Kiln shelf at top of kiln.

[HenryBurlingame]

Does anyone here put a kiln shelf at the very top of their electric kilns over all of the ware? I am wondering if this would help add a few degrees to the top of my kiln. The top of my kiln is just a tad cooler than the rest of it and I am wondering if there is any way to heat it up a bit without having to resort to thermocouple offsets. I would rather not have to mess with offsets if I don’t absolutely have to.

[Hulk]

Good question.

I've:
Put more mass in the middle, which should take longer to heat, and hence the top and bottom get more heat work while they wait around.*
Loaded less mass in the top and bottom.
Put thinner wares, also smaller wares in top.
Had the lowest/bottom level set tall.
Run the half shelf stagger so there's a tall open half at the top, which faces a "balcony" half shelf and a good part of the next level down - where there's lots of radiating room.

That last bit seems to work, well, help - keep the difference closer.

*In the fully manual kiln, one pyrometer reading the middle section.
In the three zone numerically controlled kiln, I still load the middle a bit heavier.
The file shows that all three levels reached peak about the same, but the top and bottom cooled off faster.

Yeah, the top and bottom also cool faster. With less mass, the drop is even faster, hmm.

[Min]

By adding a covering shelf at the top of the stack you are adding a lot of mass to heat.

I seem to remember you have a 3 zone kiln? If so, TC offsets are great to bump up or down one section of a kiln if that section consistently fires off. For bisque firing  If you are about 1/2 cone cool on the top shelf of pots and you have at least 5 or 6” to the lid then a  negative offset of 10 - 12 F would be somewhere to start the adjustment for a cone 6 glaze firing. I use a different kiln for my bisque firings, if you are bisque firing in the same kiln use some cones to check how much higher that is taking the top zone, might not be significant. 

[neilestrick]

Nothing wrong with thermocouple offsets. That's why they're available. Use thermocouple offsets to adjust evenness between the sections, use cone offsets for over all firing temp accuracy. I've got TC offsets in all of my kilns.

[Jeff Longtin]

Are you packing your kiln tight? How you pack the kiln has a great impact on how well it fires. Areas where its firing cool you might try putting more space between the pots. (fewer pots in other words)

 

 

[elaine clapper]

I have always put a top shelf over my ware when I glaze fire, not always when I bisque fire.

 

[HenryBurlingame]

15 hours ago, Hulk said:

Good question.

I've:
Put more mass in the middle, which should take longer to heat, and hence the top and bottom get more heat work while they wait around.*
Loaded less mass in the top and bottom.
Put thinner wares, also smaller wares in top.
Had the lowest/bottom level set tall.
Run the half shelf stagger so there's a tall open half at the top, which faces a "balcony" half shelf and a good part of the next level down - where there's lots of radiating room.

That last bit seems to work, well, help - keep the difference closer.

*In the fully manual kiln, one pyrometer reading the middle section.
In the three zone numerically controlled kiln, I still load the middle a bit heavier.
The file shows that all three levels reached peak about the same, but the top and bottom cooled off faster.

Yeah, the top and bottom also cool faster. With less mass, the drop is even faster, hmm.

Yeah, this last bisque fire I loaded the kiln with more mass in the middle, with lowest level set tall, etc., etc. following all recommendations I could find from L&L and Skutt etc. But my kiln has three zones and a downdraft vent, and with a three zone kiln everywhere seemed to reach the final temperature at once but then top and bottom cooled off faster so I think middle got more heatwork. So I don't think the recommendations really apply to three zone kilns. The top especially cooled off faster since I think it loses a lot of heat through the lid and probably the downdraft vent that I have makes it worse up there since I think most of the air comes in around the lid. Hence me thinking maybe a shelf at the very top over everything could solve this issue (and probably NOT following recomendations to load up the middle more and instead just keep everything even everywhere would help as well). Only drawback would be then I wouldn't have one glorious kiln-post-free shelf at the top lol.

Edited October 4 by HenryBurlingame

[Min]

Why not have the vent shut off after peak temp reached? Having the vent on isn't going to effect the top shelf of pots underfiring.

