ivor & olive lewis on wed 11 nov 09
Dear Snail Scott ,
Thanks for you note.
Has it been explained what the actual cause of element failure is ?
I have some ideas that are in deep memory. Has to do with thermal cycling.
I seem to recall that repeated heating and cooling gradually changes the
crystalline structure of a metal. This means more crystal to crystal
interfaces into which oxygen can diffuse. These extra layers of oxide reduc=
the effective cross sectional area through which electricity is conducted.
Resistance increases and temperature rises. Eventually the wire overheats
At the Seaham pottery they had electric car kilns. Their elements used to
fail so they would use Oxy Acetylene to weld up the fracture, if need be
inserting a few coils of salvaged element.
Snail Scott on wed 11 nov 09
On Nov 11, 2009, at 12:03 AM, ivor & olive lewis wrote:
> I seem to recall that repeated heating and cooling gradually changes
> crystalline structure of a metal. This means more crystal to crystal
> interfaces into which oxygen can diffuse. These extra layers of oxide
> the effective cross sectional area through which electricity is
> Resistance increases and temperature rises. Eventually the wire
> and melts.
This is exactly my understanding as well - development
of crystallization which impairs flow of electricity and
results in excessive heating of the wires and connections.
After the second wall fire (each time resulting in replacement
of the stuff with more of the same), I wrote to the electrical
supervisor in the Maintenance office, and explained in detail
all the stuff that I thought was relevant. (The guy who came
out with the repair work order didn't seem to believe me.) The
supervisor immediately understood the implications of having
infinite switches, especially when I mentioned that they might
cycle a thousand times or more during a day. The #6 wire
might have been adequate (if marginal) otherwise, at 20 or 30
feet from the box, but connections are always more vulnerable,
and that's where it burned every time. The fuses never blew -
it was the connection to the fuse box that overheated and set
the wall on fire.
There's a time and place to throw a hissy-fit, and I decided that
after the second fire might be it. ;)
Fuzzy Chef on wed 11 nov 09
Huh. So it sounds like doing straight glaze firings NOT using my
electronic controller would prolong the life of my coils.
Mind you, I don't think I can live without the controller, but it's
something to think about ...
Michelle Hamilton on fri 13 nov 09
I do a slow rise from 2000 to 2280. I set it for 100 per hour and it slo=
ws on its own to=3D20
approx 50 around 2200-2280. I then hold for 15 minutes and 8, 9 are down=
and 10 is half=3D20
way. Works well!
On Tue, 10 Nov 2009 16:29:00 +1030, ivor & olive lewis T.COM.AU>=3D20
>Would I be right in assuming that by restricting the rate of temperature=
>rise during the last couple of hundred degrees, cones in an electric kil=
>would mature at a lower temperature. If this is true then the life of th=
>elements would be extended yet the benefits of high thermal work would =
>Was reading Olsen's Kiln Book this morning. On Elements for Kilns. Some
>pretty complex ideas to grasp. But he does say that larger diameter wire=
>will deliver a higher temperature.
ivor & olive lewis on sat 14 nov 09
Dear Michelle Hamilton,
Thank you for this comment....
<slows on its own to approx 50 around 2200-2280. I then hold for 15 minutes
and 8, 9 are down and 10 is half way. Works well!>>
So your pyrometer is reading about 100 degrees F below the official maturit=
temperature of Orton Cone 10, supporting my proposition.
jonathan byler on mon 16 nov 09
temperature is based on the rate of climb. you will reach a higher
final temp for ^10 climbing at 500F/hr instead of 270F/hr. or a lower
final temp if you climb slower.
this doesn't rule out the thermocoulple reading in error, though...
my experience with type K thermocouples is that this is ALWAYS the
case at elevated temps. they are rated for about 2400F, but they also
burn up real fast at that temp.
On Nov 13, 2009, at 11:48 PM, ivor & olive lewis wrote:
> Dear Michelle Hamilton,
> Thank you for this comment....
> <> it
> slows on its own to approx 50 around 2200-2280. I then hold for 15
> and 8, 9 are down and 10 is half way. Works well!>>
> So your pyrometer is reading about 100 degrees F below the official
> temperature of Orton Cone 10, supporting my proposition.
> Best regards,
> Ivor Lewis,
> South Australia