Jump to content
grayfree

temp for opening kiln?

Recommended Posts

My new kiln takes a day and a bit to cool to 200C so slow in fact that a number of my glazes have changed significantly and some not for the better.. I haven't the recipe here but a blue/green has become so metallic and matte that I will have to play to get it where I want it or give it the flick.

Unless one of the readers of this post has any ideas.....taking plugs out and crash cooling at the top end?? What harm would this do to elements and bricks?

Share this post


Link to post
Share on other sites

I have no science behind this, but if you're crash cooling at the top end of your temp you're going to end up pulling in significantly cooler air through your peep holes - could possibly thermally shock hot elements and brick local to the peeps.  I do it because I already know I'll be working on my kilns continuously through the year, some home ceramicists might not ever touch their equipment until it breaks.

 

Sometimes you just gotta do what you gotta do.  I've been known to crash cool the student's work in a big gas kiln firing - just leave the damper and burner ports fully open and remove the spys.  Below maybe 500* I'll crack the door open and put on the blowers.  Usually this is only when we are on a time restriction, usually for critique.

Share this post


Link to post
Share on other sites

 

High Bridge Pottery

Joel Edmondson

Posted 06 March 2014 - 02:24 PM

You probably have a lot more pieces in a bisque firing to hold the heat in the kiln compared to the air in a glaze firing.

Great point, it all plays a roll in how fast the kiln cools. The scientific term is "thermal mass" for those who want to know in technical terms. The simple illustration is: two pots of water boiling on the stove covered. One has one quart of water, the other has 1 gallon of water; obviously the one gallon will take a lot longer to cool. Simple premise- but you get the point.

A cheap way to track temps on the low side of cooling (400F and under) is using an oven thermometer. Just stick it in a peep hole or under the lid. Can buy them most anywhere for $3-5 and most have a 3-5" probe.

Nerd

 

Note: Was just reading the technical specs on K-23 IFB. Very low thermal conductivity 0.17% @ 725F. Basically what that is saying is: at 725F the brick is storing little heat- everything else in the kiln however is.

Share this post


Link to post
Share on other sites

I see this is an old subject but it pertains to my situation so here goes;

I fired a load of pottery to cone 6 Saturday at noon. Skutt kiln 1027-3. I attempted to open the kiln Monday afternoon after work at 5:00pm. The kiln internal temperature was 251°f.  After removing the first piece  I heard  "a choir of pinging and popping". My shop temperature was 75°f and from what I have read, opening the kiln at that temperature is fine. I removed the top shelf of pots and stopped since I didn't want anything to bust or have cracked glazes. Was there just too wide a difference between the internal and external temperatures and that's why all the noise when removing pieces? Also, will the cracking and popping damage the pieces?

Share this post


Link to post
Share on other sites

I used to unload the kiln in a hurry years ago, with the pinging and popping sounds. It seemed at the time that everything was fine, yet I found that pieces we used at home seemed to be crazed quite a bit, and the glaze would actually get rough. Nowadays I take the time to allow the kiln to cool til the lid is cool, and then crack the kiln to check inside, if the air coming out is hot, I just leave a crack until it is cool. Pots seem to have more complete glaze surfaces, and crazing is not occurring any more. Maybe it is overkill, but better safe than not so happy.

 

best,

Pres

Share this post


Link to post
Share on other sites

Yeah prob some crazing on those pots. I have that kiln and never had it take near as long to cool. It hits glaze in about 8 hours and it's below 100 half a day after that (12 or so hours). I do pop out top plug at 350 and prop lid at 100=120. Based on your times I would have been unloading Sunday afternoon with kiln under 100. Sooo I am am assuming you are leaving peeps in and with three inch brick its really holding that heat. If you have a vent running don't pop peeps (sucks in cool air) but I think most people agree that popping the top peep under 400 if fine and you won't have to wait days to get to room temp. I just turn off vent at 350ish and pop out top plug. We never seem to have a crazing problem and have been using our pots daily for over a decade now.

We sell pots so can't let one of the kilns sit around cooling for days but certainly nothing wrong with just leaving it alone until under 100. I think a lot of the kiln companies say under 200 is fine but its sure hard to handle the pots at that temp. 

Share this post


Link to post
Share on other sites

Our impatience is usually kept by our firing schedule with our electric kiln. If we spend the day firing - hit cone before bedtime. We let it cool while we sleep. In the morning it's less than 200 degrees, and yes - it's  just like Christmas!

We can't even drink a cup of coffee before we run to the kiln in the morning...

Share this post


Link to post
Share on other sites

Dave:

was your load stoneware? Stoneware is prone to develop cristobalite in cone 6 .or higher firings. ( technical explanation). Cristobalite inversion happens at the low end of the cooling cycle. The classic sign of cristobalite is pinging in the kiln in the temperature range you describe. 

