Skutt Pre-program Overfiring?

[MissyLee5]

First off, please forgive my lack of experience. I have a new Skutt KM822 kiln and have only done a few test firings as l'm new to firing my own work. I just remembered that the self supporting cones are supposed to end up even with the angled base, not kissing the shelf to indicate a perfect firing. Which means the built in cone fire program is overfiring according to the witness cones. 

Last week I did a glaze test, with about 10 test tiles on the middle shelf, firing to set to slow, with no hold or preheat. I had a self supporting ^6 on the middle shelf with a large ^5 (the large ^7 is from a previous firing with the same load and firing schedule, where the SS^6 also came out kissing the shelf) and as you can see, it overfired. If I measured correctly, the ^7 witness cone is at 40°. l'm not sure if this is happened because it was a near empty kiln firing on slow, or if 1 actually need an offset. The controller showed it reached 2226F in 14 hours and 1 minute. The glazes came out as expected, but I still don't want to be overfiring.

I went and checked the witness cones from my previous glaze tests, and they are the same. SS^6 just kissing the shelf with no additional bending or buckling. These were also fired to ^6 on the slow setting (some of the tiles were a brown bear clay body which can bloat if not fired slowly when going beyond ^5). All of the previous glazes also came out as expected, except one, and a few on brown bear pinholed. What would you say it's firing to, just under ^7?And what adjustments should I make? Thanks in advance! 

[Bill Kielb]

I would say a strong six or barely over six, almost darned near perfect. Maybe a degree or two or an extra minute or two. I would be pretty happy if my kiln fired that way all the time for that matter. You can do a small cone offset to get it to your liking, but I would do that based on a reasonably filled kiln, probably not a mostly empty one. Just wondering if the cone  was damaged at the base before firing?

Edited May 18 by Bill Kielb

[neilestrick]

It's quite close. Close enough that with variations from firing to firing it may actually come out perfect at times. If anything, do a thermocouple offset of +5, but like Bill said it's really not bad.

[Hulk]

Cone packs on all levels complete the picture?
...where some variation between levels is typical, in my (limited) experience.

Where the cone has bent to the point where the tip rests on the shelf, hard to say how much heat work there without a guard cone.
I'm not finding a resource indicating what a forty-degree bend in large cone seven means; this article has a cone ten depicted with o'clock positions and temperatures when rate is 60°C/Hour: Pyrometric Cone (digitalfire.com) ...forty degrees ~= 28°C

Where the target is cone five to "five and a half," bending the seven is too much for the buff and red clays I was using a while back.
No problem for the red clay I'm using now, and the white  and light buff clays are ok too...

Edited May 19 by Hulk
italics

[MissyLee5]

Thanks for the replies everyone. Glad to hear it's not much of an overfire. I figured kissing the shelf would be a big difference from level with the base for a self supporting cone. I'll try firing with a full kiln before I look at doing any offsets. 

 @Bill Kielb I don't believe the cone was damaged prior to firing. I have Advancer shelves and the cones stick slightly when fired directly on the shelf, so it took a bit off when I pulled the cone out.

@Hulk The red (plainsman M390) and white (standard 182) clays weren't bothered at all by the additional heat. The brown bear did pin hole on on several glazes I tried. I may have to try firing that one lower.

 

[Bill Kielb]

2 hours ago, MissyLee5 said:

Thanks for the replies everyone. Glad to hear it's not much of an overfire.

They sort of all follow that once gravity takes over they bend quickly within just a few degrees. From three o’clock on things happen pretty quickly

[MissyLee5]

@Bill Kielb Ah, I did not realize that. Very helpful diagram, thank you! 

[Hulk]

The diagram relates o'clock to degree angle - the units depicted are not temperature degrees.

O'clock to temperature would "have to" include rate, hence, o'clock (or degree angle) to temperature degree at a specified rate (temperature degrees per hour).

This is the only reference I've seen (err, remember seeing) that relates temperature rate to o'clock position.


units depicted: o'clock, angle degree, °C degree

Cone ten cone, at the specified rate (60°C/Hour), the temperature difference ~27°C (~50°F) at forty degree bend, is, err, appears to be significant.
By which, I'm suggesting the forty degree bend in the OP's seven cone may be significant.
Falling the rest of the way, as Bill points out, happens quickly.
How much more heat work is required to slump/deform the cone once the tip is supported by the shelf?