Candling temperature / Drying greenware in kiln

[Min]

1 hour ago, Bill Kielb said:

Seems to work. 

Yes it did.

2 hours ago, Bill Kielb said:

Not sure I will depart from significantly below boiling point though as I am rarely looking to save a specific amount of time and like to be cautious with the wares.

I get what you are saying Bill but just because we have always done something in a certain way does't mean other methods won't work. I'm not in the practice of wanting to blow up pots but I like to keep an open mind. When crunch time comes I can see it being really beneficial to be able to dry pots out as quickly (and safely) as possible. Perhaps for people who have access to a large studios like where you go it isn't such of an issue but for us with our own studios and less kilns I can see it being helpful for faster turnaround times.

[Pres]

Just reminded that when I was still teaching at the HS I would run a one week workshop in Ceramics for an Intermediate school Summer camp. We did Slab project, some extrusions, and some coil work. Every day I would put pots on the two downdraft tables in the room after class  at noon. In the afternoon they would be nearly bone dry, and go into the kiln with a 2 hour or so water smoke. Day 3 of class we started glazing, and by day 5 they all had about four pieces of pottery that went in the final glaze firing of two kilns. Crazy how those downdraft tables would save my bacon in end of the semester rushes.

 

best,

Pres

[Bill Kielb]

2 hours ago, Min said:

et what you are saying Bill but just because we have always done something in a certain way does't mean other methods won't work

Yes, I did not mean to make that sound negative, apology and it does seem very interesting so thanks for sharing. As you say, it definitely must save time so I would be curious how much? Again, I am just old and chicken, never gonna take someone’s full bust sculpture and not be ultra conservative - just me though. fyi I am not associated with any particular studio as well, I have unlimited access to several that I have helped over the years but they are hours of travel away.

[Min]

1 hour ago, Bill Kielb said:

never gonna take someone’s full bust sculpture and not be ultra conservative - just me though

Me neither. 

I'm going to dip my toe into the water with some not so precious functional pots. 

I did email Bartlett and ask why the 180 or 200F, if I hear anything back I'll post it here.

1 hour ago, Bill Kielb said:

fyi I am not associated with any particular studio as well, I have unlimited access to several that I have helped over the years but they are hours of travel away.

Thanks for clarifying that, wasn't sure where you worked out of, was just going by the size of the ventilation system pictures you have shown and general comments.

[Bill Kielb]

21 hours ago, Min said:

wasn't sure where you worked out o

And of course drying at home, but never in any rush with a temp hold there for sure.

[Min]

I heard back from Bartlett after asking for some info as to why they set the hold at 180 F but the answer was pretty flat, copy of their reply below. I queried their answer, asking for any data they had to corroborate the 180F protocol and just heard crickets. "bake not boil" seriously? that's as technical as they'll go?!?

"Madeleine,

Our ceramic profiles are set to 180F Preheat to keep it below boiling point. If you want a higher Preheat you will need to use a custom user profile when programming. At 240F the kiln heats the clay faster which can render issues and might not work for all customers. When running at 180 the preheat is normally longer than 3 hours. Some customers are going 4-8 hours. This adds more heat work to the clay by running it at 180 for 4-8 hours. It can be hard to explain, but you want to bake not boil.

Evan"

[Dick White]

@MinI'm not surprised at that answer. Yes, our newer controllers preheat at 180F. But I have 2 ancient ones (with the 8-segment numerals) that preheat at 200F. I guess the notion of "more heat work" by running longer at a lower temperature can be explained by considering the meaning of "heat work" as allowing heat to penetrate to the interior of the clay mass. We conventionally think of heat work at cone temperatures (faster ramps require a higher final temperature to bend the cone, slower ramps achieve the cone bend at lower temperatures) as how heat penetrates the body to attain maturity all the way through. But why not also use the word "heat work" to model temperature penetration to attain complete drying?

[Min]

24 minutes ago, Dick White said:

@MinI'm not surprised at that answer. Yes, our newer controllers preheat at 180F. But I have 2 ancient ones (with the 8-segment numerals) that preheat at 200F. I guess the notion of "more heat work" by running longer at a lower temperature can be explained by considering the meaning of "heat work" as allowing heat to penetrate to the interior of the clay mass. We conventionally think of heat work at cone temperatures (faster ramps require a higher final temperature to bend the cone, slower ramps achieve the cone bend at lower temperatures) as how heat penetrates the body to attain maturity all the way through. But why not also use the word "heat work" to model temperature penetration to attain complete drying?

