Help! with old electric kiln firing

Hi,

I have a small (approx. 30 x 30 x 30cm internal) old electric kiln. I
think it is about 30 yrs old and fully refurbished approx. 7 yrs ago.
I fitted a digital controller last year with an adjustable first ramp
and then on full to top temp.

My question is, currently I have been firing only to 1000C - 1060C,
with a first ramp of 50C an hour (bisque) or 150C an hour(glost) to
300C then on full. When it reaches 300 it goes up about 100 degrees in
about 10 minutes! and recording the temp regularly shows it slows down
massively towards the end of the firing like a 'S' shape on a graph. I
have no control over the second ramp or cooling phase so I wanted to
know if this is normal? or putting excess stress on the pieces? Should
my first ramp be a lot longer to control more of the firing?

I really want to go to 1280C and make some lovely stoneware glazes but
I feel it is already struggling quite a lot to reach 1060 and am
afraid of damaging elements or overfiring; if it takes +4 hours from
900 - 1280C.

Oh, I forgot to mention it runs on a 13 amp plug so am aware that it
might not 'Have the power'!

Any help or advice would be massively appreciated!

Thanks,

Julia

As you said, it might not be getting the electricity it needs. I have one
kiln that is a 19 amp kiln on a 20 amp circuit. It can't get to it's max
temp. and I burned the power cord off of it.

--
Connie Ryman
Cryman Studio

"Julia" wrote in message
om...
Hi,

I have a small (approx. 30 x 30 x 30cm internal) old electric kiln. I
think it is about 30 yrs old and fully refurbished approx. 7 yrs ago.
I fitted a digital controller last year with an adjustable first ramp
and then on full to top temp.

My question is, currently I have been firing only to 1000C - 1060C,
with a first ramp of 50C an hour (bisque) or 150C an hour(glost) to
300C then on full. When it reaches 300 it goes up about 100 degrees in
about 10 minutes! and recording the temp regularly shows it slows down
massively towards the end of the firing like a 'S' shape on a graph. I
have no control over the second ramp or cooling phase so I wanted to
know if this is normal? or putting excess stress on the pieces? Should
my first ramp be a lot longer to control more of the firing?

I really want to go to 1280C and make some lovely stoneware glazes but
I feel it is already struggling quite a lot to reach 1060 and am
afraid of damaging elements or overfiring; if it takes +4 hours from
900 - 1280C.

Oh, I forgot to mention it runs on a 13 amp plug so am aware that it
might not 'Have the power'!

Any help or advice would be massively appreciated!

Thanks,

Julia

Hi Julia,

You have two problems here.

A kiln of that size (about 1 cubic feet, in the old measurements) would
need at least 15 amps to get to 1280 deg C, depending on the construction.
If you are drawing less than 13 amps from the supply, you will be struggling
to get to top temp. You need to check what current the kiln is actually
drawing - you probably need to contact an electrician for that. You say the
kiln is old - what is the kiln made from? Most kiln materials from 30 years
ago used very inefficient insulators, and would require heaps of power to
get to 1280 deg C.

Also, the controller you fitted is not really suitable for the job - you
ideally need at least two ramps. To make the best of what you have, you
should program the first ramp to 600 deg C so that you don't over-stress the
pieces, particularly during bisque firing.

Cheers

Dave



"Julia" wrote in message
om...
Hi,

I have a small (approx. 30 x 30 x 30cm internal) old electric kiln. I
think it is about 30 yrs old and fully refurbished approx. 7 yrs ago.
I fitted a digital controller last year with an adjustable first ramp
and then on full to top temp.

In message , Julia
writes
Hi,

I have a small (approx. 30 x 30 x 30cm internal) old electric kiln. I
think it is about 30 yrs old and fully refurbished approx. 7 yrs ago.
I fitted a digital controller last year with an adjustable first ramp
and then on full to top temp.

My question is, currently I have been firing only to 1000C - 1060C,
with a first ramp of 50C an hour (bisque) or 150C an hour(glost) to
300C then on full. When it reaches 300 it goes up about 100 degrees in
about 10 minutes! and recording the temp regularly shows it slows down
massively towards the end of the firing like a 'S' shape on a graph. I
have no control over the second ramp or cooling phase so I wanted to
know if this is normal? or putting excess stress on the pieces? Should
my first ramp be a lot longer to control more of the firing?

I really want to go to 1280C and make some lovely stoneware glazes but
I feel it is already struggling quite a lot to reach 1060 and am
afraid of damaging elements or overfiring; if it takes +4 hours from
900 - 1280C.

Oh, I forgot to mention it runs on a 13 amp plug so am aware that it
might not 'Have the power'!

Any help or advice would be massively appreciated!

Thanks,

Julia

My electric kiln is exactly the same size as yours, and the element is
rated at a little over 3.5 kW. (Actually each time I replace the element
I nudge the power rating up a little. Clive Shellard at Roman
Environmental Instruments in Bath is very obliging, and winds elements
exactly to specification.)

My firing cycles a
Bisque: 100C/hr to 600C, then flat out to 1000C.
Glost: 150C/hr to 600C, then flat out to cone 7.

Like you, I have only the one controlled up-ramp. I always put a cone 7
somewhere in each glost firing, just to check that all went well. And
although the cone says that I have fired to cone 7, my controller is set
to switch off at an indicated 1200C. Whether this is because the
thermocouple is getting tired, or the controller calibration is a little
awry, I don't know: but the stoneware cone 7 glazes turn out OK.

My firings last about 8 hours for bisque, and 10 hours for glost.

I would have thought that your cycle, of switching to full on at 300C,
is too early. All the authorities suggest that 600C is a more suitable
temperature to start the rapid rise. And 50C/hr seems excessively
cautious, unless you're firing very thick items.

I have just checked my latest firing graphs, which I plotted after I
treated the kiln with ITC100. On both types of firing, my temperature
shoots up by more than 100C per hour when the controller switches to
full on at 600C, but the pots don't seem to mind. And by the time we
reach nearly 1200C, the rate of rise is about 50C per hour.

Hope you can make sense of these ramblings . . .
--
Jake Loddington, POULTON-LE-FYLDE, Lancashire, England.

To add to Jake's excellent response, note that the
assumption is that your mains are 220/240 volts.
Probably a safe assumption since you use Celsius.
13 amps would be too low for 110/120 as in the USA.

Also, any kiln slows down on an S-shaped curve as it
nears its top temperature. (Well, not really S-shaped,
but sorta like the top of the S anyway.) This is an
exponential curve, where the rate of rise is proportional
to the difference between the max temperature and
the current temperature. The closer you get to the max,
the slower the rise, until it becomes essentially flat at max.

The max temperature is directly related to the amount
of insulation as well as power input, so if you really need
a little more temperature you might consider if there is a way to add
insulation, maybe a ceramic fiber blanket.

But you also probably don't need full 1280C (cone 10) either.
Cone 6 (1200C or so) can give really nice results, and there are
tons of glazes for this range.

Best regards,




Bob Masta
dqatechATdaqartaDOTcom

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