Share this post


Link to post
Share on other sites

Stephen, I do have an electric vent running the entire time and I did remove all the peeps at the 250° temp. I also propped the lid open about 6" but it was still popping and pinging. I ended up waiting until the next day after work to remove my stuff. 

Share this post


Link to post
Share on other sites
1 hour ago, davesnipersdad said:

Stephen, I do have an electric vent running the entire time and I did remove all the peeps at the 250° temp. I also propped the lid open about 6" but it was still popping and pinging. I ended up waiting until the next day after work to remove my stuff. 

Well I don't get it. My 1027 is a 3" brick as well and has never taken anywhere near that long to cool. If I read right you fired at noon on Saturday and Monday afternoon it was at 251.  I'm not even sure its possible for that temp to hold at 250 for so long with peeps out and lid cracked. Even with a slow glaze run that sounds like almost 48 hours to cool to 250 and like 3- 4x what it takes ours. It sounds like the same routine (although we do use porcelain). Maybe the therocouple  is reading bad? Perhaps your glaze fit is bad in which case you would hear the crazing regardless of when you unload and may hear some for some time afterward.

Be sure and turn off the vent before removing those peeps, it will immediately draw cool air from the room. 

Edited by Stephen

Share this post


Link to post
Share on other sites

This is a new kiln. This firing was its 6st time being used. What's even stranger is I used all dipped glazes, Ancient Jasper, Frosted Turquoise and Indigo Float. Everything glazed with Ancient Jasper ran and stuck to the shelves. Two large pieces cracked, one of which is still adhered to a shelf. It's a fine mess. The other glazes did fine. I'd attach a photo but its too large to upload. 

Share this post


Link to post
Share on other sites
2 hours ago, davesnipersdad said:

Stephen, I do have an electric vent running the entire time and I did remove all the peeps at the 250° temp. I also propped the lid open about 6" but it was still popping and pinging. I ended up waiting until the next day after work to remove my stuff. 

turn the vent off when fire is over-keep peeps in keep lid closed-let the glaze cool slowly so glazes crystals can form. Open lid 3 inches when kiln is cool-say 200 Degrees-be patent. If your pots are cracked you are rushing things.

Share this post


Link to post
Share on other sites

If you are running a computer, and don't have and image processing program like photoshop to resize images, download the Gimp, an open source free image processor that will allow you to resize images.

 

best,

Pres

Share this post


Link to post
Share on other sites

@davesnipersdad, ouch on a glaze mess with a new kiln. I'm thinking there is some confusion over when you started and finished the firing in regards to length of time to cool. Did the firing start or finish at noon on Saturday?

Did you have cones in the firing to check what the kiln actually fired to?

Have you used those glazes on that clay before without issues? Excess running could be a glaze thickness issue. Pinging could be glaze fit. 

Share this post


Link to post
Share on other sites

Min, I started the kiln on Saturday at noon. I didn't use cones in the firing. This dipping glaze was new. I mixed it the previous Saturday. The larger pieces I had to pour the glaze over since they were too large to actually dip. At one point I did sit the one down in the glaze to readjust the tongs. I'll bet that's what caused the glaze to get thick on the bottom and stick to the shelf. It was only a moment but apparently that does matter. The other smaller pieces that were dipped were only in the glaze for a count of 5 seconds. The glaze seemed quite watery after remixing. I've used the premixed ancient jasper and brushed it on pieces in the past and had no problem. 

Share this post


Link to post
Share on other sites
2 hours ago, davesnipersdad said:

Min, I started the kiln on Saturday at noon. I didn't use cones in the firing. This dipping glaze was new. I mixed it the previous Saturday. The larger pieces I had to pour the glaze over since they were too large to actually dip. At one point I did sit the one down in the glaze to readjust the tongs. I'll bet that's what caused the glaze to get thick on the bottom and stick to the shelf. It was only a moment but apparently that does matter. The other smaller pieces that were dipped were only in the glaze for a count of 5 seconds. The glaze seemed quite watery after remixing. I've used the premixed ancient jasper and brushed it on pieces in the past and had no problem. 

A couple of observations for what its worth since there is no right wrong, just different approaches so these opinions are just that, opinions.

I'm not sure you should pour glaze on a pot, actually I'm fairly sure you shouldn't unless of course that's the point of your design. I don't see how that could end up with a uniform covering without unpredictable overlaps and blotches where the glaze hit and ran.  Brushing, dipping or spraying are I think the norm. I have poured in a cavity and then dumped out for liner. I'd research that. I'm also not clear on how you are doing bottoms. You said you sat a pot down in glaze and then put it on the kiln shelf? You generally don't glaze bottoms at all  unless you have a way for it to be raised, such as a carved foot and then the inner area that is raised can have a thin coat of glaze. You also need to run test to know if its right and how much the glaze runs and then wipe enough of a ring of glaze around the bottom to account for the glaze run. I think it makes the bottom look nice but also it keeps glaze off your shelves. You have to use a grinder or chisel to get the glaze off your shelves and that usually chips them up and takes forever. You also end up with an unsightly bubble drip on the pot that either ruins it or requires being sanded off, again takes forever. 

