WardBurner@aol.com on thu 16 may 96
Hey Vince and All,
Vince questioned my post about oxidation in an electric kiln. (I would
repost, but already trashed it.) Anyway a note was sent to both of us off of
ClayArt that I'll repost without signature. I figure they didn't want to post
to the list as a whole with signature. Here's Vince's post, then a response
from another ClayArter, plus a few more thoughts on the subject.
Vince wrote:
<<
WHOA! An electric kiln is a neutral atmosphere, neither oxidizing nor
reducing??? I respect your level of knowledge, and appreciate all that you
post to the list, but I stumbled on this one. An electric kiln is not
hermetically sealed, so oxygen is always entering the kiln, and whenever an
atmosphere contains abundant free oxygen, it will oxidize any compound
chemically able to accept more oxygen atoms. And of course, heat greatly
accelerates the process. How about all this?
- Vince>>>>
I received this note:
<<<continuously vented by forced air fan, then the internal air is largely
stagnant once the moisture comes out of the pots and slowly leaves the
kiln by the buildup of internal pressure (gas law effects). Most
commercial electric kilns are surprisingly quite well sealed when the
lid/door is closed. And so a non-vented kiln will likely not suck in fresh
oxygen as the temperature rises, perhaps only as the kiln cools, so any
oxidative activity would be rather slight and superficial (on the surface
molecular layers only). So the kiln atmosphere, as Marc W says, is
essentially neutral (non-oxidative) during the heat-rising stage, the
phase of firing that has the major effect on glaze formation.>>>
The key here is combustion. Combustion is a violent process on the molecular
level.
Oxygen, which almost always exists with two atoms bonded together, combines
with carbon and hydrogen (hydrocarbons) to produce carbon dioxide and water.
One other thing is produced; HEAT. This heat is produced by the liberating of
the energy that held the molecules together. No matter is created, no matter
is destroyed, just a slight change of arrangments. So, what this has to do
with electric kilns is this; The tiny bit of oxygen that enters the kiln and
what is already there is bound in twos (hence O2). This form of oxygen is
nowhere as reactive as a single atom stripped out of the molecule. This
single atom's main goal is to find a molecular bond. You can get single
oxygen atoms (oxidation) roaming around in a gas kiln because there isn't
enough carbon dioxide or hydrogen to bond with. It's pretty tough to make
this happen in an electric kiln. The opposite of this is reduction. You have
all this extra carbon & hydrogen looking for their missing oxygen buddy to
help them form water (H20) and carbon dioxide (CO2). It's not in the
atmosphere, so they break the bonds in the oxides (iron oxide, copper oxide,
manganese dioxide, ect.) and steal the oxygen leaving behind elemental
metals. They can make this theft because all these molecules are vibrating
(heat) like crazy and the bonds between atoms are weaker.
So this is the difference between a neutral atmosphere in an electric kiln
and an oxidation atmosphere in a gas kiln. The heat in the electric kiln
excites the molecules but without the violent molecular process of
combustion, these oxygen molecules stay bonded and far less reactive.
Marc Ward
Ward Burner Systems
PO Box 333
Dandridge, TN 37725
423.397.2914 voice
423.397.1253 fax
wardburner@aol.com
Don Jung on mon 20 may 96
Marc, Tom, Vince,
Your responses to the question regarding oxidation in an electic brought
on some additional questions for me:
Does air circulation play a significant role in an electric kiln firing?
I'm under the impression that a certain amount of air circulation will
improve heat distribution and the lack of good circulation may cause
warping in some pieces and glazes to to be 'cloudy' or dull. Is this
correct?
In a normal firing, gases of many kinds are released from the clay and
glazes, most will find there way out where there is any kind of opening.
I suspect that these gases, some of them very corrosive, will cause the
metal parts of the kiln to deteriorate...explains the sheet metal that's
starting to crumble, flake off and rust out. Is this also correct and
would a vent/exhaust improve or eliminate this?
Then, assuming we put an exhaust vent and circulate air through an
electric kiln firing...would this change the neutral atmosphere to a true
oxidizing one? (not that it matters much it seems)
I'm on a roll...if I were able to control the 'air' into an electric kiln
(using an intake vent combined with the exhaust vent) and substitute
carbon monoxide or some other gas (other than a combustible hydrocarbon),
would it create a reducing affect on any of the glazes? I suspect
combustion may still be necessary to cause the oxygen atoms to get
stripped away from the oxides, but are there any reactions through less
volatile means?