[neilestrick]

2 hours ago, HenryBurlingame said:

then top and bottom cooled off faster so I think middle got more heatwork. So I don't think the recommendations really apply to three zone kilns.

This is the case in all kilns. The middle cools slower because it's trapped between the top and bottom. The top cools fastest because of heat loss out the lid. If you need them to cool at the same rate, program a slow cooling cycle. Do a drop of 100 degrees then cool from there. When the kilns shuts off, all 3 sections begin cooling immediately and drop like a rock, so the middle would not be getting more heatwork.  If that were the case you'd see it in all kilns. Additional heatwork is achieved through holding temp, climbing temp, or cooling very slowly from the peak. But as fast as the temp drops at the end of a firing it's not going to build heatwork, even in the middle. The vent speed up cooling a little bit, but not a lot.

[Hulk]

"...three zone kiln everywhere seemed to reach the final temperature at once but then top and bottom cooled off faster..."

That's what my file says* too!

Arranging the furniture, adjusting the load very much seemed to help in the single zone scenario, where almost a cone difference could be held to less than a half cone difference between levels.
Per prior, I'm arranging/loading the three zone numerically controlled kiln about the same, however, it's not "the same" - for the control system is much better at twiddling the switches to get a more even heating, much more even ...definitely considering using offsets to heat the top more and the bottom a bit more. Thanks for that.

*Where blue is target
red is top
yellow is bottom
gray is middle

The cones indicate significantly more heat work in the middle and lower middle.
...perhaps less than half a cone, yet enough to clearly (see and feel) impact bare red clay color, glaze finish, and one clay experiment's drastic faulting (severity and instance, waay more in the middle).
The graph lines are very close together, except for this excerpt.
How would you explain the heat work discrepancies?

[Callie Beller Diesel]

I have an unvented, cone sitter kiln at home (it’s outdoors in a shed), and I fire brand new ConeArts with 3 thermocouples at work. I will sometimes put a top shelf on the one at home, if the kin isn’t super full and the weather is really cold. I would never with the ones at work. They behave very differently.

You’re right. The 3 computer controlled zones make the usual advice about loading counterproductive. Make the pack even, and try not to put too much thermal mass in front of the thermocouples, or it’ll error out. My kiln programs at work involve a drop and hold, because they’re usually quite full. My bottom shelf there is usually 4-6”, middle is dictated a bit by the size of available pieces, but I try not to put a super flat shelf in front of the middle thermocouple. We don’t get a lot of super tall things, so those do tend to wind up on the top, but often on a split shelf. I haven’t checked the top to bottom differences in a while, but they’re only about a year old and fire very evenly. The glazes are consistent, so that’s the important part. 

 

[neilestrick]

22 hours ago, Callie Beller Diesel said:

The 3 computer controlled zones make the usual advice about loading counterproductive.

In 3 section kilns, the middle doesn't have to work as hard as the top and bottom because it's not affected by heat loss out the lid and floor, and it also gets some heat from the top and bottom sections (especially the bottom). This is true whether the kiln has 3 control zones or just 1, so the usual advice of packing the middle tighter still applies.

If you give the thermocouples an inch clearance in all directions you'll have no issues with errors.

[HenryBurlingame]

On 10/5/2024 at 8:27 AM, Hulk said:

How would you explain the heat work discrepancies?

That looks just like mine, no differences in temperature between the different sections until cooldown and the cones show differences in heatwork between sections. So it has to be from slower cooldown in the middle (and bottom to a lesser extent) section. I think I am just going to try loading the top and bottom more than the middle (and try a kiln shelf on top) to see if it evens things out with the 3 zone kiln. Doing a slow cooling cycle from the very top temperature is hard to time right without adding too much heatwork or having the kiln throw errors at you if you try to go too fast... its a lot easier to do 9999 drop to like 1900F and then start your slow cooling, but it might be better to try and figure out the best rate for the former for more uniformity between kiln loads.

