daniel on thu 25 mar 04
Hi John,
I have completed reading your slides. Great talk, really interesting to
read about eletric kiln firing. Not least because I never do it. I did a
little analysis of your firing schedule to see what it looked like and it
prompted a number of questions which I hope you don't mind addressing.
I drew a graph of your firing schedule in Excel to see what it looked like.
It was a lot longer than I thought it would be, about 14 hours. Here are the
questions.
1. It seems that the 100F rise in the first section is primarily aimed at
free water burnoff without risk of damage or popping off the glaze. Is that
correct or is there more to it ?
2. I am intrigued by the slow down before peak temp for about 1 hour 45 min
or there abouts. Is this simply to allow a soft glaze to melt more
completely ? It seems that you could continue your 350 ramp to 2185 and then
hold the soak a little longer instead, so I presume there is some difference
in result perhaps, or am I reading too much into this ?
3. Cooling. At peak it seems that the natural cooling rate of your kiln
must meet or exceed 500 F/hr - unless your force cooling it somehow. I
realise that cooling of a hot body is substantially faster at higher temp.
Are you firing down over this period or naturally cooling to 1900 F ?
4. I assume that by 1400 when you shut off that the glaze is essentially
"set". Is that correct ?
5. Finally, I am guessing that the kiln controller uses a number of
thermocouples distributed about the kiln to determine what is going on and
to know when to turn on/off your elements. Does it have several or is it
using just one and the radiant nature of the element heating is even enough
given the allowance for packing etc. ?
Apologies for the perhaps seemingly pointless questions but my mother
always said I asked too many questions :)
Thanx
D
John Hesselberth on thu 25 mar 04
Hi Daniel,
See my comments below.
On Thursday, March 25, 2004, at 07:57 PM, daniel wrote:
>
>
> 1. It seems that the 100F rise in the first section is primarily aimed
> at
> free water burnoff without risk of damage or popping off the glaze. Is
> that
> correct or is there more to it ?
That is correct. I'm just being extra cautious. You could probably
speed this up significantly.
>
> 2. I am intrigued by the slow down before peak temp for about 1 hour
> 45 min
> or there abouts. Is this simply to allow a soft glaze to melt more
> completely ? It seems that you could continue your 350 ramp to 2185
> and then
> hold the soak a little longer instead, so I presume there is some
> difference
> in result perhaps, or am I reading too much into this ?
I'll make no claim to have optimized this. It is what works for me.
There are lots of profiles that will work through this zone. Read the
page on Ken Russell; he does his soak at 1900. What is important is to
give the glaze enough time to degas and get the level of glaze
migration or intermingling you want. If you have multiple glazes and
want more intermingling, soak longer at higher temperatures. There is
no right way to fire a kiln. You have to figure out what is right for
your glazes and the effects you are trying to achieve. What I am trying
to point out is that you must learn that if you want high quality
results. Those who turn their firing over to a KilnSitter are limiting
themselves in ways they don't realize--unfortunately most people firing
electric kilns do turn their firing over to a KilnSitter.
>
> 3. Cooling. At peak it seems that the natural cooling rate of your kiln
> must meet or exceed 500 F/hr - unless your force cooling it somehow. I
> realise that cooling of a hot body is substantially faster at higher
> temp.
> Are you firing down over this period or naturally cooling to 1900 F ?
Naturally cooling. My small kiln will drop about that fast, but the big
one cools a bit more slowly. Nothing much happens in this temperature
range on the way down so I don't think it is critical
>
> 4. I assume that by 1400 when you shut off that the glaze is
> essentially
> "set". Is that correct ?
I haven't found significant crystal growth in a cone 6 glaze below this
temperature. 'Set' might be too strong a word. Glazes are not really
solid glasses until about 1000-1200F
>
> 5. Finally, I am guessing that the kiln controller uses a number of
> thermocouples distributed about the kiln to determine what is going on
> and
> to know when to turn on/off your elements. Does it have several or is
> it
> using just one and the radiant nature of the element heating is even
> enough
> given the allowance for packing etc. ?
My big kiln has a 3 zone controller; my small one is single zone.
Regards,
John
http://www.frogpondpottery.com
http://www.masteringglazes.com
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