Questions, questions, I hope one day to provide a few answers to.
thanks
Don Jung
dojun@axionet.com
Vancouver B.C. Canada
(where we're still getting teased with a bit of sun now and again.)
WardBurner@aol.com wrote:
>
> ----------------------------Original message----------------------------
> Hey Vince and All,
>
> Vince questioned my post about oxidation in an electric kiln. (I would
> repost, but already trashed it.) Anyway a note was sent to both of us off of
> ClayArt that I'll repost without signature. I figure they didn't want to post
> to the list as a whole with signature. Here's Vince's post, then a response
> from another ClayArter, plus a few more thoughts on the subject.
>
> Vince wrote:
> <<>
> WHOA! An electric kiln is a neutral atmosphere, neither oxidizing nor
> reducing??? I respect your level of knowledge, and appreciate all that you
> post to the list, but I stumbled on this one. An electric kiln is not
> hermetically sealed, so oxygen is always entering the kiln, and whenever an
> atmosphere contains abundant free oxygen, it will oxidize any compound
> chemically able to accept more oxygen atoms. And of course, heat greatly
> accelerates the process. How about all this?
> - Vince>>>>
>
> I received this note:
> <<<> continuously vented by forced air fan, then the internal air is largely
> stagnant once the moisture comes out of the pots and slowly leaves the
> kiln by the buildup of internal pressure (gas law effects). Most
> commercial electric kilns are surprisingly quite well sealed when the
> lid/door is closed. And so a non-vented kiln will likely not suck in fresh
> oxygen as the temperature rises, perhaps only as the kiln cools, so any
> oxidative activity would be rather slight and superficial (on the surface
> molecular layers only). So the kiln atmosphere, as Marc W says, is
> essentially neutral (non-oxidative) during the heat-rising stage, the
> phase of firing that has the major effect on glaze formation.>>>
>
> The key here is combustion. Combustion is a violent process on the molecular
> level.
> Oxygen, which almost always exists with two atoms bonded together, combines
> with carbon and hydrogen (hydrocarbons) to produce carbon dioxide and water.
> One other thing is produced; HEAT. This heat is produced by the liberating of
> the energy that held the molecules together. No matter is created, no matter
> is destroyed, just a slight change of arrangments. So, what this has to do
> with electric kilns is this; The tiny bit of oxygen that enters the kiln and
> what is already there is bound in twos (hence O2). This form of oxygen is
> nowhere as reactive as a single atom stripped out of the molecule. This
> single atom's main goal is to find a molecular bond. You can get single
> oxygen atoms (oxidation) roaming around in a gas kiln because there isn't
> enough carbon dioxide or hydrogen to bond with. It's pretty tough to make
> this happen in an electric kiln. The opposite of this is reduction. You have
> all this extra carbon & hydrogen looking for their missing oxygen buddy to
> help them form water (H20) and carbon dioxide (CO2). It's not in the
> atmosphere, so they break the bonds in the oxides (iron oxide, copper oxide,
> manganese dioxide, ect.) and steal the oxygen leaving behind elemental
> metals. They can make this theft because all these molecules are vibrating
> (heat) like crazy and the bonds between atoms are weaker.
> So this is the difference between a neutral atmosphere in an electric kiln
> and an oxidation atmosphere in a gas kiln. The heat in the electric kiln
> excites the molecules but without the violent molecular process of
> combustion, these oxygen molecules stay bonded and far less reactive.
> Marc Ward
> Ward Burner Systems
> PO Box 333
> Dandridge, TN 37725
> 423.397.2914 voice
> 423.397.1253 fax
> wardburner@aol.com
Tom Buck on mon 20 may 96
Don J: My view is that by venting the kiln via a forced air system one
distributes the heat more uniformly, and that in turn avoids some pots
being erratically heated and therefore subjected to stress and warping
especially in a fast firing (a rise of over 150C/300F deg/hour). Also,
some of the corrosive off-gases being removed quickly will certainly help
to prevent metal corrosion. But, as Marc W says, the chemical activity
(ie, the gas/solid interaction) in an electrically-fired kiln is low or
near-zero and thus does not affect the glaze colour, etc.
Good potting. TomB Hamilton Canada
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