James Freeman on fri 6 nov 09
I just read the technical column in last month's CM wherein it was stated
that reducing in an electric kiln will shorten element life by stripping
away the protective oxide coating. I have heard and accepted this same
caveat since I began in ceramics, but now am starting to wonder if this is
true.
I understand and accept that the elements form a protective oxide coating.
I understand and accept that burning organic stuff in the kiln requires
oxygen, and that if it cannot get enough from the atmosphere, it will
attempt to pull it from glazes, elements, whatever. My pondering centers
around this question:
If a reducing atmosphere is indeed created, and if the combustion does
indeed pull oxygen from the element's protective oxide coating, then rather
than "stripping" this coating from the element, wouldn't it actually conver=
t
the oxide back into metal, just as reduction of a copper raku glaze or a
luster converts the oxide into metallic copper or gold or whatever? If thi=
s
is the case, and I believe it is, then wouldn't this actually have no net
effect whatsoever on element life, as the new metal will again turn to oxid=
e
in the next firing, or just in humidity?
Is there some science that I am missing? Has anyone ever tested this
contention side by side, two kilns with identical elements, one fired
normally and the other fired with combustibles, then measure element
resistance over time, or is this just logical-sounding hearsay?
Thanks.
...James
James Freeman
"All I say is by way of discourse, and nothing by way of advice. I should
not speak so boldly if it were my due to be believed."
-Michel de Montaigne
http://www.jamesfreemanstudio.com
http://www.flickr.com/photos/jamesfreemanstudio/
http://www.jamesfreemanstudio.com/clayart/
James Freeman on sat 7 nov 09
I have received a couple of off-list responses to my question from some
actual, credentialed science folk, both saying the same general thing. Thi=
s
lead me to some further reading, and I thought I would share what I have
learned with anyone so interested.
First, there seems to be no actual study or experimental evidence on the
topic of element degradation in a reducing atmosphere, at least none that I
could find, so this does seem to be hypothesis (conjecture) rather than
theory. In a reducing atmosphere, the element's oxide coating would indeed
be reduced back to metal, but this is where the fun begins. The problem
centers around a phenomenon called "metal dusting", which is a type of
corrosion caused by carbon rather than oxygen. Once the metal is exposed,
graphite (pure carbon) is pulled from the carbon monoxide from the burning
reduction material (a simplification), and is deposited onto the metal
surface. This graphite coating pulls atoms of iron from the heating elemen=
t
and forms iron carbide molecules (Fe3C). As things progress, the carbon
leaves the carbide molecule, leaving the iron atoms free, but these iron
atoms are now separated from the metal of the element and exist as a
metallic "dust" on the surface, hence the name "metal dusting". (There is
also a form of metal dusting that does not involve the intermediate
formation of carbides, but this does not meaningfully change things in our
discussion.) This metal dust is then subject to mechanical dissociative
forces, and one respondent suggested that it would also re-react with carbo=
n
monoxide to form iron carbonyl, which is highly fugitive. (If I have
misunderstood anything you have said, you have my sincere apologies.)
Upon further reading on the fascinating topic of metal dusting, I learned
that this corrosive process is highly temperature dependent. It operates i=
n
a range from 300 degrees C (572 F) through 1050 C (1922 F), with the
meaningful portion occurring between 300 C (572 F) and 850 C (1562 F).
Outside of this temperature range, metal dusting does not occur. Given
this, it seems that one could reduce in an electric kiln at a temperature i=
n
excess of 1562 and avoid most of the corrosion problem, or at a temperature
above 1922 and avoid all of it. I further learned that in industry, the
easiest methods of avoiding metal dusting is to alter the temperature of th=
e
process such that it is outside of the critical range, or to introduce stea=
m
into the atmosphere. Thus it would seem that one could perhaps burn moist
combustibles even within the critical temperature range and avoid or
minimize the problem (conjecture on my part).
Someone asked me why I even want to reduce in an electric kiln in the first
place. The honest answer is that I'm not sure I do want to. I had always
accepted the potters' folk wisdom that reduction would destroy the elements=
,
and since I am too cheap and too lazy to be changing elements all the time,
I had simply eliminated this process from my range of possible aesthetic
exploration. Electric kiln reduction was "off limits". If, in fact, the
deleterious effects of burning crap in an electric kiln can be mollified,
then this is just one more possibility returned to consideration; one more
arrow in the quiver, if you will. Always good to have options.
If anyone has information to add, I would sincerely appreciate it. If not,
I shall return to my fascinating study of the physics of a rolling bowling
ball. I had assumed that one simply rolled the ball at the pocket then sat
down and drank another beer, but it seems there is so much more involved.
Very interesting stuff.
All the best.
...James
James Freeman
"All I say is by way of discourse, and nothing by way of advice. I should
not speak so boldly if it were my due to be believed."
-Michel de Montaigne
http://www.jamesfreemanstudio.com
http://www.flickr.com/photos/jamesfreemanstudio/
http://www.jamesfreemanstudio.com/clayart/
On Fri, Nov 6, 2009 at 10:52 AM, James Freeman m
> wrote:
> I just read the technical column in last month's CM wherein it was stated
> that reducing in an electric kiln will shorten element life by stripping
> away the protective oxide coating. I have heard and accepted this same
> caveat since I began in ceramics, but now am starting to wonder if this i=
s
> true.
>
> I understand and accept that the elements form a protective oxide coating=
.
> I understand and accept that burning organic stuff in the kiln requires
> oxygen, and that if it cannot get enough from the atmosphere, it will
> attempt to pull it from glazes, elements, whatever. My pondering centers
> around this question:
>
> If a reducing atmosphere is indeed created, and if the combustion does
> indeed pull oxygen from the element's protective oxide coating, then rath=
er
> than "stripping" this coating from the element, wouldn't it actually conv=
ert
> the oxide back into metal, just as reduction of a copper raku glaze or a
> luster converts the oxide into metallic copper or gold or whatever? If t=
his
> is the case, and I believe it is, then wouldn't this actually have no net
> effect whatsoever on element life, as the new metal will again turn to ox=
ide
> in the next firing, or just in humidity?
>
> Is there some science that I am missing? Has anyone ever tested this
> contention side by side, two kilns with identical elements, one fired
> normally and the other fired with combustibles, then measure element
> resistance over time, or is this just logical-sounding hearsay?
>
> Thanks.
>
> ...James
>
> James Freeman
>
> "All I say is by way of discourse, and nothing by way of advice. I shoul=
d
> not speak so boldly if it were my due to be believed."
> -Michel de Montaigne
>
> http://www.jamesfreemanstudio.com
> http://www.flickr.com/photos/jamesfreemanstudio/
> http://www.jamesfreemanstudio.com/clayart/
>
>
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