I have made dozens of glazes and nothing should seem watery. Basically our glazes have a whole/2% milk consistency and other than a couple they get mostly a three count dip. I guess watery with a longer dip might work but I have found that way gets more runs. It also depends on your bisque firing. Hot bisque means the pot will absorb less glaze. The way you said 'seemed watery' sounds like you might not be being methodical about it so I thought I would mention. Glazing I found became a much less frustrating part of the process when everything became consistent. We also end up with very few glaze defects. It all becomes predictable.

My process is the glaze mixed slowly adding water and checking until hitting the consistency desired. I use an inexpensive drip tool and use that for first mixing. Just spent two days reconstituting  a couple dozen glazes that had the water pulled off for transport. Same glaze as we have used for a decade but different water source and climate so all new test. Using the drip tool I took them all to a single drip count and then in a test kiln ran test tiles of each. Making adjustments this weekend to a few and another test tile on the ones changed then will take a fresh hydrometer reading and note that on bucket so I can check and adjust when needed over time.

Hopefully some of this ramble may help. Good luck! 

Share this post


Link to post
Share on other sites

Much as Stephan says will aid you in future glazing.

I will say that there have been times when I have had to pour on glaze, especially when the piece is too large to dip, and I did not have a proper sprayer. However, no matter how hard you try you will always get inconsistencies. The best strategy if you have to pour glaze is thin the glaze a bit, and do multiple pours. Do this quickly, and as evenly as possible leaving a bit of time between pours.

Sometimes it is helpful to change the type of container you are using to dip your pots into. For plates now that are 10 to 14 inches in diameter, I use a large 6-8 inch bin to dip. I use staple removers to hold the plates, and slide/dip the plates through the glaze getting an even coat with the 1, 2, 3 count.  Works well. 

Anytime you find you have left a pot fall into the glaze, set in the glaze or other mishap, wash the glaze off the pot, let it dry, and re-glaze.

All of this glazing thing takes time, patience, and experience. I have spoken to some folks, that believe that those of us that do not painstakingly brush designs and details on a pot have it so easy and that the effort is just that. . . effortless. However, dip/pour/spray glazing is an art in itself and takes time to learn just as firing a kiln gas or electric, as fuming pottery or doing terra sigilita. 

So don't despair, and good luck with your future endeavors.

 

best,

Pres

Share this post


Link to post
Share on other sites
5 hours ago, davesnipersdad said:

I didn't use cones in the firing.

Just adding a bit to what Stephen and Pres have said. Cones are super important to measure the heatwork in the kiln. With a new kiln it's especially a good idea to use cones to see if the thermocouples need calibrating and or firing schedule needs adjusting. They are also good for seeing if there are cold areas in the kiln.

 

5 hours ago, davesnipersdad said:

The glaze seemed quite watery after remixing. I've used the premixed ancient jasper and brushed it on pieces in the past and had no problem.

A dipping glaze will seem "thin" compared to a brushing glaze. I would suggest making up some large test tiles and dipping one coat on the top 2/3 of the tile then double dipping the top half of that then a third dip on just one top corner of the test tile.  Leave lots of room at the bottom of the test tile for the glaze to run.  Do you have an accurate set of scales? I would measure the specific gravity of the glaze when mixed according to the directions that came with it. If no instructions then how Stephen describes the consistancy as whole milk is a fairly common way of describing what it should look like. In the future when you use that glaze check the specific gravity again. If the number goes higher then you need to add water to the glaze, if the number is lower then water needs to come off the glaze slurry. If you don't want to weigh and measure it you can use a glazing hydrometer to measure it. Info on measuring specific gravity here if you need it.

 

Share this post


Link to post
Share on other sites

Interesting comments since industry often goes from room temperature to cone ten and back again in hours. So the mystery of cooling looks to be not necessarily how fast but how uniformly fast. With respect to pinging in general if it pings the glaze fit was not close enough and would craze eventually anyway. Now take them out warm, say 300 degrees and set them on a nice cold conductive metal or stone shelf / table and you may exceed the tensile strength of the pot  where it is being cooled rapidly. So rate is always key and evenness is as important.

 I have waited until the hundreds and also been forced to crash unload at 400. With rate in mind, I have really never had a problem after that hot platter I set on the cold metal cart cracked within seconds of being set there. It was a great cause and effect lesson.

My suggestion, think like industry, remove as necessary and cool as uniformly as practical.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×

Important Information

By using this site, you agree to our Terms of Use.