inhibits the oxidation, but this seems less likely.

Notes:
The tests were originaly planned for cone 6 and boron added that is usually sufficient for it. Although the magnesium mix melted at cone 6, all mixtures with it turned viscous and foamy as soon as they melted. Cone 10, the maximum for my kiln, helped with some, but was still inadequate for a few.
Every stray cat in the neighbourhood was attracted to the kiln outside air vent by the smell of the corn oil!

Microphotos

Here is the setup I use to take my microphotos. My microscope is a basic full-size frame - no condenser, fine focus, stage movement or anything else expensive. The objective is a 5x 0.1 NA - low NA gives large depth of field, important for viewing solid objects. The eyepiece is a 10x periplan, selected from a dozen brands in the store for clearest view over the field with this objective.

My camera is a basic Canon PowerShot, set to focus and meter only in a central square that is shown on the LCD in P mode. This is the least expensive camera I've found that gives pictures as sharp as its pixel count. Many made by electronics companies have poor resolution lenses and/or autofocus. Its 3.2 megapixels is ample for web photographs.

In use, the camera is placed on the eyepiece, moved from side to side until all the circle of light from the microscope is visible on the screen, then zoomed until the edge of the circular field just touches the edges of the view. That standardizes the magnification of the photos so only one session with measuring scales is required. The microscope is focussed until the image is sharp on the LCD, then the shutter pressed.

Total cost for microscope plus camera - $340 in Canada (). Of course, both microscope and camera can be and are used independently of each other.

If 50x is too high a magnification, remember: a camera works just like a human eye. If you can see it you can photograph it! Try holding magnifying glasses in front of the lens - I find a 4x jewellers' loupe especially useful for photographing small insects.

Author's permission to republish

All materials presented on my site, except those specifically credited to other authors, are Copyright © John Sankey, -, under the Berne convention, solely in order to protect the right of all to continue to use them freely. Anyone may copy, link to, or distribute any of them as much as they wish, as long as this notice of copyright and permission to further copy is distributed with all copies. That's the only restriction I put on them - that they remain absolutely free to all. No one may restrict their further use in any way, by collection copyright, physical copy prevention, or any other means. The courtesy of a reference link or credit is always appreciated. John Sankey

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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//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, , 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, and , 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 ) 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, 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, and , 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 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, at 2:23 PM, Robert Harris wro=
te:

> I looked at the MSDS for "synthetic RIO" (I think it was ) 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, 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, and , 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//12/large-persimmon-canist=
er.jpg

and

http://www.etsy.com/view_transaction.php?transaction_id=3D

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, 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, and , 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 ( -
). It was first published in Ceramic Review No 186 in and
is available from an archived copy of his website:
http://web.archive.org/web//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 ) 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/ 12:50:01 PM Eastern Daylight Time,
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//01/03_100ppi.jpg
>
> hope this helps
>
> Robert

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 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 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, 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 ( -
> ). It was first published in Ceramic Review No 186 in and
> is available from an archived copy of his website:
>
> http://web.archive.org/web//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 ) 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 -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.
http://www.scielo.br/scielo.php?pid=3D3DS-&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/.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 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. mol Fe2O3 (a-hematite)
produced or we could say that 0. 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
C ( =3D96 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. 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 C). 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, 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. mol Fe2O3 (a-hematite)
> produced or we could say that 0. 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
> C ( =3D96 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//04/promethean-pottery-mos=
=3D
sy-mahogany-canisters-watermarked.jpg


and iron blue glaze

http://www.etsy.com/listing//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, 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 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 (), -.

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--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