Delayed dunting crack delayed crazing crack

[FelicityPalumbo]

Hello,  I am trying to solve an issue that only appears in about 1 of 50 functional pieces I make using J Cast porcelain casting slip and a clear glaze (Silica 3.35, potash feldspar 3.01, whiting 1.90, Kaolin, 1.39 and talc 0.35) Thermal expansion on digitalfire insight of 6.9.  A fine hairline crack sometime appears weeks to months after firing and multiple uses and the piece make the dull thud when tapped as opposed to a ring.  I have done multiple ice water freezer/boiling water checks and cannot recreate the crack and no crazing happens.  I sell a lot of my pieces so I don't know how they are treated but have had some returned with this hairline crack that I can't reproduce.   When I get dunting in my kiln its always a proper crack of a plate but never a cup or bowl.  When I accidentally break a cup on my concrete floor it breaks into only a few clean breaks.  Could anyone suggest whether this a firing issue (I fire to 1280 degrees celsius very slowing with controlled cool until 500) or a glaze/body mismatch.  Thank you.

[Bill Kielb]

Generally delayed glaze issues are a result of enough of a fit mismatch. Very hard to predict and ……… there is no “for sure” test or way to predict when they will occur. Interestingly crazing would be an indication that the glaze recipe is trying to keep the ware in compression. Light compression makes wares considerably stronger. Too much compression results in the glaze crazing. So of the flaws potters often prefer to adjust out of, crazing is preferred because it has the good chance of leaving the ware glaze composition in  slight compression.

In my experience, published or calculated coefficients are just that. Useful perhaps for looking at trends but cannot be correlated well to whether things will match at a certain number. Testing for future defects is a really good practice but also not a for sure thing as well. Adjusting a glaze to stop it from crazing is often the only easy inferential way or indication that your combination is in compression, which then would imply it is as strong as it can reasonably be.

With respect to firing, this is interesting because it could affect this combination you have used. I would fire to cone, not peak temperature as it ought to be the best way to ensure your combination is as mature and strong as practical. Ceramics mature by time and temperature so there is the chance that your combination is not as strong as it could be fully matured or it’s final fired characteristic will be stronger when fired with the right amount of heatwork. No way to know that right now since we only have a peak temperature and no indication of heatwork used.

Also interestingly it looks like you do not glaze down the entire outside of your mug. Partially unglazed areas and the difference in tension / compression locally can weaken the final product. You might try glazing down a bit further and see if this improves things over time.

What cone does your clay fully mature at?

[Mark C.]

I agree with Bill on firing to a cone temp. (at least test cones) and the unglazed surface will also make for more issues.

I think you need to test the fisnished ware more. You can pour boiling water into them and see if this cracks them . Do this test a few times as once may not be enough.

Plates can crack in a micro wave  pretty easy as well.  Bodies with talc also can be prone to this type of cracking

[Min]

3 hours ago, Bill Kielb said:

Too much compression results in the glaze crazing.

Say what? Could you expand on this please?

@FelicityPalumbo, just to clarify, in the first image above the glaze is only going down the outside of the pot for the first 1/3 or so? If this is the case then the mismatch of glaze to clay fit is likely the cause of the dunting. To only glaze part of the outside greatly increases the chances of dunting unless the glaze fits extremely well. Root of the issue would be the mismatch between clay and glaze but since it sounds like you are right on the edge of a good glaze fit then contributing factors could be glaze thickness as compared to clay wall thickness, possibly the cooling speed of the kiln and/or overfiring the claybody.

Instead of the theoretical analysis for both the potash feldspar and the kaolin what are the actual materials you are using? Does the J Porcelain casting slip you use come with the blue stain added or is this something you add?

Welcome to the forum.

[Bill Kielb]

2 hours ago, Min said:

Say what? Could you expand on this please?

