Jump to content

Pots Cracking Way After Firing

Recommended Posts

Hello everyone,
I have some cracking/breaking issues after glaze firing and i would like to discuss it with someone who had similar problem.
I slipcasted the pots (lots of them), than bisque fired them, after glazing, fired again. After the kiln cooled down loaded off the kiln than the pots began cracking, breaking one after another. Even a day after they're continuosly breaking. I've already made a 100 of them about a year ago did everything the same and i don't know what went wrong this time. Could somebody give me some advice in that concern?
Thanks & regards

Link to post
Share on other sites

There is a discussion of a similar issue here: http://community.ceramicartsdaily.org/topic/5888-success-then-failure/


However, in the case of that topic, the cracking was probably related to cooling too quickly. In your case, I'd wager it was a matter of improper glaze fit. The glaze and clay body are expanding and contracting a different rates, during heating and cooling. The stress from that doesn't always manifest itself immediately. Glazes can craze hours or weeks later, and dunting in the clay body can as well. I had the same thing happen to a small vase I made a couple months ago. It was fine out of the kiln, and even a couple hours later. Sometime after that, the tension between the glaze and clay body was too much, and CRACK! I have no one to blame. It was a new form I was trying along with a new glaze combination, bad things were not only possible, but likely.

Link to post
Share on other sites

Could there have been a change in an ingredient in your casting slip? Or in the glaze ingredients? Or are both made from the same materials as the earlier 100 pots? Did you have to buy any new materials to make either the slip or glaze? Sounds like a glaze fit issue; and if things were okay before, then something in the materials used for the slip or the glaze may have changed.

Link to post
Share on other sites

Thanks for your replies! We fired the day before yesterday again (glaze firing) and opened the kiln only today. The temp. was at 26 C, it should be good for opening the kiln, i suppose...There were already two pieces broken/cracked in the kiln. Now we wait if the rest will crack or not. The slip is the same type but new mixing (though it should be the same as earlier - manufacturers mix). The glazes are also the same type but new mixing. - These were mixed followed the same measurements as earlier (by the gram, weighed and calculated).
-Is it possible that something went wrong at bisque firing (too)? Though none were (seemingly) damaged after bisque firing.
-Made some photos so as you can see the crackings. The inner glaze is not that thick and it nicely fired, i mean there are no flowing. The inner glaze and the outer glaze is also transparent. The inner is lead free, the outer contains lead.It makes the color difference. Till now we did not have problems with this pairing. - Manufacturerer suggested these two together, because their "expansion" is the same.
Thanks again for your support!



i am not allowed to upload photos "in this community" the system says. So i uploaded four images to our website:  http://szeghalmys.com/index.php?main_page=index&cPath=67


thanks again !!!

Link to post
Share on other sites

Your photos look like the vases (cruets?) are undergoing dunting, most likely on the cool down. Is the thickness of the glaze application on inside and outside the same? If one is thinner and the other thicker, you might be creating tension in glaze fit that could be resulting in the dunting. Is the cracking along the bottom common? Might the glazes also be thicker near the bottoms -- maybe pooling during firing? Any unevenness in glaze thickness there could be a potential problem. The shape of the ware may affect cooling; the temperature on the inside will stay hotter longer as it has a relatively narrow opening at top to allow heat to escape; while the outside may feel cool, it would still be warmer on the interior. Looks like you are using a porcelain slip(?) Are the bottoms of the wares sticking to the kiln shelves? If so, that sticking may be preventing/hindering the vase from expanding and contracting during firing, resulting in dunting during the cool down.


You might find a copy of Harry Fraser's Ceramic Faults and Their Remedies a handy reference to troubleshooting problems. Lots of photos illustrating problems and loads of considerations/suggestions for remedies.

Link to post
Share on other sites

Yes they are cruet like vases (for holy water) Their neck is really narrow. The inner glaze is pured the outer is coated with brush. Yes it cracks starting from the bottom. It is not a porcelain slip, ceramic (fires at 980-1040 C). The bottom doesnt stick to the shelves - it is waxed. (the bottom rim).

The cruets from the second firing started cracking since my last post (two pieces damaged till now).


The kiln cools "naturally". But theres an option to influence the cooling in the program. Do you think it could be useful?


