Kind of equipment, kind of clay related: Overfiring? Too fast?

[cadenrank]

So, I've been having an issue that I'm pretty sure I've pinpointed what the issue is, but I'm not exactly sure the routes there are to solve it. So backstory: I've posted other topics of these project in the past, and thus far, all of these projects have worked great at making my one kiln more efficient, safe, and larger. (infinite switch conversion, then blank ring to powered ring conversion.)

The only issue I've had going forward now, is that the kiln fires really fast to cone 6 (which is the exact opposite of the issue I had before) getting there in about 5 hours with a relatively full load, compared to almost 14 hours before all of these projects. I find that a lot of my glazes actually like the fast firing, but I've been extending it out to about 6-7 just so it's not going too quickly and roughly matches my digital kiln. And that it's been overshooting cone 6 (almost cone 7)  in the middle of the kiln where the blank ring that is no longer a blank ring is (for note: This ring is very slightly more powerful than the top and bottom sections of the kiln). Again, both of these issues are just reference points, moving into the problem: 

The last couple firings with the blank ring in, I've noticed that some pieces end up with almost a very small bubbled surface. Not pinholes,  maybe a mild form of bloating, but I'm not positive. I originally thought maybe this was just something in the glaze, or it was because it was firing really fast, so I sieved the glazes that were used that had the issue, and then during my firing yesterday, I made sure to extend it out to about 6 and a half - 7 hours to cone 6. This firing I believe pointed to the issue. When I opened the kiln, the top shelf cone pack was perfect cone 6, everything looked good. I moved to the middle shelf, and once again, the cone 6 witness was pretty much flat on the shelf, and cone 7 was about halfway into the bend, and everything on the middle shelf had this texture inside and out, but glazes relatively looked good (see photos in the imgur link below. The pink bowl was from a few firings ago when I first noticed the issue, and the interior of the bowl on the bottom is a black glaze from this firing, and the small white bubbles all over it are on the inside and outside).  and then the bottom shelf was again, a perfect cone 6, with little to no movement in the cone 7, and everything on the bottom shelf was as expected. So I believe this points to a too hot issue. Let me know what you think, I'm just making sure I'm chasing the right issue. Also in reference, glazes in the top, middle, and bottom had a few of the same ones, where the middle had done this, but the same glazes on top or bottom did not. Also, same glazes and clay used in my digital kiln.

For reference here, I use Highwater's P5 porcelain, which is a mid range cone 5-6 porcelain, and I've been using it for years without issue. I've read in other posts that this clay has a history of not liking being overfired and bloating as a result, but I've never experienced this, and I'm also not sure if this is what I'm experiencing now, because when I imagine bloating, this isn't what I picture. 

For this firing, I left the middle ring on 6 (2 notches below HI) up until the last hour or so of the firing, when I turned all the switches to high. If I wouldn't have turned all the switches to HI at that point, just based on pyrometer readings, I think it probably would have taken a lot longer. Around 2040 ish degrees F with the middle ring on position 6 it seemed like it was moving very very very slowly upwards at that point. Where when turning it all to HI it began moving upwards at a good pace again (as expected) 

So if the consensus is that this is an overfiring issue with the clay (which seems to be the case, given the data, but I'm here to get more opinions on that to know if I'm chasing the right thing.) would the solution be to fire maybe with a cone 5 in the sitter instead of the 6? I'm not sure what else would solve it, if it is a overfiring situation. The other option is that it's a speed issue, but I fire my digital kiln (granted, it's probably ramping a bit differently) to cone 6 on medium and it takes roughly the same amount of time as this last firing. And speed doesn't really explain the issue only being present in the middle shelf, or whichever shelf fired way hotter at least. I don't know the positions of the other pieces from past firings, but this last one, it was only the middle shelf, so I'm going to assume it was the middle shelf.

Images here.

[Min]

It looks like bloating / blebbing from over firing to me. If you can get more mass in the hotter part of the kiln by densely packing shelves it might slow down the heating enough to even the temperature out. Use the taller pots (so less shelves) in the cooler top and bottom of the kiln. If you are using mini cones and not bars in the sitter you can tweak it a bit by moving the cone over a tiny bit.