When.a Glaze crazes it puts the body in compression.. Slight compression of the body adds strength to the ware. As the glaze goes deeper and deeper into tension and the body does not shrink, the glaze pulls itself apart causing little fissures in the glaze. A big mismatch, more craze lines.

EFFECT OF GLAZE VARIABLES ON THE MECHANICAL STRENGTH OF WHITEWARES

• January 2003

Authors:

JENNIFER L. BENSON

 

 

[Mark C.]

Bill that link was not working

[Bill Kielb]

Just  screenshot to the article, here is the link to the screenshot below. It basically behaves like asphalt roadway, winter, it shrinks faster than the base and cracks the asphalt, Summer it grows faster than the base buckles and delaminates. Glaze in tension compresses the clay. Slight compression strengthens the ware, similar to prestressed concrete. Compressed work has been measured up to 20% stronger, right up until the glaze crazes.

https://www.researchgate.net/publication/255660995_EFFECT_OF_GLAZE_VARIABLES_ON_THE_MECHANICAL_STRENGTH_OF_WHITEWARES

[PeterH]

1 hour ago, Bill Kielb said:

EFFECT OF GLAZE VARIABLES ON THE MECHANICAL STRENGTH OF WHITEWARES

Full text at https://tinyurl.com/2p98akx3

[Min]

I believe this is the complete article you are linking Bill?  https://aura.alfred.edu/bitstream/handle/10829/289/Benson2003.pdf?sequence=1&isAllowed=y

54 minutes ago, Bill Kielb said:

When.a Glaze crazes it puts the body in compression..

This is what I have problems with.

55 minutes ago, Bill Kielb said:

Slight compression of the body adds strength to the ware.

Yes.

From page 15 of the above link from Benson on page 15 1.1.1 Coatings & Strength :  "As a glazed whiteware piece is cooling after it has been fired, stresses build up in the glaze and body due to the thermal expansion mismatch between the two. The body wants to contract more then the glaze, which causes the glaze to be placed in compression and the body in tension.6 This should cause an increase in strength because of the fact that ceramics break when placed in tension.4 If the glaze is placed in too much compression, however, shivering can occur. Shivering is when a sliver of glaze with body flakes off the piece, because the body has fractured. When a glaze is placed in a state of tension, i.e., when the glaze has a higher coefficient of thermal expansion then the body, a result called crazing can occur. Crazing occurs because the glaze wants to contract more then the body during cooling; it is therefore placed in tension, causing cracks, crazing, in the glaze."

As the last two sentences state, a crazed glaze is in tension, not compression. 

[neilestrick]

The glaze on the piece in the photo above appears to be relatively thick. Have you noticed the cracking occurring on pieces where the glaze is thicker than on those that don't crack? Since you're not getting crazing, I'm leaning towards the cracking coming from part of the piece being unglazed. It could also be that a weak spot was created at the lip during handling of the piece before bisque firing. Usually when I see cracks like that coming straight down from the lip it's because the piece was flexed and cracked before it was fired. However that type of crack usually shows up in the glaze firing, not delayed.

What's the thickness of the piece like? It appears to have a thin lip, but is the rest of the piece also relatively thin? Thin porcelain and thick glaze on only one side is a recipe for getting ripped apart.

13 hours ago, FelicityPalumbo said:

Could anyone suggest whether this a firing issue (I fire to 1280 degrees celsius very slowing with controlled cool until 500) or a glaze/body mismatch.

Post your firing schedule. From a glaze standpoint, there's no reason to slow cool below 800C, as the glaze is set at that point. If you're slow cooling below that to avoid dunting, that shouldn't be necessary unless your kiln cools very quickly. Like VERY quickly. Any normal kiln shouldn't have that problem. Even in little test kilns which cool from 1200C to unloading temp in about 6 hours, dunting isn't a normal thing.

[Bill Kielb]

3 hours ago, Min said:

Crazing occurs because the glaze wants to contract more then the body during cooling; it is therefore placed in tension, causing cracks, crazing, in the glaze."

