Paul Borian on wed 22 nov 06
I have two main versions of a slip glaze I make from local clay - about
50% clay and the rest is varied amounts of feldspar, silica and whiting
and occasionally some talc. One version uses black iron and comes out
like a temnoku; the other uses red iron and bone ash and is an iron red
quite similar to Ohata red, which I also use. In fact I could probably
substitute one for the other and most customers would not notice, that's
how similar they look.
Anyway, the iron red usually has about 8% RIO plus the natural iron in
the clay and about 8% bone ash and I recently took a gallon of this
glaze, set it aside and added another 1-2% RIO and the small bowl I used
it on had a more matt surface than the glaze normally has. I did a
similar test a while back and had the same kind of thing happen - a
fairly glossy glaze came out more matt (or maybe satin matt is more
accurate) when high amounts of red iron were added. I fired to cone 11
in reduction and my kiln cools very slow.
I would have thought RIO is more of a flux so I am trying to figure out
whether maybe both of these samples I did were just in unusual parts of
the kiln to get these results. Or is there some strange affect RIO can
have on the surface of a glaze? Can anyone elaborate?
Thanks,
Paul
Timothy Joko-Veltman on wed 22 nov 06
On 11/22/06, Paul Borian wrote:
> Anyway, the iron red usually has about 8% RIO plus the natural iron in
> the clay and about 8% bone ash and I recently took a gallon of this
> glaze, set it aside and added another 1-2% RIO and the small bowl I used
> I would have thought RIO is more of a flux so I am trying to figure out
> whether maybe both of these samples I did were just in unusual parts of
> the kiln to get these results. Or is there some strange affect RIO can
> have on the surface of a glaze? Can anyone elaborate?
Paul,
Oxidation or reduction firing? In oxidation, iron oxide (black, red,
or otherwise) is refractory. In reduction, it is a powerful flux.
Some colourant oxides are fluxes (copper, cobalt), and others are not
(nickel) are refractory. Iron is the only one I am aware of that is
both, depending on firing conditions.
Regards,
Tim
Dave Finkelnburg on thu 23 nov 06
Paul,
What you are witnessing is the limit of iron
solubility in your particular glaze.
To understand solubility, think of sugar in water,
as in syrup. If you put in enough sugar, with heat,
you can get the sugar to dissolve, but as the syrup
cools sugar precipitates out as crystals in the bottom
of the container. Who hasn't had syrup crystallize
like this at some time? The crystals form because
there is more sugar in the solution than the solution
can hold.
The same thing happens with iron. Up to 6% iron or
so, as a crude minimum, iron is soluble in most high
fire glazes. For your glaze the solubility is a bit
higher. Finally, though, with that bit extra iron you
went over the limit and iron crystals are now forming.
That is exactly how you make a matte glaze. You
supersaturated the glaze with iron.
Now you know how much iron to put in your glaze to
get an iron matte, or, how much iron not to put in if
you want to avoid an iron matte.
Solubility, by the way, is not necessarily (nor
even usually) a function of kiln atmosphere.
Good potting!
Dave Finkelnburg
--- Paul Borian wrote:
> I have two main versions of a slip glaze I make from
> local clay - about
> 50% clay and the rest is varied amounts of feldspar,
> silica and whiting
> and occasionally some talc. One version uses black
> iron and comes out
> like a temnoku; the other uses red iron and bone ash
> and is an iron red
> quite similar to Ohata red, which I also use. In
> fact I could probably
> substitute one for the other and most customers
> would not notice, that's
> how similar they look.
>
> Anyway, the iron red usually has about 8% RIO plus
> the natural iron in
> the clay and about 8% bone ash and I recently took
> a gallon of this
> glaze, set it aside and added another 1-2% RIO and
> the small bowl I used
> it on had a more matt surface than the glaze
> normally has. I did a
> similar test a while back and had the same kind of
> thing happen - a
> fairly glossy glaze came out more matt (or maybe
> satin matt is more
> accurate) when high amounts of red iron were added.
> I fired to cone 11
> in reduction and my kiln cools very slow.
>
> I would have thought RIO is more of a flux so I am
> trying to figure out
> whether maybe both of these samples I did were just
> in unusual parts of
> the kiln to get these results. Or is there some
> strange affect RIO can
> have on the surface of a glaze? Can anyone
> elaborate?
