Help! with old electric kiln firing

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.

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

In message , Julia writes

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 are: 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 . . .

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.

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 D A Q A R T A Data AcQuisition And Real-Time Analysis