Temperature vs heatwork - Kiln setting

[Edda]

Hello all,

I've been getting some mixed results from my electric kiln lately (front loading, Pottery Crafts - 2015). 

I'm firing to 1260° C and using witness cones (Orton 7 & 8). Trusting the cones I've been shutting the kiln off at 1220 - 1230° C of late. Feels like a big gap between controller temp and heatwork.

The controller setting is:

1. 120 to 600

2. 250 to 1260 

The kiln seems to struggle from 1000, really slows down, and takes a long time to reach target temp. This might be impacting the heatwork? Maybe 250 is too low?

I've been reading up on this and it seems like many set their cut off time of a 1260 firing to 1235 (Clayman Supplies) - 1250 (Stuart Carey), sometimes with a long soak. I find this very confusing. 

Any insight would be highly appreciated.

Fyi. The elements are fairly new and we had the kiln looked over by a professional, who found no fault with it.

Hope this makes sense.

Thanks!

[Bill Kielb]

Just my take on this: Heat work is time and temperature. Not so much the entire firing but during the period where the fluxes help the melting process. So when does that happen? Orton solved this by making cones from glaze and calibrating them to bend in the last 100c of the firing based on a firing rate and peak ending temperature. Prior to that, the fluxed reaction was not really happening to a significant extent.

So for predictability and the important part of heat work, the rate in the last 100c was where the important heat work was going to take place. The published ending rates to make Ortons cones bend at the predicted peak temperature: 15, 60, and 150 C per hour in the last 100C of the firing.

 

This worked out great because these rates , especially 60C (center column) were achievable at the end by most kilns and ceramic wares heated pretty evenly at this rate. Orton did not calibrate a column for an ending speed of 250C per hour. Most kilns can’t keep this rate at the very end, often go some random speed that they can achieve, and going this relatively high speed does not improve the evenness of the temperature of the wares.

When I follow the published Orton rates and use the corresponding peak temperature in the last 100c , my cones bend consistently and predictably with the right amount of heat work.  Most home kilns can make 60c per hour at the very end and when they wear by 10% or more begin to struggle so 60c is a decent achievable ending rate for most kilns.

Peak temperature schedules, ending soaks, really high speed firing can work, but most kilns really don’t maintain that speed. If you follow the Orton ending speed, my experience you get predictable heat work results. On the other hand, you are free to go whatever speed your wares will tolerate throughout most of the firing, so 250 c per hour or more may be fine except the last 100c, go the published Orton rate.

So in the end I agree and find ALL those schedules very confusing and more difficult than just following the Orton tested stuff. Still, there are folks that make it work for them some way.

Since your kiln is experienced, it’s likely a good time to measure the element resistance. Most kilns when their elements have risen in resistance by 10% or more need new elements to achieve reasonable end of firing rates.

Edited February 26 by Bill Kielb

[Edda]

Thank you Bill, that's very helpful.

[Babs]

1 hour ago, Edda said:

Thank you Bill, that's very helpful.

250°C way too fast!

I'd go 150°c / hr to about 80° below shut down   then  slow down to 80° C / hr to target temp.

The lower target and soak at end you write about is prob more economic fuel wise and kinder on your kiln, some glazes will love it too.