Yes, but heat work at 240F vs 180F (or 200F as the case may be) what's the difference insofar as benefits vs detriments, that was what I asked them about and if they could point me in the direction of testing results etc. 240F will also penetrate to the interior of the clay mass, if the rate of climb stays constant what we are looking at is how much of a time savings is there if the end result is the same, ie clay mass being dry enough to cease the candling and preheat segment and move the schedule along to the next segment?

I get that it seems logical to stay below 212F but it seems there are mitigating factors that come into play, perhaps the chemistry of the water within the clay, speed it evaporates off, how the clay platelettes are laying... don't know. There seems to be a lack of data concerning this. 

And another edit: It also makes sense that Bartlett is playing to the lowest common denominator here, keep the temp below 212F and customers can't complain if their pot explodes or cracks or whatever, their behind is covered. 

Edited August 25, 2022 by Min

[C.Banks]

.

 

Edited August 26, 2022 by C.Banks

[Bill Kielb]

Sorry this has been here for a bit - just got back to it.
This is not difficult so no worries. In general: bisque firings burn out all organics and to do that, the firing goes relatively slow (8-10) hours or even more for very “dirty” clays. What temperature (cone) are you working at and which JenKen controller / kiln do you have? Picture of the control would be helpful as well.

Edited September 24, 2022 by Bill Kielb

[Audrey Young]

Hi Bill,

I have a Jenn ken kiln that only fires to 1800F I have never worked with cones. I've previously worked with PMC, enameling and fused glass.  Guess I need to pick up some cones. To expand, I also own an Olympic kiln that is set up for glass.  I am able to program it for clay.  My 3rd kiln is a Paragon 2 which has a controller that can be programmed as high as is needed. My plan is to make jewelry. I can send you pics of all 3, but I must admit, being tech  savvy is not my strong suit. I will do my best. 

[Min]

@Audrey Young and @Bill Kielb, I split Audrey's first question off to a new thread in the Equipment section here. Going forward it will be easier to find in a search.

[shawnhar]

On 8/14/2022 at 7:43 PM, Min said:

In a recent thread the subject of drying less than bone dry pots in the kiln came up in which the suggestions were to keep the temp at or below 180F to avoid blowing things up. This jogged my memory of an article by Tony Hansen where he states it's okay to candle / dry out greenware up to 240F without issues.  I know it seems logical to keep the temp below the boiling point of water but is it necessary? If one can get the same results with drying the pots out at 240F rather than below 212F it seems logical to do so. In a perfect world we would have temperature, humidity and air flow control to have the perfect drying conditions but that's not likely for most studio potters.

I haven't gone up to 240F in temperature for candling greenware, wondering if anybody has and were there any issues?

I have forgotten to candle and fired wet pots, does that count?

 My experience leads me to believe you can throw and fire the next day without candling at all, so it makes sense you could candle at a higher temp. But, but but... one of my pots blew up, a thick one. So... seems to me the thickness dictates everything. I have broken greenware that "seemed" dry, but the thickest section still had moisture inside, so throwing thin with little water should be fine, but a newbie pot that is thick and wet maybe not.

 Neil made an interesting point about pots never blowing up until after 500f or so. My experience has been the same.

  I feel like the real issue is the "escape" of water content below a certain threshold prior to reaching that higher temp where the mechanical water starts to vaporize/steam, if the regular water content is too high then steam pressure doesn't have time to release through the body. I have steamed the hell out of pots on the wheel with a heat gun and they don't blow up. I have also seen another potter use an acetylene torch to heat up sections really fast so they would explode, but the whole pot didn't explode, just the surface. ( Well, sometimes the pot was lost, lol)

[Kelly in AK]

Apologies if this has been discussed earlier in the thread, but something occurred to me I had never taken into account regarding the idea of candling a kiln higher than the boiling point of water. I had mostly thought of how the clay took time to heat through its entire thickness so the interior of a pot’s wall might not be above boiling point while the kiln could be, and the structure of the clay and it’s ability to vent steam. The thought I had tonight was that evaporation takes heat. As long as water is evaporating from a pot it’s cooling itself off by some degree. It’s another variable in the fun game of “How hot can I get this pot before it explodes?” 

[PeterH]

4 hours ago, Kelly in AK said:

Apologies if this has been discussed earlier in the thread, but something occurred to me I had never taken into account regarding the idea of candling a kiln higher than the boiling point of water. I had mostly thought of how the clay took time to heat through its entire thickness so the interior of a pot’s wall might not be above boiling point while the kiln could be, and the structure of the clay and it’s ability to vent steam. The thought I had tonight was that evaporation takes heat. As long as water is evaporating from a pot it’s cooling itself off by some degree. It’s another variable in the fun game of “How hot can I get this pot before it explodes?” 

Yes indeed. While it takes 100 cal/gm to heat water from freezing point to boiling point it takes another 541 cal/gm to turn it into steam.

