Joanne L. Van Bezooyen on mon 19 jul 99
When I wanted to do this, my university leader told me I would ruin the
coils in my electric kiln. What do you think?
Joanne in Tucson
Larry Phillips on tue 20 jul 99
Joanne L. Van Bezooyen wrote:
> When I wanted to do this, my university leader told me I would ruin the
> coils in my electric kiln. What do you think?
I think he is absolutely right. The coils last as long as they do
because they are cooled slowly. The rapid thermo-cycling of raku will
ruin the coils in no time.
--
Did you know that "prickly heat" has nothing to do with
the location of the rash?
http://cr347197-a.surrey1.bc.wave.home.com/larry/
Ben Shelton on thu 22 jul 99
How would it ruin them? If you simply fire in the kiln and reduce elsewhere,
what could it hurt??
-----Original Message-----
From: Joanne L. Van Bezooyen
To: CLAYART@LSV.UKY.EDU
Date: Monday, July 19, 1999 1:33 PM
Subject: firing raku in electric
>----------------------------Original message----------------------------
>When I wanted to do this, my university leader told me I would ruin the
>coils in my electric kiln. What do you think?
>
>Joanne in Tucson
>
Tom Buck on thu 22 jul 99
Dear Mr Phillips:
Pray tell, where in the Kanthal Corp manual does it say what you
just declared as the gospel on Clayart?
Lifetime of elements (Kanthal A-1 alloy is most common) is
strictly a function of time-temperature -- the higher you fire and the
times it takes to do so will decide the lifetime, see the Kanthal graph in
the manual.
If you cycle very marginally at 900 oC (+/-) it is unlikely to
alter in any significant way the lifetime of the elements at 1000 oC,
usually rated above 800 full cycles (on/off).
And if one allows more raw air to enter the kiln by raising the
lid frequently at 900+ oC, the elements are essentially unaffected since
the Kanthal alloy is designed specifically to be inert to oxygen - there
is an impervious Al2O3 (alumina) coating on the surface of the element
wire.
There really is only one hazard in using an electric kiln for Raku
-- the danger of striking the hot (and soft) elements with the Raku tongs
used to remove hot pots. Some have voiced worries over being electrocuted,
but since we rakuers use heavy mitts, we are well insulated; we won't be
damaged by the electricity, but the elements and the kiln could be if we
bang into elements.
And finally, as has been noted previously by many, the main reason
NOT to use a standard top-loading electric is to avoid replacing the lid
more ofen than one would wish.
Tom Buck ) tel: 905-389-2339
(westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
Michael Banks on fri 23 jul 99
I tend to agree with Ben. How can rapid cooling harm Kanthal elements? It
may well do this in practice, but does anyone know a good technical reason
why?
I've always regarded the failure of heating elements to be mainly due to
loss of metal mass. This I've attributed to the boiling off of metal vapour
from the surface at high temperatures. The higher the temperature, the
higher the vapour pressure of the metal. This can be viewed as a direct
positive relation between temperature and the proportion of atoms of the
element alloy exceeding the particular escape velocity of the composition.
Over 1300 degrees C, the proportion of atoms exceeding escape velocity for
say, Kanthal A1 becomes significant and element life becomes poor.
Oxidation firings result in a refractory oxide coat which may protect the
elements by effectively increasing the threshold escape velocity. Reduction
firings remove this protective coat, which can be replaced by subsequent
oxidation firings, but some of the metal originally bonded to the oxygen is
probably lost.
One possible mechanism of crash-cooling hurting elements, might be the
cracking of the oxide coat by thermal shock. This cracking might penetrate
into the element along metal crystal boundaries, as elements are known to
become increasingly brittle tempered (more crystalline) from firing cycles.
But does anyone actually know if crash-cooling hurts element life?
Michael Banks,
Nelson,
NZ
Ben Shelton wrote:
> How would it ruin them? If you simply fire in the kiln and reduce
elsewhere,
> what could it hurt??
> -----Original Message-----
> From: Joanne L. Van Bezooyen
> To: CLAYART@LSV.UKY.EDU
> Date: Monday, July 19, 1999 1:33 PM
> Subject: firing raku in electric
>
>
> >----------------------------Original message----------------------------
> >When I wanted to do this, my university leader told me I would ruin the
> >coils in my electric kiln. What do you think?
> >
> >Joanne in Tucson
> >
>
Julia M. Townsend on sun 25 jul 99
I have a friend who does electric kiln raku all the time. He says the only
thing that really is damaged is the lid... from the rapid cooling, it will
cause it to crack and then it will become weak. He seems to think its worth it
for him.
Julie
Michael Banks wrote:
> ----------------------------Original message----------------------------
> I tend to agree with Ben. How can rapid cooling harm Kanthal elements? It
> may well do this in practice, but does anyone know a good technical reason
> why?
>
> I've always regarded the failure of heating elements to be mainly due to
> loss of metal mass. This I've attributed to the boiling off of metal vapour
> from the surface at high temperatures. The higher the temperature, the
> higher the vapour pressure of the metal. This can be viewed as a direct
> positive relation between temperature and the proportion of atoms of the
> element alloy exceeding the particular escape velocity of the composition.
> Over 1300 degrees C, the proportion of atoms exceeding escape velocity for
> say, Kanthal A1 becomes significant and element life becomes poor.
>
> Oxidation firings result in a refractory oxide coat which may protect the
> elements by effectively increasing the threshold escape velocity. Reduction
> firings remove this protective coat, which can be replaced by subsequent
> oxidation firings, but some of the metal originally bonded to the oxygen is
> probably lost.
>
> One possible mechanism of crash-cooling hurting elements, might be the
> cracking of the oxide coat by thermal shock. This cracking might penetrate
> into the element along metal crystal boundaries, as elements are known to
> become increasingly brittle tempered (more crystalline) from firing cycles.
> But does anyone actually know if crash-cooling hurts element life?
>
> Michael Banks,
> Nelson,
> NZ
>
> Ben Shelton wrote:
> > How would it ruin them? If you simply fire in the kiln and reduce
> elsewhere,
> > what could it hurt??
> > -----Original Message-----
> > From: Joanne L. Van Bezooyen
> > To: CLAYART@LSV.UKY.EDU
> > Date: Monday, July 19, 1999 1:33 PM
> > Subject: firing raku in electric
> >
> >
> > >----------------------------Original message----------------------------
> > >When I wanted to do this, my university leader told me I would ruin the
> > >coils in my electric kiln. What do you think?
> > >
> > >Joanne in Tucson
> > >
> >
Christopher J. Anton on tue 27 jul 99
(Previous writer had mentioned problems with lids cracking as a result of
rapid cooling in electric kiln raku firing.)
It just struck me that there may be a solution available. Has your friend
considered building a fiber lid for use in raku firing? At school we had
a couple of electric kilns whose lids' hinges had died. We still used the
original lids, but a well-designed fiber lid could have also been used?
| |
|
