Jump to content
Sue T

What temperature is OK to crack kiln open

Recommended Posts

Usually I don't open my kiln door until it's cold.  

I'm wondering, on a glaze fire, cone 7-8 if anyone has experience of maximum

temperature they vent the kiln without causing damage to ware.

Many thanks!

Share this post


Link to post
Share on other sites

Well below 400* F for me. No pyrometer on my kiln; if a piece of paper ignites when stuck into kiln its over 451; if it smokes and yellows its around 451. Cheap and easy way to tell temp in your kiln. Usually though I just hold my hand up to spyport; if its too hot for my hand to be comfortable, its too hot to crack. If I can touch the brick without burning myself I open it up.

On bisque fires, I often am trying to force cool a kiln; I will crack (1/4" or less) around 600; 1" around 475, 4-6" at 400, all the way open at 350. Never had any damage to my BISQUE doing it this way. Glazed wares that have been vitrified now have crystalline structures which dont like rapid heating/cooling around 450. More sensitive to thermal shock once vitrified.

Share this post


Link to post
Share on other sites

Industry goes from room temp to cone 10 to room temp in hours. How do they do that? Even heating and even cooling. So rate is very important. To answer your question we crack the lid at 250 and open at 200. Have I cooled quicker? Yes, but always in a way to not create a rapid change. Some folks crack it above four hundred and move on from there. Neil’s  300 mark seems to be totally reasonable and time tested by him, how cool is that?

lastly there are many that feel if you have a specific quartz formation (Cristobalite) then the material cannot be cooled before approximately 450 degrees.  Just a thought here it’s hard to develop crystobalite below cone 14 and really really unlikely in a cone 6 kiln. So crystobalite in regular pottery generally is very unlikely.

Edited by Bill Kielb

Share this post


Link to post
Share on other sites
33 minutes ago, Bill Kielb said:

lastly there are many that feel if you have a specific quartz formation (Crystobalite) then the material cannot be cooled before approximately 450 degrees.  Just a thought here it’s hard to develop crystobalite below cone 14 and really really unlikely in a cone 6 kiln. So crystobalite in regular pottery generally is very unlikely.

Dunno what crystobalite is, but cristobalite is common in non vitreous stoneware bodies and forms above 1100C (2000 F) which is well within the the temp range of a cone 6 firing. 

In studio pottery it’s more about how much flux is in a body. 

You are right, though, even cooling is the key thing.  Cristobalite inversion is just the last significant cooling change in a clay body and going through it unevenly is asking for risk.

Edited by Tyler Miller

Share this post


Link to post
Share on other sites

Ok, we should be seeing pots cracking everywhere everyday all the time then, considering the cooling rates of most kilns. I have not seen it yet, even in refires so after a year of researching this, my conclusion:  cone 14 for a significant amount of time. Just sayin!

The other popular myth (in my opinion) is the go slow through quartz inversion. What is slow? Since many commercial schedules traverse this upwards of 550 degrees per hour every single day I would  say that is slow enough else we would be cracking thousands of wares daily. Again, I see many that say slow down to 150, 200, you name it. Not sure how this stuff starts really the pots just don’t seem to care.

Share this post


Link to post
Share on other sites

Bill,

I see the confusion you’re having now.  You’ve equated the possibiity of fast firing with the reality of cooling rates being irrelevant, without taking into account the effect of unevenness of cooling.

Fast firings are indeed possible, but under specific conditions that demand a very even heating and cooling of a kiln.  This is accomplished by using a kiln of low thermal mass, but very well insulated (ie they’re made of rcf wools etc).  Also, they tend to use clay bodies specifically formulated to fast firings—that is,  bodies with very little free silica.

The issue isn’t the cooling rate, but the evenness of cooling.  Cristobalite inversion cashes out in the real world as a dramatic change in the dimensions of a pot, and it occurs over a narrow range.  So, open the kiln at the wrong point and remove the stabiliIng thermal mass of the lid, and you could have a pot with one side above and one side below the temp of inversion, then PONG!  That red stoneware platter is now two pieces.