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Ivor and Olive Lewis on fri 24 nov 06
Dear Tim,=20
You say <refractory. In reduction, it is a powerful flux.>>
Are you able to explain the Chemistry behind this proposed reversal of =
properties ? Why would the addition of Carbon monoxide or Hydrogen =
enhance the ability of a glaze to melt.
Could it be possible that Paul Borian's results might be due to =
placement in the kiln where there was a cool spot or inadequate remixing =
of the bulk of the glaze from which he took his sample gallon. It is =
possible that the glaze is mature and slow cooling has resulted in =
crystallisation.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Vince Pitelka on sat 25 nov 06
Ivor wrote:
"Are you able to explain the Chemistry behind this proposed reversal of
properties ? Why would the addition of Carbon monoxide or Hydrogen enhance
the ability of a glaze to melt."
Goodness Ivor, why do you do this? As you well know, in reduction red or
ferric iron reduces to black or ferrous iron, which is a powerful flux.
This happens even in oxidation once you reach adequate temperature. In
reduction, iron is capable of reacting with silica at remarkably low
temperatures, and is a primary cause of carbon coring when there is early
reduction or inadequate air circulation in a bisque firing.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Timothy Joko-Veltman on sat 25 nov 06
On 11/24/06, Ivor and Olive Lewis wrote:
> Dear Tim,
>
> You say <>
>
> Are you able to explain the Chemistry behind this proposed reversal of properties ? Why would the addition of Carbon monoxide or Hydrogen enhance the ability of a glaze to melt.
Yes. Fe2O3 (which makes up most of what we call red iron oxide) is
refractory, while FeO is a flux. Thus, in reduction, where there is
an abundance of CO
Fe2O3 + CO -> 2FeO + CO2
This happens because CO is very reactive, while CO2 is not (viz., it's
not "hungry" for more atoms to fill any valencies). As a result, this
reaction is unlikely to take place in an oxidation/neutral
environment, where there is plenty of CO2 (from carbonates), but not
much CO floating about. And vice versa in a reducing environment,
where the flame is busy taking up oxygen from anywhere it can ...
including from CO2 (thus producing CO).
> Could it be possible that Paul Borian's results might be due to placement in the kiln where there was a cool spot or inadequate remixing of the bulk of the glaze from which he took his sample gallon. It is possible that the glaze is mature and slow cooling has resulted in crystallisation.
Sure could. But then, I didn't offer my response as the *only* cause
of what Paul is seeing. Just a beginning to understanding it. For
example, if it's a cool spot in an oxidizing kiln, the matteness is
likely to be even more exaggerated - but why? Because of the
refractory nature of Fe2O3, which caused the glaze to be slightly
under-fired. Finding a hot spot, or prolonging the soak, could fix
this. Crystallisation is also possible - and maybe not even because
of slow cooling - because most glazes can hold more iron in liquid
solution than they can when solid, and any cooling will cause iron
crystals to form. Then again, slow cooling COULD be the culprit,
especially in a high Ca glaze. But Paul didn't give us the glaze
recipe or unity formula, so we can't say much about that.
I don't want to pose as a Know-It-All (I know for sure that I don't) -
but I feel strongly that the Chinese saying is right: "Give a man a
fish, and feed him for a day. Teach him how to fish, and feed him for
a lifetime."
Regards,
Tim
Edouard Bastarache Inc. on sun 26 nov 06
Hello,
I just googled this, melting points :
1-Fe2O3 :1565 C
2-FeO : 1370 C
Hope this help in this discussion.
Later,
Edouard Bastarache
Le Français Volant
The Flying Frenchman
Sorel-Tracy
Quebec
edouardb@sorel-tracy.qc.ca
www.sorel-tracy.qc.ca/~edouardb/Welcome.html
http://perso.wanadoo.fr/smart2000/index.htm
http://www.pshcanada.com/Toxicology.htm
http://www.flickr.com/photos/30058682@N00/
Ivor and Olive Lewis on wed 29 nov 06
Dear Edouard Bastarache,
Yes, those temperatures are important.
Potters use the term "Flux" in a free and easy manner. As far as I care =
it just means that things flow, which they do if you can get them hot =
enough. So to make black Iron oxide flow you have to get the temperature =
up to 1370 deg C.
I support Vince in his claim that there is a reaction between Iron oxide =
and Silica. Iron Meta-silicate (2FeOSiO2) melts at 1205 deg C which =
seems to be ignored in the text books.
Best regards
Ivor Lewis.
Redhill,
South Australia.
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