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Kiln temp vs room temp - safe margin to open & unload?

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I've searched the forums, and found some threads discussing "at what temperature can I open my kiln" - but all that I've looked at are focused on internal temperature of the kiln, and how cool does it have to be before you open it.

My question is in a slightly different context:  What is the maximum recommend differential between internal kiln temp, and ambient (room) temperature when opening ? 

My kiln is in an unheated garage, that currently has a "room temperature" of around 30*F.   I know if I start unloading too soon, I'm risking problems from thermal shock.  Obviously, I could just open the lid & see what happens - but I get plenty of chances to learn from my own mistakes (like plates that split when I dried them too fast) - so I try to learn from mistakes made by others whenever possible.  ;-)

 

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After (below) the silica inversion point at 256C, which can affect clay and glazes and at least in principle needs to be passed through slowly, it really depends on what is in the kiln.  

For functional ware generally, how hot does the water in your dishwasher get?  How hot is the boiling water you pour in for your cup of tea?  Your mugs will probably be experiencing these temperatures again and again, so if they can’t hack it there... If it is baking or oven ware, even more aggresssive thermal shock in their future...well you get the point.

If large, flat platters I would wait until below 100 C for sure - too much thermal shock (ie, sudden cooling) across the surfaces of large flat things sitting on too hot kiln shelves no good.  Particularly, glassy (read inflexible) porcelain bodies are sensitive to thermal shock - open stoneware bodies with more grog and large particles seem to have a bit more staying power, with some thought that larger particles in those bodies terminate micro-cracks before they open up into major splits.

For more “robust” work, I asked one veteran potter who was running a community studio this same question and he/she said “how badly do you need the work?”  Since they were talking about student work I took this to mean that even quite high temperatures are OK to open at if the work is small and thick, and/or it is likely that the people looking at the work will not be able to differentiate cooling dunts from poor making techniques.   In this instance the priority was how quickly the kiln needed to be turned around and reloaded at the end of term.  Yes, appalling I agree.  Needless to say, I do not put anything larger than test tiles in those kilns anymore.  But the relevant point for me was that a kiln can often be opened at quite high temperatures without obvious damage to the work (damage to the kiln may be another matter?), and I have seen amateurs get away with this in seeming complete ignorance on more than one occasion.

Since I see glaze crazing as largely a chemistry issue, I don’t think opening early makes this more or less likely unless you are opening at crazy high temps.

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What are the damages to a kiln in this situation? I would think opening a kiln at 250-300F with a 30-40F room would be a shock. Is there typical damage done? It's interesting to think about because a kiln drops from cone 6 super fast if let cool naturally. So is the shock between 2200 and 1500 greater than 250 and 30?

I am genuinely unsure and would like to know more. I opened my kiln at 500F all the time but I don't unload until 250ish. I have a top shelf on all my work so it still takes a while to cool. I guess I am damaging my kiln?

Edited by Joseph F

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When I attended a workshop of Steven Hill's our sprayed bisque ware was packed into the kiln as soon as we finished spraying around 6 pm Saturday. They took the kilns up rather quickly but did the Stephen Hill glaze fire schedule. Sunday morning at around 800*F they propped the lid. When they unloaded a few hours later we had to use towels to keep from burning ourselves.

Watching Matt Katz' video on crazing he said it doesn't matter when taking things out of the kiln. If a glaze is going to craze it will not be because of early kiln opening. curt, I am with you, it is chemistry.

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Quote

Watching Matt Katz' video on crazing he said it doesn't matter when taking things out of the kiln. If a glaze is going to craze it will not be because of early kiln opening. curt, I am with you, it is chemistry.

This is what I have found so far as well. But limited testing.

Edited by Joseph F

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I agree fully about the crazing thing being chemistry, not early kiln opening. My pots don't ping out of the kiln, and I put my glazes through some pretty extreme thermal shock tests to see if they'll craze.  While I don't like doing it often, I pulled pots 8 hours after the kiln shut off because I needed them for show stock last Thursday. I melted a Rubbermaid bin transporting the work because I should have left the lid off the bin. (Oops.  Don't recommend that part!) I had no pots "singing" and the ambient temp outside where my kiln is was approximately 5* above freezing. I am a stoneware user.

If you're going to push your luck like that, don't set the ware down on a cement surface: opt for wood, or something else insulating like that.

 

Edited to add:

I really don't recommend pushing your luck like I did in this example. It's not my normal practice, and it's likely only through sheer luck that nothing happened. I did it because I straight up bit off more than I could chew this show season, and didn't plan the way I needed to.

Edited by Callie Beller Diesel
Added last paragraph.