[cadenrank]

14 minutes ago, Min said:

If you are using mini cones and not bars in the sitter you can tweak it a bit by moving the cone over a tiny bit.

This is what I tried to do during the last firing. Normally I use the cones over more towards the hotter side, because the kiln used to fire colder, but I put it as far over towards the thin end of a cone 6 as I could with it still being on the cone rests, and that still wasn't enough. 
 

16 minutes ago, Min said:

 If you can get more mass in the hotter part of the kiln by densely packing shelves it might slow down the heating enough to even the temperature out. 

Normally this is the way it's packed. The bottom and middle usually have more mass. This last firing that wasn't the case, but prior to it was. 

[neilestrick]

Looks like bloating to me, too.

In terms of the middle being more powerful, that's exactly where you don't need the power. If the top and bottom are at 100%, the middle may only need to be working at 50% to achieve the same heat since the middle doesn't have to deal with the same heat loss as the top and bottom, and benefits from the bottom. Is it possible to move the middle ring to the bottom? I think you may want to invest in a pyrometer that can read 2 or 3 thermocouples so that you can get an idea of what's happening in each section.

[Min]

6 hours ago, cadenrank said:

I left the middle ring on 6 (2 notches below HI) up until the last hour or so of the firing, when I turned all the switches to high.

Have you tried not turning the middle ring to high and just leaving it on 5 or 6 for the entire firing? Can you place your cone packs so you can see them (using eye protection) towards the end of the firing? 

[cadenrank]

 

3 minutes ago, neilestrick said:

Looks like bloating to me, too.

That's what I thought too. 

3 minutes ago, neilestrick said:

In terms of the middle being more powerful

Just to touch on this in case, by more powerful (in comparison) I mean by like .85 of an amp. If we divided the numbers up, (each full section (9 inch section) is 12 amps, two elements in each section, so divided roughly would be about 6 amps per each 4.5 inch section in the top and bottom) where the  middle ring is just the single height 4.5 brick, and comes out to about 6.85 amps. 

8 minutes ago, neilestrick said:

 Is it possible to move the middle ring to the bottom? 

I don't think that would be an issue, it would just put the sitter in the middle of the kiln, and the permanently mounted thermocouple in the bottom. (Thermocouple is mounted in the middle ring)

 

9 minutes ago, neilestrick said:

If the top and bottom are at 100%, the middle may only need to be working at 50% to achieve the same heat since the middle doesn't have to deal with the same heat loss as the top and bottom, and benefits from the bottom. 

This was why I had tried to leave the middle ring on 6 instead of HI for most of the firing, but I could tell by watching the pyrometer that each time the infinite switch cycled off, it lost 3 or 4 degrees once it was nearing 2000F, and it seemed like it was taking forever to increase in temp. This was also the first time I had tried that, and wasn't exactly aware that the middle was the hot spot. 

 

5 minutes ago, Min said:

Have you tried not turning the middle ring to high and just leaving it on 5 or 6 for the entire firing? Can you place your cone packs so you can see them (using eye protection) towards the end of the firing? 

See the above response ^, but to answer directly, no. I tried leaving it on 6 until the last hour, but I turned it on once I got to 5 and a half hours as to try to keep the 6-7 hour firing time. 

I can see my top shelf cones from the peephole, but I can't see the middle or the bottom. 

[cadenrank]

16 minutes ago, neilestrick said:

In terms of the middle being more powerful, that's exactly where you don't need the power

Adding to note on this as well, while logically that all makes sense to me, I find it interesting because for the size of the kiln, the element configuration is (again, relatively in regards to relation of numbers, not exact numbers) pretty much the same way that Skutt "balances" their elements resistances in this size of kiln, with the 3rd (or middle) elements being higher resistance. 

Just reflecting back on this, I haven't looked at a diagram of the wiring of the Skutt configurations, so maybe they're wired differently than what I'm thinking. But end amperage comes out to the same as what I'm achieving with Skutts configuration for this size kiln as well. 

