Martin Schiller on wed 19 nov 97
Seems like the reduction/oxidation dialogue has run it's course, whether
by loss of inertia or careful moderation.
But I'm going to miss the "clay as analogue for physical science"
presence. I went to the dictionary this afternoon and tried to find a
chemical representation for alumina and aluminum, but all I found were
definitions. I don't know whether glaze formulae represent molecular
structure or atomic...or what the difference is between the two....but I
like the exchange that goes on about physical theory perhaps more than
other applications of the "clay as analogue school."
Not to say that I won't read stories about freezing clay (from MN?), but
I'd sure like to hear someone theorize about considerations for
"reducing" alumina.
regards, Martin
Cupertino, Calif
Gavin Stairs on thu 20 nov 97
At 05:42 PM 19/11/97 EST, Martin wrote:
....
>I'd sure like to hear someone theorize about considerations for
>"reducing" alumina.
Hi Martin,
Here's my bit.
In chemistry, oxidation means removing an electron from a cation (a metal,
for example), usually by means of an acceptor anion, like oxygen, hence the
name.
Reduction means the opposite.
This is the fundamental basis for what is called redox chemical analysis.
In general, this analysis is used for ionic chemistry.
In the case of a kiln atmosphere, if there is an excess of donor species
present, for instance oxygen in elemental form (O2 or rarely O, O3 or Cl2
or some other), the atmosphere is said to be oxidizing, since any cations
present will tend to react to move toward a more oxidized state. Thus, Fe
+ O2 -> Fe3O4 + O2 -> Fe2O3 + O2 -> FeO2 (nonstoichiometric - I haven't
balanced the equations).
If the species of cation is already in its most oxidized state, it will not
oxidize further. Thus Al2O3 + O2 -> Al2O3 + O2. Nothing happens.
However, just because there is no (excess) oxidation happening doesn't mean
the atmosphere is neutral. The atmosphere is still oxidizing if excess O2
is present.
One further note: Cl2 is more oxidizing than O2. Therefore Cl+ may replace
O++ in some cases, should there be an excess of Cl2 in the atmosphere.
This is of some relevance to salt glazing, since the chlorides tend to be
more mobile than the oxides.
The opposite case, a reducing atmosphere, is usually present in a kiln as
an excess of CO in the kiln atmosphere. In this case, CO accepts an
additional O from the oxide, leaving the oxide less oxidized (the reverse
of the above series in the case of iron).
A large excess of CO may result in the deposition of C by the reaction 2CO
-> C + CO2, but that should not be common. Most blackened ware is probably
the result of C deposited as soot. Black core is most probably heavily
reduced iron and iron oxide, along with a few other transition metals in
heavy reduction. You can test for heavily reduced iron with a magnet. See
if the black particles are picked up by a bar magnet.
That's about it. This means that the atmospohere in a ventilated electric
kiln is oxidizing, even though no oxidation reactions may be taking place.
In an unventilated electric kiln, the atmosphere may be less oxidizing,
since oxidation reactions may deplete the oxygen in the atmosphere, but it
seems a stretch to go all the way to reducing without adding some carbon
source, like a stick of wood. In any dynamic atmosphere, with circulation
of reducing and oxidizing gases, there may be pockets of heavier or lighter
reduction or oxidation. This is what makes a wood fired kiln so
interesting (along with the fly ash, of course).
Anyway, I hope this is of saome use.
Gavin
stairs@echo-on.net
http://isis.physics.utoronto.ca/
416 530 0419 (home) 416 978 2735 (work)
Toronto, Ontario, Canada
Bill Aycock on fri 21 nov 97
Ah- an opportunity to clarify a definition- namely:
"clay as analog for physical science"
Clay work, to be successful, must embody a lot of Physical science, whether
the practitioner calls it that or not. It is no analog- it IS.
When you try something, then change your process based on the results- then
refine your understanding-and improve your understanding as you go -
That IS science, Lab coat or not.
Science is a dynamic process of understanding, using what you believe, and
changing what you do, based on the results you get. Its no big mystery,
except to those who fear it. And--- its great fun.
At 05:42 PM 11/19/97 EST, you wrote:
>----------------------------Original message----------------------------
>Seems like the reduction/oxidation dialogue has run it's course, whether
>by loss of inertia or careful moderation.
>But I'm going to miss the "clay as analogue for physical science"
>presence. I went to the dictionary this afternoon and tried to find a
>chemical representation for alumina and aluminum, but all I found were
>definitions.
Aluminum is an element, which merely means it is a substance that cannot be
separated into smaller parts by chemical means. It is a metallic element,
which has to do with the way it is malleable, conducts electricity, and
creates positive Ions in solutions. (Yes, Marc, Ions can be created without
heat) Generally metals are lustrous. (cribbed directly from the CRC handbook)
Alumina is an oxide of aluminum, and is whitish in color. The name of the
oxide of many metals is made by shortening the end of the name of the
metal, and adding "a" eg, Baria is an oxide of Barium, and Soda is an
oxide of Sodium. (I know, baking soda, washing soda and soda pop are not
just oxides, but, its a mixed up world out there.)
