lewis ramage on sun 6 jun 04
I'm having a real problem firing to desired cone using my kiln =
controller.
I know my heating rate - it's 60C/hr at 1100-1200 dropping quickly to =
40C/hr
@1280.
My first firing to Cone 10 was set to 1280C and was way overfired Cone =
10
was a horizontal droopy blob - as would be expected with that slow =
heating
rate (which I couldn't know about until I'd monitored it, of course).
My second firing to Cone10 was set to 1240C and was a bit over fired, =
Cone
10 was bent right over. I had used the Orton Conecalc program to find =
that
temperature, and it should have been a good Cone 10: it was close =
enough. .=20
My third firing to Cone 6 was set to 1222C (60C / hr at that temp) and =
cone
7 was laying flat out - could have been Cone 9 or 10 I suppose. I used =
the
Orton "wallchart"
(http://www.ortonceramic.com/center/pdf_files/wall%20chart_degreeC.pdf) =
to
set that temperature, but it was clearly way wrong. =20
My fourth firing to Cone 6 set to 1209C (60C/hr) was a perfect Cone 8! =
I
had used the data in =
http://www.ortonceramic.com/center/pdf_files/60C.PDF to
set that temperature, as it was much lower than the one shown in the
wallchart! (Why????) but it was clearly way wrong.=20
(BTW, yet a 3rd Cone 6 temperature is shown in
http://www.ortonceramic.com/center/cone.chart.C.html as 1220 (OK, not so =
far
from 1222, but still different!)).
So: I am really at a loss now. There are two things I can think of, =
and
I'd really appreciate some guidance -
1) The ambient temperature around my controller will be say 40C (well =
above
30C anyway, which is what it is after several hours cooling). I fire
overnight in my garage. Is the thermocouple simply not taking account =
of
the ambient temperature? It should, I thought (it has some kind of
'compensation lead' at the end).=20
2) Am I supporting the cones properly (I can't quite decipher what I am
meant to do) - The clay support I press them into is less than =BD inch =
thick
- I think I saw on one of the Orton Charts that the 'regular cones' =
should
be "mounted" at 2" above base (footnote to
http://www.ortonceramic.com/center/pdf_files/ortoncones.pdf) , which is =
odd
as they only measure just over 2" anyway! =20
That's me completely baffled for now. I have no idea what temperature =
to
fire to to get Cone 6 now, so I'll try for 1182C when I do another test
firing tomorrow. So far I haven't dared fire anything but tests! =20
Any feedback would be most welcome!
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