In domestic terms you put an uncovered stew in the oven at 180C and it fairly rapidly reaches boiling point, but then quietly sits there evaporating  for several hours at 100C.

The downside of course is that water expands by a factor of 1600 when it turns to steam, which needs to escape.

Don't know the figures for chemically combined water.

Latent Heat and Freezing and Boiling Points https://www.e-education.psu.edu/earth111/node/841

PS Loosely related

The Art, Science, and Allure of Spain’s Water-Cooling Jugs https://www.atlasobscura.com/articles/botijo-jug
But behind its apparent simplicity, the details of the botijo’s cooling system are incredibly complex. In the 1990s, two Spanish scientists developed a mathematical model to explain the details. They placed botijos in an oven and measured the water evaporation rate and the temperature drop over time. In the end, they came up with two lengthy and complicated differential equations to explain the mechanism.

An ancient method of cooling water explained by mass and heat transfer http://quim.iqi.etsii.upm.es/botijo.pdf
... they placed a botijo full of water at 27.5C in an oven at 39C (simulating a hot day), seven hours later the water was at 24C.

... and you certainly don't want to follow the maths, but I don't expect that for candling pots is any simpler.
 

Edited September 29, 2022 by PeterH

[Bill Kielb]

5 hours ago, PeterH said:

In domestic terms you put an uncovered stew in the oven at 180C and it fairly rapidly reaches boiling point, but then quietly sits there evaporating  for several hours at 100C.

Hmm, interesting stuff so If I don’t start at freezing does it take less calories? …. And as it “vaporizes” is there any particular pressure associated with the vaporization? And finally how can this be used practically when drying wares?

[PeterH]

11 minutes ago, Bill Kielb said:

Hmm, interesting stuff so If I don’t start at freezing does it take less calories? …. And as it “vaporizes” is there any particular pressure associated with the vaporization? And finally how can this be used practically when drying wares?

In answer to your question. For water below it's local boiling point inward heat-flow causes the temperature of the water to increase, when the the local boiling point is reached some of it is converted to steam. The local boiling point is a function of the local pressure, which in turn will be influenced by any back-pressure generated as the steam tries to escape.

AFAICT bara = absolute pressure in bars (atmospheric pressure is around 1 bar).

My post was really just applauding @Kelly in AK for making the point that the thermocouple temperature doesn't indicate the current temperature of everything in the kiln, rather than offering a solution to the candling problem. And throwing in a little physics to indicate the complexity of the situation when evaporative cooling is involved.

Hence just looking at the kiln's thermocouple  probably doesn't give much insight into what is happening at the water-air interface within a damp clay body, and certainly not the temperature there. So - for example - wondering if the thermocouple can safely show a temperature just above boiling point (at atmospheric  pressure) probably isn't a useful question. BTW I calculate that the  240F mentioned in the first post is the boiling point at ~2bar, so it cannot produce an back-pressure of more than about ~1bar. Hence 240F could well be safe for "normal" pots.
... figure generated via  https://www.engineeringtoolbox.com/boiling-point-water-d_926.html?vA=2&units=B#
... I would certainly agree with treating "demanding/thick" and "high-invested-effort" items such as sculptures more conservatively.

What I certainly don't understand is a point I heard Ian Gregory make. That he could fire (without candling) pots a few hours after he made them, but otherwise has to wait about a week to do so.  A tentative guess is that a half-dry pot could have a dry (and thermally insulating) outer layer that lets the outer layer to become "too hot" before evaporative cooling kicks in, while a fully damp pot supports an evaporatively cooled interface  moving gradually from the outer surface inwards.

Anyway Ian Gregory's comment suggest that all damp pots are not equal, so we probably have to allow for the likely worst case in the load. In which case for normal pots perhaps a temperature picked to be incapable of generating "high" back-pressure might be a starting point, and a time derived from experience. Which is pretty well where we started.

PS Has anybody done IR temperature measurements on pots while they are candling?

[Min]

One variable that hasn't been explored is the heating up rate of 80F/hr to the target temp of 240F that I used. What is happening to the surface of the pot during that phase? What is the relationship between the clay thickness and the warming up period? What happens when this rate is increased? How much non chemically bound water is lost during heating up to 240K?  At what rate did Ian Gregory heat his freshly thrown pots? 

[PeterH]

1 minute ago, Min said:

One variable that hasn't been explored is the heating up rate of 80F/hr to the target temp of 240F that I used. What is happening to the surface of the pot during that phase? What is the relationship between the clay thickness and the warming up period? What happens when this rate is increased? How much non chemically bound water is lost during heating up to 240K?  At what rate did Ian Gregory heat his freshly thrown pots? 

