Test Fired new Kiln: hot and cold spots

[mudified]

The first test fire of my new Skutt KM822 yielded uneven cone indications as shown in the photo of the pyrometric cones (04 target and 03 guard) left to right in photo equals top to bottom kiln shelves. I placed each  pair of cones (I don't have any 05 cones right now) about three inches from the brick, and the center shelf cones about three inches from the thermocouple.  All cones were facing toward the center).   I don't have safety glasses yet, so I didn't look into the spy holes to observe at what time in the firing the cones dropped; and maybe that should be my next step--it seems the kiln temperature was much higher than 1946°  despite what the controller display showed. I'm looking for tips to even out the results. Would a 5 to 10 minute hold at the end of the Cone Mode program make a helpful difference? Perhaps that and running another test with Cone 05 as the objective given the excessive drop of the overly bent target cone where Cone 04 was the objective? Guess I need to wait until I can obtain some 05 and 06 cones.

Or, am I overly obsessing?  Should I go ahead, I wonder, and do a bisque fire with the appropriate preheat and a 5 or 10 minute hold at the end? Thoughts anyone? 

Edited March 22, 2023 by mudified

[Bill Kielb]

Can you post the schedule, in this graph for the last hour or so it is firing on the order on 50 Degrees f per hour which seems be a bit odd for a cone fire schedule. The typical way would to hit a cone would be to fire about 100 f per hour for the last 200 degrees of the firing, so this seems different. Also can you tell us if this kiln was loaded or empty and why the top peep is out till the end of firing? Finally i assume this is top, middle, bottom left to right so do you have a shelf and spacers on the bottom of the kiln?

[PeterH]

24 minutes ago, Bill Kielb said:

Can you post the schedule, in this graph for the last hour or so it is firing on the order on 50 Degrees f per hour which seems be a bit odd for a cone fire schedule. 

 

Isn't that 120f in the last hour and 100f in the preceding 58m?

(1948-1828)f in (16:18-15:18)
(1828-1728)f in (15:18-14:20)

[Bill Kielb]

Yes,, I think it is! 60 degrees in the last 30 minutes. Bad math! 120 per hour more reasonable, maybe even a touch fast actually for evenness. Interesting, that makes more sense especially if an empty kiln. I wonder if that is slow, Medium, Fast cone fire? still want to know loaded or empty and if the order of the cones are correct. Expectation would be cooler top, then bottom, then middle based on radiation and convection but no way to know the element position / apportionment.

That graph is a bit odd though 14:58 - 15:18 (20 minutes) = 60 degrees. Not sure that data is accurate.

Edited March 22, 2023 by Bill Kielb

[neilestrick]

It looks to me like it fired about how I would expect a single zone to fire when empty- the middle ran a little hotter than the top and bottom. There's nothing you can do in the controller to even that out because all the elements cycle at the same time and there's only one thermocouple. You can try firing to a lower cone and using a hold to achieve your target cone because sometimes things will even out a little when holding,  but what I would do is pay attention to how you load the kiln. Load the middle really tight- lots of small pieces, plates on short shelves, lots of mass- and load the top and bottom lighter- bowls, tall pieces, things that take up a lot of space but don't have much mass.

[mudified]

I really appreciate the comments. Bill, I posted the schedule below out of the operating manual--it seems fairly similar to my log (except the schedule shows a five-minute hold at the end which I didn't do.)  I left the top peep open because the  operating manual indicated, "If your kiln is not equipped with a down-draft vent system ... Leave the top peep plug out throughout the entire firing."  Yes, I do have a shelf about an inch above the kiln floor; then two more shelves placed so that the cones could be set in the vertical center and a few inches below the lid. The firing speed was "medium". 

Neil, thanks for your recommendations! Likewise, I thought this morning about firing to Cone 05 and using a hold. After I get some safety glasses and the enough cones for a pack with a "guide, target, and guard" I'll attempt to actually "witness" the witness cones so that I can make any manual adjustments as the firing process draws toward it's end during the last half hour or so.  And I'll do what  you said about loading the kiln. Thanks again!

Edited March 22, 2023 by mudified

[Bill Kielb]

Since this was empty, I think Neil’s point is very relevant. I think you will need to load the kiln and test. I mention speed because slower generally means more even so given the choice between 120 and 108 per hour (Center column  Orton chart) is a good way to get predictable heatwork…… it is slower than 120 degrees so will tend to work more evenly across a single zone kiln.

Almost all the heatwork takes place in the last 200 - 250 degrees, prior to that, generally everything is reversible. Slower heating tends to even things out. At some point all burnout has occurred so leaving the top plug out the entire firing can cause extra losses / extra energy use in the top section.

Automatic cone firings tend to compensate for varying firing speeds and attempt to slide up and down the Orton chart depending on the speed the kiln can achieve for a given load. They can be helpful.