[neilestrick]

3 hours ago, cadenrank said:

Adding to note on this as well, while logically that all makes sense to me, I find it interesting because for the size of the kiln, the element configuration is (again, relatively in regards to relation of numbers, not exact numbers) pretty much the same way that Skutt "balances" their elements resistances in this size of kiln, with the 3rd (or middle) elements being higher resistance. 

Just reflecting back on this, I haven't looked at a diagram of the wiring of the Skutt configurations, so maybe they're wired differently than what I'm thinking. But end amperage comes out to the same as what I'm achieving with Skutts configuration for this size kiln as well. 

.85 amps doesn't sound like much, but it's about 14% more power in a section that has to work half as hard. I'd move it to the bottom, where it will do more good. Plus you really want the Sitter in the middle of the kiln anyway.

Skutt sets the very top and very bottom elements running hotter than the middle elements. How much varies depending on the model, and on a couple of models they don't do it at all.

[Bill Kielb]

Joined this late but try this for some thoughts after skimming through …….power (watts) = current squared X the resistance  so it went up in current by about 14% (.85) which is actually significant,  but it also went up in wattage by about 22% in heating energy because it’s exponential. Everything indicates over fired and I am just curious why turn up the center section to max - ever?  Seems the number say never turn higher than 80%, or always have it lag the others by about 22%.
 

Also - when an element is higher in resistance, it draws less amperage and less wattage or heat energy. Not sure that helps as I just skimmed through this.

Just to add, I think Neil posted same while I was posting except for the exponential part which is another few percent. I also would pick top or bottom for the hotter section based on some pyrometer readings. Not sure which would need it more.

[cadenrank]

17 minutes ago, neilestrick said:

.85 amps doesn't sound like much, but it's about 14% more power in a section that has to work half as hard. 

Do you think this would solve the issue? My concern is that it would just move it so that the bottom section over fires. I understand the logic, but just making sure. I'll see what I can do in regards to switching it. Really it's just a matter of figuring out the logistics of where the sitter and thermocouple will be in that configuration. 
 

3 minutes ago, Bill Kielb said:

Everything indicates over fired and I am just curious why turn up the center section to max - ever?

Welcome! So my concept here was from watching the pyrometer. I had started the "high" section of the firing with top and bottom on high, and middle on 6. (switch goes up to HI being in the 7 position) and once we got to about 2000 ish degrees, the cycles of the infinite switch would drop the temperature to about 3 or 4 degrees lower, and then slowly climb back up when it cycled back on, only to loose 3 or 4 more again after that. It seemed like it took forever to move beyond the 2000 mark, and we had already been going for almost 6 hours at that point, so that's when it was turned to high (again, all of this is based off of my limited knowledge of how fast it will take or short it will take when I change switch positions, and not wanting to fire too slowly) where it began to again increase steadily in temperature, until eventually dropping the sitter about an hour later. Mostly just trying to keep it under 7-8 hours for the glaze firing, because 2 or 3 of my glazes pretty much just turn brown when we start passing 8 hours, and I was (am) concerned that it would take way way longer just based on watching the pyrometer. 
I mean, logically, I know that with it on 6 or lower, and the other two on hi, that it would still eventually drop the sitter. I've run this kiln with the blank ring (no elements in it) to cone 6 on just the two sections but it took like 16 or 17 hours to drop the sitter. But still, I'm obsessing over keeping it below 7 hours, just because my glazes seem to look best at those ranges.

[neilestrick]

5 minutes ago, cadenrank said:

Do you think this would solve the issue? My concern is that it would just move it so that the bottom section over fires. I understand the logic, but just making sure. I'll see what I can do in regards to switching it. Really it's just a matter of figuring out the logistics of where the sitter and thermocouple will be in that configuration. 

I think it's less likely to over fire at the bottom. The bottom is the coldest part of the kiln- lots of heat loss out the floor, and the heat moves upward to assist the middle. 