All embarrassingly good questions, I'll only answer the last one. Stuntman, singer and potter, he was somewhat of an exhibitionist and - at least at exhibitions - fired fast. His shopping-trolley kiln took about 10 mins for bisquit or raku, and 30 mins for stoneware. My vague memory was that his little-rocket reached cone 9 even faster. [Not certain if he used temperature or cones.]

[Kelly in AK]

3 hours ago, Min said:

 At what rate did Ian Gregory heat his freshly thrown pots? 

The two times I tried this the pots went directly from the wheel into a red hot kiln. No explosions. 

[PeterH]

4 hours ago, Min said:

How much non chemically bound water is lost during heating up to 240K 240F?

Most of it according to the first diagram I could find. Their ramp looks faster than yours, but I've no idea how "dry" the clay needs to be before starting this bisque firing.
https://cawstudiopotters.wordpress.com/exp/kilns-firing/

Hamer & Hamer gives:
The water of plasticity dries out from the clay in the atmosphere.
The pore water is driven off by heat up to 120C (248F).
The bound water remains as part of the clay compound (Al20O3,2SiO2,2H2O) until red heat, 600C (1112F).

Elsewhere H&H say: the point at which the clay is theoretically bone dry is 120C (248F). 
I'm not sure what is meant by theoretically here. Is it the same thermodynamic style of  argument that says diamond spontaneously decays into carbon (which it probably does, but it takes geological timescales to do so).

I've no idea where (on the above diagram) all the water of plasticity has been expelled. Nor how much might be present before candling starts.

PS Looking athttps://www.lakesidepottery.com/HTML Text/Tips/Clay drying and firing process.htm
When the water has evaporated form between the clay particles, and all the remaining clay particles are in contact, drying shrinkage is complete. This is called the leather hard stage. The particles themselves are still damp, but their drying will not cause any additional shrinkage.

So maybe the terminology is as simple as:
Wet clay has water of plasticity, pore water & chemically combined water.
Leather hard clay has only pore water & chemically combined water.
Bone dry clay has only chemically combined water.
Bisque clay has no water at all.
... which still leaves the practical matter of telling when an actual pot reaches these idealized states.

Edited September 29, 2022 by PeterH

[Bill Kielb]

5 hours ago, PeterH said:

The local boiling point is a function of the local pressure, which in turn will be influenced by any back-pressure generated as the steam tries to escape.

No I  think I agree with pretty much everything said thus far. What I was asking specifically was : the vapor pressure of the water will be positive with respect to the local pressure, hence vaporization and it leaves by pressure, not necessarily quietly sits there evaporating.  Calories are interesting also, but tying that to temperature presents it’s own issues..  My concluding question was how do we use this in a practical way?

As far as understanding it with any precision I believe it is beyond practicall pottery which includes: measurement precision, local boiling point, pure water, porosity of the clay, thickness of the clay, energy available to heat, and so on …. It’s too difficult an issue for me so to candle below boiling is conservative IMO. 

[PeterH]

1 hour ago, Bill Kielb said:

It’s too difficult an issue for me so to candle below boiling is conservative IMO. 

I'm a great believer in "If it ain't broke, don't fix it". So I'm in total agreement that keeping to your tried and true process is the smart thing to do.

To play devils-advocate. You are ensuring that there won't be problems during candling (by keeping below boiling point), and relying on your experience to know that any residual pore-water won't cause problems during the bisque. While somebody candling to 240F is ensuring that there won't be problems during the bisque (by removing virtually all the pore-water) , and relying on their experience to know that this won't cause problems during the candling. IMHO both are admirable strategies. 

[Bill Kielb]

1 hour ago, PeterH said:

While somebody candling to 240F is ensuring that there won't be problems during the bisque (by removing virtually all the pore-water) ,

Could be but my guess is pore water migrates with time as the surrounding air becomes very dry and the kiln will will go through 240f at some rate anyway. In the end whatever you have success with, but I do remember the benched firing schedule with the quartz inversion hold. After probably millions of firings automatic controllers seemed to have made a good case that  quartz inversion just happens, clay Is tough stuff.

[Pres]

While the science is quite interesting, the best physical evidence that the kiln is done water smoking is the feel of dryness from the peep, If there is still a dam feel from the peep, do not begin the firing.  I know that in this day of programmable kilns that one would like to set all of the parameters up to have a program for wet ware, leather hard ware, bone dry ware, and clunky ware, but in the long run checking before the kiln  goes to full firing will save a bit of worry about whether the ware was ready for the program. Sometimes the characteristics  of the ware being fired is beyond programming, especially when dealing with student pottery hand built or thrown.

 

best,

Pres