I believe I over-fired

[JohnnyK]

I have used this firing program before without problems, but for some reason, the glazes on most of the pieces were darker than usual and the cones shown in the photo give me the sense that instead of ^6 I'm looking closer to ^7. Can you give me any insight?

[Dick White]

Yep, well past 6, almost 7. What is your firing schedule?

[neilestrick]

Almost 7 on top, 6 on bottom.

What model kiln, and how was it packed?

[JohnnyK]

The firing schedule was: 

Ramp1: 100*/hr  to 200*

     “    2: 350* to 1045*

     “    3: 50* to 1100*

     “    4: 325*to 1976*

     “    5: 200* to 2200*

Hold:  20 min

Normal cooldown 

Total firing time was 9 hours and 25 minutes which was consistent with previous firings of this schedule.  

The kiln is a Cress FX-23-P, which is a ^10 kiln that has never been fired above ^6. It has an Orton Auto Fire AF4000 digital timer.

As for packing...the bottom and middle shelves had some large bowls and mugs, while the top shelf was fairly tightly packed with lotion bottles. The only real difference here was that I had mistakenly punched in ^4 for the bisque firing instead of ^04. I wonder if that would affect the final outcome of the glaze firing.

[Hulk]

Interesting!

My guess, bisqued to a higher cone may (almost certainly) have some influence on the final product*, however, I'm not seeing how it would influence the glaze firing's heat work.

My second guess, that twenty minute hold once 2200°F is achieved would push to the next cone; what I don't get is why it hadn't previously...
 

*glaze thickness comes to mind, however, likely you'd already adjusted for that!

[Babs]

Yes the higher bisque is prob main reason if not the only one. Your ware would not have the same thickness of glaze and so would take on a different appearance. See the recent post re mistakenly firing to cone 5 bisque by Elaine Clapper

Are your cones significantly different than previous firings?

[Dick White]

The reason I asked for your firing schedule is I suspect a technical nuance is at play here. Cones deal in heatwork - time and temperature. Nothing new with that idea. If the ramp rate in the final 2 hours is aggressive (relative to the Orton table, which depending on who you ask, what book you read, or what kiln controller you have, medium speed to cone 6 is 120F/hr to 2232F), the cone bends at a higher final temperature, and vice versa, a slower ramp will bend the cone at a lower temperature. Nothing new with that idea either. Your ramp 5 is set for 200F/hr to 2200F, which is pretty aggressive. Putting that into the Orton cone calculation spreadsheet, that would give you a cone 5.5, and even the 20 minute hold doesn't quite get to 6. However, the nuance that isn't always seen is that as the elements wear with usage, the actual ramp rate attained by the worn elements falters (and in the most extreme case, simply can't heat the kiln in the final segment and the controller errors out). While the cone-fire programs in the Bartlett and Orton controllers have an adaptive feature that monitors the actual final ramp rate and adjusts the target temperature to produce a good cone bend, the custom ramp hold programs do not have that capability. In ramp hold, the controller will keep chugging until the programmed temperature is reached regardless of how long that might take (unless it determines the rate is futile and it errors out). As the elements wear, the final ramp rate slows down and the cone should bend at a lower temperature, but the controller keeps going until the programmed temperature, and now the cone is overfired. Going back to the Orton spreadsheet, a ramp of 100F/hr to 2200 with a 20 minute hold should give you a cone 7, which it did. So, my suspicion is that your elements are wearing out, the programmed 200F/hr final ramp rate is bovine droppings, it's actually only making 100F/hr, and from here, it will get worse. Check the resistance of the elements and be ready to replace them as they drop too far out of spec. In the short term, drop you final temperature to 2185F and that should give you a nice cone 6 when the kiln is chugging along at the best it can do.

[Bill Kielb]

you may find this helpful
Just a late add here, @Dick White summed it up well. For the final segment ….. generally folks use the center column of Orton and most kilns can maintain with elements in reasonable condition.. So 108f per hour is a nice final segment speed in the last 180f or so to hit the predicted cone at the corresponding peak temperature within that column. (See chart) The added benefit of reduced speed in the final segment is the tendency for firings to be more even compared to higher speeds.

Edited November 14, 2023 by Bill Kielb

[JohnnyK]

I appreciate all of your input! I've had the feeling that my elements may be fading and will make the time to check their resistance and, if necessary, replace them. Then I will move on to doing the testing necessary to re-establish a firing pattern.

[Dick White]

27 minutes ago, Bill Kielb said:

[snipped a bit] generally folks use the center column of Orton and most kilns can maintain with elements in reasonable condition.. So 108f per hour is a nice final segment speed in the last 180f or so to hit the predicted cone at the corresponding peak temperature within that column. (See chart) [snipped some more]

Yes, the Orton table shows 108F/hour for the medium speed. Bartlett programs the cone-fire profiles at 120F/hr. That's why I caveated my comment with "depending on who you ask..." I have no clue why Bartlett is using a different rate, but my kilns seem to bend the cones just right at 120F/hr. And yes, I know they are going at 120F/hr - I have Genesis controllers and I download the log files to keep track of exactly what the the kiln is doing.

Edited November 14, 2023 by Dick White
Added last sentence. No substantive change.

[Bill Kielb]

2 hours ago, Dick White said:

Yes, the Orton table shows 108F/hour for the medium speed. Bartlett programs the cone-fire profiles at 120F/hr.

Yes of course. I thought your explanation spot on. I mention 108f  because if all else fails look at the chart and follow it! regardless I have seen kilns wear enough from 120 degree speed to usually over fire and eventually error out. 120 is fine and is part of Bartlett cone fire design. Eventually though many kilns won’t even do 120.  Too slow often ends up ugly. 200 is probably pretty hard for most kilns to hit unless new or lightly loaded. 2200 is sort of a cone 5 finish at 200 degrees per hr final segment. Add a 20 minutes hold for good measure. Lots of compensations from Ortons design can lead to reduced consistency.

Edited November 14, 2023 by Bill Kielb

[Dick White]

@Bill KielbYup, and it gets worse. The 120F rate is embedded somewhere in all the Bartlett cone fire profiles with 108F only in a few slow bisque profiles. At least the cone-fire method has the adaptive algorithm to fall back on for an appropriate final temperature. Then we have the all-famous Skutt pinhole solution of a slow cool manual ramp-hold program circulating all over the interwebs that specifies the 120F rate to the top temp. And to make it worse yet, Skutt has removed the slow cool checkbox from their version of the Bartlett touchscreen. Their earlier 12-key Kiln Master had the standard Bartlett option to append a stock slow cool schedule onto a cone-fire, but now with their touchscreen, one must construct a manual ramp-hold schedule with whatever slow cool you want at the end. Is it better to give the user a quick all purpose slow cool checkbox on the cone-fire profile that has the adaptive function, or a fully user-customized slow cool preceded by a recommended 120F ramp to a specific temperature that will likely fail? Ackkkk...

[JohnnyK]

What would be the best source for new elements should they have to be replaced?

[neilestrick]

1 hour ago, JohnnyK said:

What would be the best source for new elements should they have to be replaced?

Check the manufacturer first. If they're super expensive then check Euclids.