Cameron Harman on thu 13 nov 97
I have been trying really hard to keep my mouth shut on this
one and use the subject for more discussion in a future
e-zine. Now that Bill has nailed it I can breathe again.
After all there is so much to say on the subject that it is
almost impossible to put enough in a small coorespndence and
make it clear. Bill has done a credible job of that.
Well stated, Bill.
Another way to say the same thing:
A kiln atmosphere is either oxidizing or reducing, as a
practical consideration it is not neutral. However, you can
create a neutral atmosphere in an electric kiln if it is
well sealed and a special gas, such as nitrogen is
introduced under pressure. That is how they make a nitride
bonded silicon carbide refractory, for example. For potters
this is not a useful consideration.
The reactions inside the kiln ,for the most part, don't know
or care if the energy source is fuel, electricity or the sun
for that matter. If a chemical needs oxygen for a reaction
and *must* have it, it will take it from the oxides of the
nearby ceramics. That is where common black coring comes
from. The only reason this occurs more readily in an
electric kiln is because the atmosphere is relatively still
and there is little fresh oxygen coming into the area of the
particular piece.
If you have a lot of smaller pieces crowded together, for
example, they may need more oxygen that the still air of the
electric kiln can deliver to the central part of the load.
The outside of the load may be OK since there is a lot more
air nearby. The same thing can, and does, happen in gas
kilns.
The problem with windy days is that they change the draft on
the uncontrolled kilns most often used by potters. Changing
the draft during a reduction burn is often enough to change
the kiln's internal atmosphere from a reducing atmosphere to
an oxidizing atmosphere. In fact it is likely to be a local
effect. that is the atmosphere surrounding the outer parts
may shift, but the inner parts may not, or some other
peculiar event like that.
A properly designed kiln will have automatic draft
protection in order to maintain the correct pressure inside
the kiln at a constant point. Herein lies one of the main
differences between an art potter and a production potter.
While both are artists, the production potter often has a
need to produce ware having a very similar appearance time
after time. This requires more control and more careful kiln
design. The art potter is often interested in a certain
effect, but is not terribly bothered when something
interesting occurs that was unplanned.
If you have a customer demanding a certain thing, you need
to be sure you are going to get it, hence the need for
better design and better control. It is neither a good
thing nor a bad thing, it is simply a realization of a
subtle, but important difference that exists.
With all the talk about the construction of kilns, I need to
remind you that there are fewer than six persons in the
United States that really understand the elements of heat
transfer and kiln design well enough to design a proper
kiln. None of these persons has written a book, they are all
too busy working at it to write about it.
On the other hand there are dozens of persons designing and
selling kilns. You draw your own conclusions.
--
**********************************************************
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
**********************************************************
Ron or Sue Corl on fri 14 nov 97
Cameron,
I guess we all should assume, according to your commments, that the
Fredrick Olsen kiln book should be completely disregarded as invalid?
I am certain that plenty of 'amateurs' have built wonderful kilns by
accident and by study.
I am not expert bashing here. Just asking for some slack because we all
do not have to be experts to do things well.
Thanks,
Ron
Big Baby Head Studio
> With all the talk about the construction of kilns, I need to
> remind you that there are fewer than six persons in the
> United States that really understand the elements of heat
> transfer and kiln design well enough to design a proper
> kiln. None of these persons has written a book, they are all
> too busy working at it to write about it.
WardBurner@aol.com on sat 15 nov 97
Folks,
Here's the problem....we disagree on the definition of oxidizing. I do not
know what definition of oxidizing atmosphere ya'll are refering to.
Here is the definition I use and the basis of my position.
North American Combustion Handbook. Vol. I
"oxidizing flame = oxidizing fire, a lean flame or fire; that is, one
resulting from combustion of a mixture containing too much air or too little
fuel. This kind of flame produces an oxidizing atmosphere."
There. That's it. Bingo. This is what I'm talking about.