[Bill Kielb]

29 minutes ago, cadenrank said:

so that's when it was turned to high (again, all of this is based off of my limited knowledge of how fast it will take or short it will take when I change switch positions, and not wanting to fire too slowly) w

So you need the total power to get your 6-7 hour firing, but ya have too much of it in the center. In reality logic would dictate the top and bottom ought to assist the middle so lower resistance top and bottom, higher in the center.

I would try no higher than 8 for the center, but it’s not an answer for the imbalance. FYI, if you can maintain anywhere near 100 degrees per hour in the last 200f, glazes ought to turn out normal. That would be say 1.5-2 degree climb per minute

An 8 hour glaze fire is very ordinary BTW.

This could also be sped up by going faster in the beginning and middle of the firing. The last 200 f though should be in the 100 degree per hour range. This speed or a bit less improves the evenness of the firing as well.

[Bill Kielb]

17 minutes ago, neilestrick said:

think it's less likely to over fire at the bottom. The bottom is the coldest part of the kiln- lots of heat loss out the floor, and the heat moves upward to assist the middle. 

I think I would agree, but have seen it inverted as the top lid  has a giant crack for infiltration. Not much air movement in kiln nor air. At 2000 degrees air is very thin and cannot hold much energy. At 2000 degrees mostly radiation, so can be counter intuitive depending on how the elements radiate to other parts of the kiln. 

[Bill Kielb]

Sorry squared should be approximately 22% more power, not 20%

[neilestrick]

9 minutes ago, Bill Kielb said:

I think I would agree, but have seen it inverted as the top lid  has a giant crack for infiltration. Not much air movement in kiln nor air. At 2000 degrees air is very thin and cannot hold much energy. At 2000 degrees mostly radiation, so can be counter intuitive depending on how the elements radiate to other parts of the kiln.

I was just going off what I see on my kilns. On both my 4 cubic foot and 10 cubic foot 3 section kilns, the bottom has to work about 3-5% harder than the top, and the middle runs at about 50-60% of the top/bottom. A large enough crack at the top could definitely flip that, but I think that's a problem that should be dealt with if it's happening.

[Bill Kielb]

3 minutes ago, neilestrick said:

was just going off what I see on my kilns. On both my 4 cubic foot and 10 cubic foot 3 section kilns,

I have seen it both ways. Always wanted to just throw 1/2 “ rigid under the bottom shelf with cutouts for posts just to get a few percent savings back. Can always skirt the bottom under the kiln with fiber for a firing or two and see how much it will improve. Have squeaked  out a firing or two that way when someone was super desperate.

[cadenrank]

While I understand the overall solution, would it be counterproductive to try doing a cone lower in the sitter and keeping the middle switch to 6 or so? 

My thought process is that I'd probably get to cone 5 on the bottom shelf and top shelf, and cone 6 in the middle.. which doesn't sound too awfully bad to me, but of course the ultimate goal is to get balance across the whole kiln, but am just wondering what the implications of doing so would be, besides just maintaining that imbalance, and not over firing. 

I'm not sure if it makes any difference overall, but this kiln also runs a downdraft vent. 

[Bill Kielb]

Hmm, as I recall the downdraft turned out to be an issue in the overall length of time it took to complete a firing so I am assuming that has been fixed. If not, a downdraft that draws too much definitely has the potential to ruin any firing. So if the downdraft is over drawing, I would suggest that gets fixed first. Turning it off towards the final third of the firing seems like an easy test.

knowing what we know now, I am not sure that I would not keep the setting on the center section set 20-25% less than top and bottom. So never turning it up beyond 7 or 8 would be something I tried first. I think If it were me, trying to go about 90 - 100 degrees per hour in the last 200 degrees would be something I would check as well. Generally that speed provides plenty of time for things to fire evenly and if that temporarily fixes it you could always go faster early in the glaze firing to stay in your timeframe.

The idea of firing to cone five and keeping the middle switch at 6 or so is a form of the above I guess so maybe try it. For me I am methodological so knowing I could set a final rate at about 90- 100 degrees per hour with some switch combination would be first in my list. Trying it at cone 5 is just as valid, just not stepwise enough for me. Both approaches will provide information though.