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Unloading while super hot is just a bad idea.If your pots are burning wood surfaces they are to hot.

About 44 years ago a teacher suggested  that if I wanted to find out what pots do if you cool them down quickly try opening the door after turning off kiln. At that time I had in my backyard (before I bought my current place in 1973) so this was in 1972 ,I built a small catenary arch hard brick kiln at a rental I was in. I fired to cone 11 and then unblocked the door. I leaned that that thermal shock kills most clay bodies and does shorten the kiln furniture as well..

About 10 years later I found myself with a large 2x2 box fan blowing air over a hot load of pots  that where about 800 degrees  on my car kiln and I though I needed then NOW-the results where less drastic than the cone 11 to cool in 2 hours 10 years earlier. 

My point is unless you Really really need them let them cool until you can handle them-its better on the wares the furniture and the kiln.The pots are really the cheapest thing you are stressing-bricks and shelves have more value and replacement costs are high.

Joseph-I to have burned my fair share of wood tables but I no longer do any of that. No need to if you work smart and plan ahead.

 

I have mentioned learning the limits before and its good to know them so the above story is all about testing those limits-now you know what not to do as I have already done it.

Just let it cool down enough to at least oven temps or better still hand holding temps so no gloves are needed for the pots

Edited by Mark C.

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I should have said I don't usually do this. It is just every now and then when the entire load is nothing but test pots and nothing I care about. I am always super careful and use protective gloves etc. So I am not trying to say what I did was a good idea or anything, just curious what the downsides were. I didn't think about kiln damages until I started thinking about how the softbrick handle the sudden shock. I figured brick are brick and it wouldn't matter, but then the more I read about soft brick(that salt/soda kiln post) the more it made me think that maybe I am damaging my kiln. I am not in a hurry about my pots, just I didn't see a point for waiting when it is all test tiles and so forth. That being said I will continue to wait until its in the 250-500F range and open the lid from now on.

 

 

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A person that sells kilns told me that you damage the fire bricks when you open at higher temperatures.  I mentiond 250 degrees and he said that was way too hot and said that extreme temperature changes make the fire bricks crack, etc.   I have no idea as to the validity of this idea.

I never open above 250.  Usually around 150.

 

Edited by DirtRoads

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Kiln bricks can handle the expansion and contraction of heating and cooling better than pots and kiln furniture. Soft brick don't hold heat the way dense materials do, and they have much greater flexibility, allowing them to deal with the thermal expansion/contraction issues. I usually open my kilns at 300F, and my kilns don't seem to mind. Nor do my pots (porcelain) or kiln furniture. The lower the temp the better, but I think there's a point where the effects of fast cooling can't be measured.

For larger pieces, I do wait until the kiln gets down to 250F or lower.

Edited by neilestrick

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Opened the kiln today at 350* after a nine hour cool down from what the TC told the Orton controller was 2225*. It was supposed to be a ^6 firing, but when I opened the kiln, I found all 3 witness cones bent over with the ^7 guard cone bent all the way over...My guess is that the kiln over fired to at least  ^8, maybe higher. The bowls came out OK but there was some blistering on some of them.  I'm  going to get some #8 cones and do another firing this afternoon, but I'm going to do a -45* offset on the thermocouple to see what happens. I did a 15 minute hold at the end of the firing ...could that be what put it over the top? This is only the third time I've used the controller. The last time, the #7 cone in the kiln sitter bent and tripped the sitter and shut the kiln down, but because the controller didn't know the elements weren't getting any power and tried to continue running the program, it didn't give me the top temp reached in the process. The witness cones were bent properly for a ^6 firing and there was no blistering in the glazes.

Do any of you have any suggestions on how I should proceed beyond the TC setback?

Thanks,

JohnnyK

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JohnnyK, a few things here. I will speak from similar experience, as I fire in my studio, which is a brick one car garage separate from the house. Electric heat. In PA Winters I usually do not work in the shop. . . the one year I did, I had a $500 electric bill for one month.

So kiln and ambient temp? First your kiln probably heats your garage up to at least 100F. Secondly, even after a 36 hr. cool down you should be fine. Now as to opening at 350F. I would think too early to open the kiln, I believe it could cause crazing or worst yet, dunting. I usually do not open my kiln until I can place my hand on the lid and leave it there, just for warmth, not so hot that I would have to remove it. Remember that I do not fire with pyrometer. 

As far as checking the kiln, you may want to put in some cone packs to check your setter/controller. If you have peeps, you can make the packs in a row, but instead of having them  set to fall the length of the clay support have them fall to each side perpendicular to the support. This will allow you to see all three cones through the peep.