Without combustion, you do not have an oxidizing atmosphere!
Give me your definition and the source.
Marc Ward
Ward Burner Systems
PO Box 333
Dandridge, TN 37725
USA
423.397.2914 voice
423.397.1253 fax
wardburner@aol.com
Michael W Greenfield on sun 16 nov 97
Well, here goes my two cents on this one. Please keep in mind that this
is coming from someone who has no specific training, but a common working
knowledge of basic chemical reactions. Also, I have no background in
glaze chemistry other than following a glaze recipe. ( Although I do
desire to learn as much as I can regarding glaze chemical reactions to
formulate my own).
Ward, I think you can't limit the definition of an oxidizing atmosphere
to one that requires a flame source. True, you can have an oxidizing
flame, but flame does not an atmosphere make.
In combustion, the fuel is rapidly combined with oxygen producing what is
basically oxidized fuel (both gaseous & solid, depending on the fuel),
heat and flame(plasma). The heat/plasma whatever, serves as a catalyst
for continuous combustion(oxidation) of your fuel source. So, yes, the
ratio of fuel to oxygen determines the type of atmosphere obtained in a
fuel fired kiln.
>From what I've been able to follow in this thread. In a fuel fired
kiln, you can only have 2 types of atmospheres. Oxidation & Reduction
(to avoid argument I am omitting neutral since no potter is willing to
match moles of gas and oxygen to obtain perfect combustion). In
Oxidation, all fuel is consumed with an excess of oxygen which passes
through the kiln having a 'neutral' effect on the glaze. (It seems from
what I've read on this thread so far that glaze chemicals have all the
oxygen, chemically, that they will ever need (please, some one, correct
me if I'm wrong)). In Reduction, an insufficient amount of oxygen is
provided causing an incomplete combustion. Now what happens is a
partially combusted (CO) atmosphere moves through the kiln looking to
complete or stabilize its state by finding oxygen to react with (and
bring wonderful Carbon into your glaze).
In an electric kiln, there is no plasma or source of fuel, only oxygen &
heat. Now using the above reference to this thread, the oxygen in the
kiln would have a neutral effect on the glaze chemicals. However, this
is an Oxidizing atmosphere.
An oxidizing atmosphere to me is a situation where oxygen exists in a
state able to bond (oxidize) with other elements. This describes the
atmosphere of an electric kiln. To create an even greater oxidizing
atmosphere, higher concentrations of oxygen and/or heat must be added.
This would increase the rate of oxidation of any material not impervious
to oxidation.
>From what I understand about chemical reactions. The only difference
between the oxidation atmospheres of a fuel and electric kiln is a fuel
kiln is filled with plasma and the "waste" solids and/or gasses as the
result of combustion. But both have the critical element Oxygen.
Michael Greenfield
A Lease Administrator
who has neither a gas nor electric kiln
In Upstate NY, digging out of 1 1/2 feet of the white stuff...
On Sat, 15 Nov 1997 09:50:27 EST WardBurner@aol.com writes:
>----------------------------Original
>message----------------------------
>Folks,
>
>Here's the problem....we disagree on the definition of oxidizing. I do
>not
>know what definition of oxidizing atmosphere ya'll are refering to.
>
>Here is the definition I use and the basis of my position.
>
>North American Combustion Handbook. Vol. I
>
>"oxidizing flame = oxidizing fire, a lean flame or fire; that is, one
>resulting from combustion of a mixture containing too much air or too
>little
>fuel. This kind of flame produces an oxidizing atmosphere."
>
>There. That's it. Bingo. This is what I'm talking about.
>
>Without combustion, you do not have an oxidizing atmosphere!
>
>Give me your definition and the source.
>
>Marc Ward
>Ward Burner Systems
>PO Box 333
>Dandridge, TN 37725
>USA
>423.397.2914 voice
>423.397.1253 fax
>wardburner@aol.com
>
| |
|