The glaze on the inside isnt pooling really but maybe it is a bit thicker. Would it help if we would add some more water?


Its a horror now because these same "settings", process used to work a year ago....



Thanks again!

Link to post
Share on other sites

Thin with water . . . maybe. Might be better to try a drop or two of Darvan or sodium silicate to make the glaze flow better without adding water.


Or, just before glazing the insides, pour in some water and empty quickly. That will add some moisture to the bisque and it will not absorb as much glaze when you pour that inside.


How long does the kiln take to cool? A natural cool may be sufficient if that is not too short of time. That you are getting cracking long after removing them (and 26C is like room temperature) would point more towards glaze thickness as an issue.

Link to post
Share on other sites



Unfortunately we have neither Darvan nor sodium silicate at hand, so stays the water solution.  We will try it. And will try to remove the glaze (unfired) from the inside.


The kiln cools from 1020 C to 150 C in about 17 hours. In the first two hours after turning off it drops about 300 C. Then it slows down.


Thanks again for your help.

Link to post
Share on other sites

I agree as this is classic dunting

When you refire a tight glaze piece you always have more risk of problems.

Can you explain why you need to reire these twice-if you can do one glaze fire this may help this issue.

The lead free glaze can go inside as a liner dry the glaze then glaze the outside and fire. Are these two glazes different temperatures? and if so can you fire to same temp?

It sounds like the samebody is used from a year ago?There has been no body changes?

Both glazes look like shiny clears?


Link to post
Share on other sites

Hello Mark,


thanks for your reply. I dont really understand the "refire" thing. First firing is bisque. Second firing is glazed firing. The glazes are the same temp. glazes. Both are transparent. One is lead free (inside) other contains lead.

There has been no body change. I slipcast the pieces from the same mould. - if thats what you meant.

"The lead free glaze can go inside as a liner dry the glaze then glaze the outside and fire." - this is exactly how we do it.



Link to post
Share on other sites

My mistake -I reread your post again -I got confused . You only have a single glaze fire.

My suggestion is to read up on dunning. Thin your liner glaze to start.


It could be the glazes  are pulling your pots apart .

Any chance to have the same glaze on inside and out?


Link to post
Share on other sites


We will try to thin the inner glaze.


The problem is that the inner glaze cannot be used on the outside because it is leadfree and doesnt "flow together" - it doesnt look nice.

The outer glaze cannot be used inside because it contains lead.

This is the current situation, but if we can solve it with thinning the inner glaze. Maybe we should make a drastic change.....:(



Link to post
Share on other sites

The slip is the same type but new mixing (though it should be the same as earlier - manufacturers mix). The glazes are also the same type but new mixing. - These were mixed followed the same measurements as earlier (by the gram, weighed and calculated).

 - Manufacturerer suggested these two together, because their "expansion" is the same.



+1 for it being dunting. I would take a few pieces into your supplier and show them the dunting. Ask them to check with the slip manufacturer and the glaze(s) manufacturer on formula changes. Something has changed. 

Link to post
Share on other sites
Guest JBaymore

Hi from Milwaukee.... at NCECA now.  Headed home tomorrow.  It is over..... so finally some time to get on here.


Sounds like classic dunting.  The real question here is the source of the issue.


In low fire wares...... typically it is a glaze induced relief of stress.  But it could be a body formulation issue also.


Slowing cooling really never solves either crazing or shivering or dunting cause by glaze/glaze/body issues.  It just makes the issues show up later in time.  That is actually worse.  You don't get to see the issue........ but your customer does.  And they may or may NOT tell you.  Bad for business.


The first test is exactly what Mark suggested..... glaze the inside and the outside with a single glaze.  These are not to sell...... these are tests.  But I'd add in some pieces fired in a glaze cone firing with no glaze anywhere also.  That will give more data points to look at.


Lead fluxed glazes are easy to put into compression on lowfire bodies.  Lead-free are very hard to do that.......and most craze.  Heavy crazed high COE glaze on the inside, and compression from the outside......... good formula for creating dunting.  If the casts are thinner than usual..... that decreases the strength of the body to withstand the mis-match... and BOOM. 