[neilestrick]

For people who fire heavy loads like tiles and plates, L&L offers adding an element to the floor. ConeArt puts an element in the floor in place of one wall element. The bottom is where power is needed.

[cadenrank]

24 minutes ago, neilestrick said:

For people who fire heavy loads like tiles and plates, L&L offers adding an element to the floor. ConeArt puts an element in the floor in place of one wall element. The bottom is where power is needed.

How much of a difference would it make if the thermocouple was on the bottom in this configuration? I think that's the only thing I don't like about the idea, just because where it is in the middle is already hard to work around along with the sitter, but it also has been a great addition to this kiln. After installing it, I can't even imagine what it was like not being able to see the numerical temperature value before. 

 

[Bill Kielb]

32 minutes ago, neilestrick said:

For people who fire heavy loads like tiles and plates, L&L offers adding an element to the floor. ConeArt puts an element in the floor in place of one wall element. The bottom is where power is needed.

Hmm, I didn’t think I disagreed actually when I said “I think I would agree” but the top and bottom are identical except the top has an open seam all around. They don’t put elements in the top (save low temp glass kilns) because it’s just too hard to maintain for midfire and above. Both areas could use extra power to offset the losses, just one is harder to do.

I’ve seen and measured both to be an issue but I still think I agree moving the overpowered section to the bottom is a good choice.

[cadenrank]

 

19 minutes ago, Bill Kielb said:

I’ve seen and measured both to be an issue but I still think I agree moving the overpowered section to the bottom is a good choice.

and @neilestrick would I still run the bottom switch at limited power if the ring was moved to the bottom? Just for reference bill, HI is in the 7 position on this switch. So it only goes to 7. see attached picture of the switch knob. 

[Bill Kielb]

7 minutes ago, cadenrank said:

and @neilestrick would I still run the bottom switch at limited power if the ring was moved to the bottom? Just for reference bill, HI is in the 7 position on this switch. So it only goes to 7. see attached picture of the switch knob. 

I think trial might be necessary, you may get away with running it lower. Funny that switch is labeled 1-7 but has 13 positions! Since you know it’s 25% more power than the others I think you could do the surface area math to include the area of the bottom to get you close but starting testing full or near full is probably a good guess. You could always measure with you pyrometer as it’s firing.

Confusing things a bit, Infinite switches traditionally start out at about 22% but you are not really interested in low end operation so no worries.

[neilestrick]

35 minutes ago, Bill Kielb said:

Hmm, I didn’t think I disagreed actually when I said “I think I would agree” but the top and bottom are identical except the top has an open seam all around. They don’t put elements in the top (save low temp glass kilns) because it’s just too hard to maintain for midfire and above. Both areas could use extra power to offset the losses, just one is harder to do.

That wasn't directed at you, I apologize if it seemed like it was. I was just making the point that the bottom is a good place to have extra power.

15 minutes ago, cadenrank said:

and @neilestrick would I still run the bottom switch at limited power if the ring was moved to the bottom? Just for reference bill, HI is in the 7 position on this switch. So it only goes to 7. see attached picture of the switch knob. 

I agree with Bill, trial and error is going to be necessary here.

39 minutes ago, cadenrank said:

How much of a difference would it make if the thermocouple was on the bottom in this configuration?

Leaving it at the bottom won't matter if you're simply using it to get an idea of rate of climb and approximate temperature. But I think it would be a good idea to have a second thermocouple in the middle or top so that you can compare the small ring to them until you get the settings dialed in. You could jut get a second thermocouple and plug/unplug them to the one pyrometer if you don't want to invest in another one.

[cadenrank]

10 minutes ago, Bill Kielb said:

Confusing things a bit, Infinite switches traditionally start out at about 22% but you are not really interested in low end operation so no worries.

I could be wrong, but I believe this model of infinite switch (all three switches on this kiln are the same model) the low is 5%. 

I think I will swap things around, and see what it looks like with those things (sitter, thermocouple) in new positions then go from there. My concern is that the thermocouple is basically going to be either touching the bottom shelf, or will be right above it if not inline with it.