Now, with the definitions out of the way, what did you really want to know
about Alumina and Aluminum?
Bill- feeling his Oats, on Persimmon Hill
Bill Aycock --- Persimmon Hill --- Woodville, Alabama, US 35776
--- (in the N.E. corner of the State) ---
W4BSG -- Grid EM64vr --
baycock@hiwaay.net
Gavin Stairs on sat 22 nov 97
Hi All,
A correction to my last post. Cameron Harmon was kind enough to mail me
off-list with the following:
Gavin,
I liked your analysis. Your were right on about everything,
except the black core. What happens is that the Fe2O3
reduces to Fe3O4 and reacts with the SiO2 to yield an iron
silicate. There is not enough reduction to get elemental
iron. Besides with all that heat it would steal O2 from
something else and revert to an oxide anyway.
....
You might be interested to know that as far as ceramics are
concerned, you can get enough reduction for achieving glaze
features with almost no CO. Certainly less than 0.5%. So
your comments about the spot reduction that occurs is
exactly right when compared to industrial research on the
subject.
regards,
Cameron
--
**********************************************************
Cameron G. Harman, Jr. 215-245-4040 fax 215-638-1812
e-mail kilns@kilnman.com
Ceramic Services, Inc 1060 Park Ave. Bensalem, PA 19020
see our web site at http://www.kilnman.com/potters.html
THE place for total kiln and drier support
**********************************************************
I think he's right on both points. In particular, with regard to lustres,
the amount of material which needs to be reduced is very slight, amounting
to less than a micron in thickness. This can be accomplished with a very
slight reduction atmosphere. I think the lesson here is that these are
subtle effects, and more can be accomplished with a feather than a sledge
hammer. Too much may blacken your heart as well as that of your pots.
Gavin
stairs@echo-on.net
http://isis.physics.utoronto.ca/
416 530 0419 (home) 416 978 2735 (work)
Toronto, Ontario, Canada
Martin Schiller on sat 22 nov 97
In a message of 11/21/97 9:58 AM Bill Aycock baycock@HiWAAY.net said......
>Now, with the definitions out of the way, what did you really want to know
>about Alumina and Aluminum?
>
>Bill- feeling his Oats, on Persimmon Hill
If you'd have quoted just a few more lines, Bill, you'd have got it. But
it isn't anything important. I was just trailing bait to lure a few more
bytes from the redux thread.
But I stand by "clay as analog". As the weather progresses into deeper
winter we may see the "clay as analog for life" position posited on
freezing and aging.
regards, Martin
Cupertino, Calif
Karl P. Platt on mon 24 nov 97
Alumina is routinely reduced in producine Aluminum. Doing so requires
extremely high temperatures -- 4,000 F-ish--and very strong reduction.
The furnaces used rely on graphite electrodes and consume *enormous*
amounts of electricity. The dams on the Columbia River, for example, were
built to feed the aluminum mills nearby.
It is not possible to reduce aluminum in the chilly sorts of conditions
that exist in studio ceramic firings.
KPP
Joseph Herbert on wed 26 nov 97
karl is right about alumina, but there is more -
The Hall process for refining aluminum ore (Alumina) does require lots of
electricity but that is not sufficient. The mineral cryolite - sodium
aluminum fluoride (?) (doing this from memory) is necessary as a flux. Prior
to the discovery of the flux jewelry was made from the metal just on the
basis of its rarity. In addition, the very tippy end of the Washington
Monument was of this special metal. From having the rarity to crown the
monument to a great early leader to having the ubiquity to litter all our
roadsides tells one of the stories of the impact of our technological
century.
Joe
JJHerb@aol.com
Gavin Stairs on thu 27 nov 97
And the standard aluminum ore is bauxite, which is a hydrated alumina.
Alumina is one tough nut to crack. Gavin
At 09:02 AM 26/11/97 EST, you wrote:
>----------------------------Original message----------------------------
>karl is right about alumina, but there is more -
>
>The Hall process for refining aluminum ore (Alumina) does require lots of
>electricity but that is not sufficient. The mineral cryolite - sodium
>aluminum fluoride (?) (doing this from memory) is necessary as a flux. Prior
>to the discovery of the flux jewelry was made from the metal just on the
>basis of its rarity. In addition, the very tippy end of the Washington
>Monument was of this special metal. From having the rarity to crown the
>monument to a great early leader to having the ubiquity to litter all our
>roadsides tells one of the stories of the impact of our technological
>century.
>
>Joe
>JJHerb@aol.com
>
>
stairs@echo-on.net
http://isis.physics.utoronto.ca/
416 530 0419 (home) 416 978 2735 (work)
Toronto, Ontario, Canada
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