Does the lead glaze craze?  Does the lead free glaze craze ?  (Check both using a heavy wash of india ink flowed on..... then wipe it off.  Look closely for the stained in lines.    





Link to post
Share on other sites

  Heavy crazed high COE glaze on the inside, and compression from the outside......... good formula for creating dunting.  


Okay, I'm confused, admittedly though that doesn't take much most days.  I've always assumed it's worse the other way around. 


From Clayart posting between Ivor Lewis and Ron Roy (and many others): 


Ron Roy on tue 4 apr 00

" I can't stand it any longer - I just have to point out what this is all about.


If a tea pot cracks when water is poured in - even if the tea pot is frozen

- then the liner glaze is in too much compression. The problem is

compounded if there is a crazed glaze on the out side. The problem is less

if the same glaze in applied inside and out.


By all means warn your customers if your tea pots, mugs and cassarolls have

the inside glaze putting too much preasure on your ware and have them warm

the pot first - it will help those pots to last longer - but the problem

will still be there - waiting.


It is not difficult to test ware to see if that is the problem - if it is -

contact me and I will fix it for you.




plus this one: 


Ron Roy on sat 9 oct 04

 "Hi Ivor, 


That is not how it works - the low expansion glaze is in compression on the 

inside and the outside - because it's bonded to the body. 


This works in favour of durable ware because it makes the ware stronger - 

if the same or similar expansion glazes are on the inside and outside - up 

to a point. At that point the strain becomes to much and either you have 

shivering and/or dunting. 


Think of it as the glaze becoming too big for the body during cooling - 

both inside and outside. 


The classic situation - hot water in poured into a tea pot - inside glaze - 

already under compression from the body - expands from the heat a micro 

second before the body and the pot cracks. Sometimes it happens the first 

time tea is made - sometimes weeks, months or years later. 


The worst situation is a low expansion on the inside and a crazed glaze on 

the outside. The crazed glaze has already started micro crackes in the body 

making the failure from a glaze under compression on the inside more 



Pots which are ment to be subject to heat shock will last longer if they 

have glazes that are just under compression - or grazed - as a crazing 

glaze does not cause failure due to heat shock. 


Keeping inside glazes on the thin side in mugs and tea pots helps minimize 

the effect the glaze has on the body for example. 



Regards - RR"




Last one from Tony Hansen:


“A worst case scenario is a large flat plate made from a high silica non-vitreous porcelain, covered with a crazing glaze on the outside and excessively compressed one on the inside, having a thin lip and a thick base, fit tightly into an electric kiln with the lip close to the elements and cooled quickly through quartz inversion.â€


Link to post
Share on other sites
Guest JBaymore

Yup those combinations are the pretty much the "worst case scenario"..... as stated.  But that does not mean that the other combination (reverse) will not do the same thing,...... as is also stated there.  It is just not as "easy" for the issue to exist to blow up the structure of the pot.  But is one side is in compression and one is in tension......... that issue is there.


I was not commenting on the "worst case scenario" or even the typical scenario..... just on an explanation for what is being observed HERE.  Something is causing this.


Ron and I are old friends and have presented together on glazes at NCECA......... we are on the same page in these comments.


There a very limited possibilities as to what is causing this.  The glaze issue is the most likely.  Secondarily is the body itself.


Process of elimination.  Only testing will tell.





Link to post
Share on other sites
  • 3 weeks later...

Hi Everyone!


We managed to solve the issue in the meantime, with your help of course!!!!! Thanks!!!!!


In the end we used the same glaze inside and outside - that was the solution. The ones which were glazed with two different glazes : many of them cracked, but the ones with thicker body didnt.

We think that it would have worked with the different inside - outside glaze, with thinner inside glaze. But we didnt have the time to try every scenario.


Thanks again for your support! I hope that this topic will be helpful for others with the same problem!



Link to post
Share on other sites
  • 3 weeks later...

When using 2 glazes did you wait till the first glazed surface was dry poss. overnight, before applying the other glaze? with thin ware you may have had quite a thickness difference if you did not do this, would have added to the problem

Yes, we waited at least one day. But in the end using the same glaze solved the problem. Some of the different glaze pieces survived, but only the ones that were thicker.

Link to post
Share on other sites


This topic is now archived and is closed to further replies.

  • Create New...

Important Information

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