Firing not matching the firing schedule

[myrtle]

Hi folks, 

I'm doing a medium speed glaze firing in my Skutt 1027.  This will be its 8th firing, new kiln, new elements. I watch the firings (maybe a bit too closely) and take notes. The medium speed cone 6 firing should take 8.5 hours according to the firing schedule profile spreadsheet I was given by Skutt. My last glaze firing took 10 hours to hit peak temperature. Tonight it seems to be lagging behind from the start.

I'm an currently 1.5 hours into the firing and, according to the schedule, it should be at 250 F but instead it is at 160F.  Ambient starting room temperature was 70F. 

Is it common for a kiln in good condition to deviate from the schedule this much? Am I just being neurotic and watching it too closely? 

Skutt ConeFire Firing Profiles copy.pdf

[Bill Kielb]

2 hours ago, myrtle said:

Is it common for a kiln in good condition to deviate from the schedule this much? Am I just being neurotic and watching it too closely

Always good to check the kiln condition, especially amperage / wattage if you have the ability. I will say, kilns are generally marginally powered and even new I rarely see  one that can strictly keep up with a medium or fast firing schedule. This becomes real obvious when you load them full of plates and shelves. A lot of the reason they are marginally powered has to do with the typical electric service available and the simple brick construction, with  lots of mass and lots of losses. No particular brand BTW, it’s kind of an industry thing. If you have the Skutt that tells you the amperage and it’s basically spot on the design then that’s all the power you have and it cannot keep up. In my experience It would not surprise me, even if new.

IMO, The good news is you understand schedules far better than most. Very few people pay that close attention.

[neilestrick]

Give it a chance. There's always some variation, especially as the elements are still warming up. If the total time is taking longer, check your voltage to make sure it isn't low, and amperage draw.

[myrtle]

Thanks as always for the good advice you two! The total firing time only took an hour longer than scheduled, I can probably chalk that up to the dense load and using lots of short posts. It looks like my kiln's diagnostic menu will show me the amperage so run that test once I've unloaded the wares. 

[neilestrick]

13 minutes ago, myrtle said:

Thanks as always for the good advice you two! The total firing time only took an hour longer than scheduled, I can probably chalk that up to the dense load and using lots of short posts. It looks like my kiln's diagnostic menu will show me the amperage so run that test once I've unloaded the wares. 

Yes, a dense load will likely run a little long.

[liambesaw]

The only time my kilns have ever adhered strictly to the schedule is when they are completely empty.

Amazing things that will drastically lengthen your firing are a preheat (60 or 100f/hr to 200), and packing your kiln very tightly or using many shelves.

So my last glaze firing was a "fast glaze" to cone 6, but with a preheat and hold at 200 for 2 hours.  It took 13 hours to complete which is actually really quick for my kiln, because it was fairly loosely loaded with large sculptures.

Without the preheat, it would only have been 9 hours, which is still far longer than the controller allots for a fast glaze cycle, but far less than my kiln with a full load (has taken 16 hours).

[Bill Kielb]

19 minutes ago, liambesaw said:

Without the preheat, it would only have been 9 hours, which is still far longer than the controller allots for a fast glaze cycle, but far less than my kiln with a full load (has taken 16 hours).

Don’t feel bad actually. Think of it this way, most kilns will not fire effectively after the elements have worn 10%. Brand new they probably cannot make the fast glaze schedule which I think is under five hours. So if you have a 10000 watt kiln losing as little as 1000 watts generally dooms the kiln till elements can be changed. So the design of the kiln was 110% at a schedule slower than medium speed. Not great actually. We don’t design rockets this tightly. Decent insulation has to be on the horizon ...... I hope,......... or sorry, you didn’t make it to orbit this time because there was a westerly breeze.

[myrtle]

1 hour ago, liambesaw said:

The only time my kilns have ever adhered strictly to the schedule is when they are completely empty.

Amazing things that will drastically lengthen your firing are a preheat (60 or 100f/hr to 200), and packing your kiln very tightly or using many shelves.

So my last glaze firing was a "fast glaze" to cone 6, but with a preheat and hold at 200 for 2 hours.  It took 13 hours to complete which is actually really quick for my kiln, because it was fairly loosely loaded with large sculptures.

Without the preheat, it would only have been 9 hours, which is still far longer than the controller allots for a fast glaze cycle, but far less than my kiln with a full load (has taken 16 hours).

Thanks for the scoop on your firing times! Always nice to compare kiln performance with other fellow kiln watchers.