All in my humble opinion, but you seem to be having a similar environment.

 

best,

Pres

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5 hours ago, dhPotter said:

JohnnyK if you do a -45* TC offset you will be asking the controller to add 45* more heat. If it is 70* room temp the controller will think it is 25* room temp. 

Thanks for the input...the controller has a hotter mode and a cooler mode. I set the offset in the cooler mode and it is reading the way it should. Looking forward to a successful firing and another 8 Christmas presents tomorrow afternoon!

JohnnyK

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5 hours ago, Pres said:

JohnnyK, a few things here. I will speak from similar experience, as I fire in my studio, which is a brick one car garage separate from the house. Electric heat. In PA Winters I usually do not work in the shop. . . the one year I did, I had a $500 electric bill for one month.

So kiln and ambient temp? First your kiln probably heats your garage up to at least 100F. Secondly, even after a 36 hr. cool down you should be fine. Now as to opening at 350F. I would think too early to open the kiln, I believe it could cause crazing or worst yet, dunting. I usually do not open my kiln until I can place my hand on the lid and leave it there, just for warmth, not so hot that I would have to remove it. Remember that I do not fire with pyrometer. 

As far as checking the kiln, you may want to put in some cone packs to check your setter/controller. If you have peeps, you can make the packs in a row, but instead of having them  set to fall the length of the clay support have them fall to each side perpendicular to the support. This will allow you to see all three cones through the peep.

All in my humble opinion, but you seem to be having a similar environment.

 

best,

Pres

Can't see the cones at 2200* even with my arc welding helmet. 

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Use a laser pointer-I have heard that works.

Seeing cones takes some practice I have found-I use a welding glasses from long ago-you can buy a #4 glass (if I recall this is the right #?)at a welding shop-much less hassle than a helmet.

Also set up a backstop behind the cones.

also do a search on this as its been covered a lot

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Yes, Mark has it right, it does take practice. If doing a search, do it in the Home area where all forums are visible. The Search will be in upper rt of the screen. Good luck.

 

best,

Pres

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Mark is right about practice and backdrop for seeing the cone. In larger kilns, I try to have the far wall brick pattern behind the cones. Someone said years ago to brush a cobalt wash on the back ridge of the cones to help see them. I couldn't see any improvement there. I have heard use a flashlight. The laser pointer sounds good, plus you can torment the cats.

For electric firing I crack the lids less than an inch below 300-200F. Then wait til cool. I have no rush.

 

Marcia

 

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I have never burned a wood table, with a hot ware.  But I've been in a few classrooms, that had marks on them, from someone who did...

Even if I was in a rush, I wouldn't set them on my current classroom tables.  Even the ones that are made to look like wood, are not wood.  It's that plastic veneer stuff.   No way, I'd be sanding those burn marks away...

There have been times, where I've had to rush unloading, just to reload again, like at the end of the school year.  I will get some pings, if I do it too quickly.  So what I've started doing is to open the lid for a few seconds, and then close it again.  So it removes some of the heat, but not drastic enough to hurt the wares.  I do that every so often, and it does seem to help drop the temperature safely, but quicker, than just letting it set.

In regards to unloading causing crazing, I have Rakuware that doesn't craze (I'm sure it does when magnified more than normal vision), when going from 1800 F to 20-30 F, so I can't imagine that anything less than that will affect the glaze.

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Thanks, again, for all your input, Since there's no real rush on this batch of bowls, I can wait until full cool-down later this afternoon. I AM anxious to see what the changes in the TC adjustment and the lower program final temp does. Since, at this point, I'm using primarily Potters Choice glazes and they are listed for ^5-^6, I feel I have a little wiggle room on the firing outcome. I'll keep you all apprised of what happens.

JohnnyK

Edited by JohnnyK

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AAARRRGGGHHH!!! 

You know that pesky little device called a kiln sitter...you know...the one that has the timer knob that you set when you're manually operating the kiln...you know...the one that, when you have an electronic controller, doesn't need an external timer...and when you use the controller, you really don't need a timer and FORGET to set it...and when you forget to set it, it shuts off the power to the elements when it times out but doesn't tell your controller...THAT  #@&%$# kiln sitter? You know...the one that is going to be bypassed as soon as this firing is done and cooled!

Just saying...

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AAARRRGGGHHH!!! 

oh ya the timer-Its gotten me more than once on my all Manuel kiln -sometimes the bisque is done enough and I use it underfired.Of course I'm only bisquing in it.

I have a fire right controler that turns it up at various ramp speeds-this thing is from before computer electric came along. I love the simple life.

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