After I get it as even as possible via loading methods I'll probably end up doing a thermocouple offset to really nail it down.

Edited October 6 by HenryBurlingame

[Hulk]

Agreed. Looks to me that
a) the (new) heavier kiln cools slower, and hence there's more heat work for similar rate up to similar peak temp*, and
b) the middle and upper bottom get more heat work where the only difference the firing file shows is slower cool - rate up to peak almost identical**.

I'm guessing a thermocouple offset is well worthwhile, per Min and Neil, above.
I'll approach it gradual though, for running the top and bottom a bit hotter - would that not slow down the middle's cooling? I don't want more heat work in the middle (or less).

*which may help explain "my new kiln over fired" refrain.
**not finding much in the literature on downward curve heat work.

[Min]

Tom, do you have the time for the X axis for your chart above?

[Hulk]

The point where power stopped - free fall begins from peak - is record #964.
Where the top begins to fire - to maintain the 2060°F hold is #1011.
Where the middle (gray) begins to fire - to maintain the 2060°F hold is #1046.

The records are written at 30 second intervals, hence
> 40 minutes for the middle to hit the hold temp,
> 24 minutes for the top to hit the hold temp.

I'll check that tomorrow for mistakes!

Gray is middle
Red is top
Note the "extra" slice of heat the top gets as the next ramp starts is still within 100°C;  

See subtopic "Determining the Effective Heating Rate" under "Converting Cone Bending Angle to an Equivalent Temperature" on "Pyrometric Cones Resources" page, here:
PYROMETRIC CONES RESOURCES | Orton Ceramic
...the example beginning "As a rule of thumb..." - where all time within 100°C of peak is included, going up, going down, and flatline.

I've found some other mention of down and flat rates in discussion of heat work, however, the Orton example is the most direct, explicit and authoritative, ahem.

What really matters, o' course, is consistent results.

Everything else being equal, looks like a heavier/thicker kiln will over fire.
If you're used to a lighter/thinner kiln, watch out for slower cooling -> more heat work. Is that reasonable?
Numeric control makes for very even heating; however, cooling is not nearly as controlled, err, controllable?

[neilestrick]

14 hours ago, HenryBurlingame said:

That looks just like mine, no differences in temperature between the different sections until cooldown and the cones show differences in heatwork between sections. So it has to be from slower cooldown in the middle (and bottom to a lesser extent) section.

If the kiln is cooling naturally, with no programmed cooling rate, the difference in the cones is due to the calibration of the thermocouples, not the middle cooling slower. The temperature drops so quickly  when the kiln turns off that you're not going to get any measurable heatwork from a natural cooling cycle. Just because the logs are showing that the thermocouples are equal, that doesn't mean that they're calibrated properly. That is why we have thermocouple offsets. Just because the controller thinks it's X degrees in the kiln doesn't mean it actually is. It can be adjusted for accuracy. The cones are more accurate than the thermocouples, so the thermocouples must be adjusted to reflect what the cones are showing. Because the natural cooling rate is not affecting the heatwork, a heavier load will not overfire compared to a lighter load. The only time the cooing rate will affect heatwork is if you're doing a controlled cooling from the peak temp, and it would affect all sections equally. If you need to control the cooling, it's best to drop at least 100F degrees naturally before starting the cooling cycle. If you're slow-cooling to get glaze effects, then you only need to do it from 1900F down to 1500F, because that is then temp zone where it has an effect.

[Min]

TC offsets also more than likely needed after every thermocouple change also. It would be nice if they all read the same but they don't.

[HenryBurlingame]

OK, so I just finished a glaze firing where I packed the kiln evenly (not spacing out top and bottom more than middle etc.) and with a kiln shelf on top over everything. It came out a lot more even than before (with no shelf on top and bottom and top more spaced out). I can't tell the difference from the glazes, but the cones in the middle still show a bit more heatwork. I think like a 5-10F offset for the middle thermocouple and all the levels will be equal. Top and bottom were pretty much the exact same this time.

Edited 3 hours ago by HenryBurlingame