It is a real and regular defect in ceramics—either with clay bodies with too much free silica, clay bodies with cristobalite deliberately added (they used to do this with earthenwares to get better glaze fit by forcing compression), or with kilns  opened too early.  Hamer and Hamer discuss this in the Potter’s Dictionary (page 120 in their discussion on dunting), and it’s also a problem in industry.  Check this article out on cooling dunts from Ceramic Industry:  https://www.ceramicindustry.com/articles/87399-drying-firing-solutions-cooling-dunts )

I find it cool that the industrial practice of firing high for the initial firing creates issues for cristobalite dunting in the subsequent re-heat.

So, I’d suggest doing some reading about firing defects and their causes.  I stand by my original statement that cristobalite inversion is the last significant structural change in a clay body that could cause failures in a firing cycle.

Peace. 

Share this post


Link to post
Share on other sites

Interesting seems correct to me. Last and could -appear consistent with real world pottery. I'm all good with last and could.

just to add 570 degrees per hour is the rate that the Bartlett V6cf has gone for years as a fast glaze firing. This controller has been in use for thousands, maybe hundreds of thousands  or more firings rebranded for Skutt, L&L, you name it.  570 degrees per hour is way too slow for industry but likely just fine for typical  pottery.

now as to what is too early to open the kiln, certainly don’t open and cool too quickly, Neil does his at 300 and it works for him. Whatever you do try and cool, cool it uniformly. Whether cristobalite is a concern or not we differ a bit and my body of evidence researching for a year,  along with lots of pots over time says for normal pottery I have not found it to be significant concern for most potters.

Edited by Bill Kielb

Share this post


Link to post
Share on other sites

Other things to consider are kiln furniture issues-My advancers shelves do not like quick cooling . I have some friends with a 30 cubic front loader gas kiln with advancers like mine. They kept having cracked corners off the back bottom of the kiln for a few years. They live in the sticks and are shelve taught . They called me after cracking many 12x24 advancers I asked about the cooling cycle. They said they always pulled the damper at 1000 degrees. I Suggested firmly to stop that-no more racked shelves. At about $250 a shelve delivered that cost them a bundle. 

They also do not like small point stilting-meaning a high sharp point on a post or shim can also put them under tension and crack the corner off-especially when the stack is 10-15 shelves tall

Pots depending on size and form can or cannot take the fast up down cycles. Nothing like a power train kiln to produce wares fast. 

Share this post


Link to post
Share on other sites

I am fine-tuning the art of patience, playing the addictive June's Journey hidden objects game. It is complex, with several core activities and the key is strategies, timing, and patience. Not going slow can kill the fun-dead, dead, dead.  I treat my cooling kiln the same way---go slow, and patience, patience, patience. I don't open it until it's done. Oh, OK, maybe just a wee peek at around 200, but real quick. :)

Share this post


Link to post
Share on other sites
52 minutes ago, Bill Kielb said:

After all this talk, I am becoming a believer in waiting until it is ten degrees below room temperature.

Bill,

I’m sorry, I’m afraid I’ve given you the wrong impression.  To clarify, cristobalite inversion is mostly done by 200C (392 F), and is traditionally considered to happened between 210 and 250 C (410-480 F) which is hotter than the above quoted 300 F (150ish C) by Neil et al.

Sorry again for the confusion.  I just wanted to frame a temperature in terms of what’s going on in the clay body/kiln, since that’s what matters.   A temperature is just an arbitrary number until the mechanics of why it matters are there—and i feel like the OP deserves that kind of answer—at what temp and why.

Sorry, I wasn’t clear with what the temp actually was—a poor assumption on my part.

 

 

 

Share this post


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

After all this talk, I am becoming a believer in waiting until it is ten degrees below room temperature.

Well just clocked 45+ C in shade of verandah you're gonna have to be a patient possum here somedays!!!

Seeing's doing or is it the other way around...

If it works do it...just in a controlled manner.....

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.