As the last two sentences state, a crazed glaze is in tension, not compression. 

This is an interesting subject as folks often mix and match tension for compression and body for glaze layer / cause and effect. I try to always speak from the perspective of keeping the body in compression just to avoid this argument of perspective because: keeping the claybody in slight compression improves it’s strength. This is true over the life of the ware so good to know for potters I think, as it can strengthen the ware. The additional important point to me would be it strengthens the pot right up to the point where it crazes. Adjusting just out of crazing is not such a bad thing for this reason.

I like to use the roadway example because it’s more intuitive and alway try to speak from the perspective of what’s happening to the clay body. It’s not perfect, but my reading of all that has been said above is fairly consistent with exception to your citation above which begins by stating the body shrinking faster than the glaze layer can put the glaze layer in compression and concludes with tension in glaze as a cause of crazing. While all reasonably true, the switch in perspective I agree is confusing.

I find it a bit disconnected and confusing for most but this is a great discussion, always looking for the simplest most understandable way to present this. Mixing them seems to mix folks up.

To me the important point: a glaze that keeps the clay body in compression will strengthen it. Glazes that craze are putting the clay body in compression. Work yourself slowly out of crazing and you may have a glaze keeping the body in slight compression.

Glazes that shiver have serious issues but also do not strengthen the ware. Are they expanding faster than the clay or is the clay shrinking faster, the glaze layer has less mass so it probably drives this dynamic more often than not. This seems to confuse things a bit though.

[FelicityPalumbo]

Thanks everybody for your help and suggestions.  @Min I stain my slip work with 2% Scarva nano colours but the worst example of these cracks were actually on unstained platters that I had made thicker (8mm) for a restaurant that were only glazed on the inside.  To test for thermal shock resistance I have spent quite a bit of time putting my pieces in boiling water for 5 minutes boiling away then straight into the freezer for 20 minutes or until frozen through and then back into the boiling water repeating many times, and I cannot make them crack/craze/shiver.  I have taken a hammer to many pieces and they are surprisingly hard to shatter and only break into 2 or 3 pieces with clean breaks.   I use these pieces in my cafe and have only had 1 cup that was about 8mm in thickness with 2mm glaze get this crack.  But this is obviously not acceptable if that one ended up as a customer purchase.  @neilestrick I very slowly fire my kiln up and down as everything just comes out better with less pinholes/dunting plates.  I use cones to confirm a temp of 1280 pretty consistently throughout my kiln.  @Min I feel like my glaze is close to a good fit, but how do I know if it may be a crack that is closer to a crazing crack due to the glaze being in tension or a crack that may be due to my glaze under too much compression, understanding that slight glaze compression is positive.  Am I right that if my glaze was under too much compression and that was the reason for the delayed crack, the wear would shatter more when hit with a hammer as pieces with too much glaze compression sometimes do?  Thanks again everyone.

[Bill Kielb]

2 hours ago, FelicityPalumbo said:

Am I right that if my glaze was under too much compression and that was the reason for the delayed crack, the wear would shatter more when hit with a hammer as pieces with too much glaze compression sometimes do?  Thanks again everyone.

I would suggest you think of it as a glaze that puts your claybody in compression strengthens it, right up until the glaze crazes. So in essence, one cannot have too much claybody compression. Sort of like the break the egg trick by squeezing between the top and bottom points using one hand. A Glaze squeezes the clay three dimensionally though so partially glazing down the outside leaves an area where stress can be present because there is no glaze to offset the force of the inside glaze.

since your glaze is not crazing, or shivering for that matter, it appears fine. I would try glazing farther down the outside to see if that improves durability.

Just curious what Cone do you fire to and what range cone is your clay rated for?

[Min]

4 hours ago, FelicityPalumbo said:

To test for thermal shock resistance I have spent quite a bit of time putting my pieces in boiling water for 5 minutes boiling away then straight into the freezer for 20 minutes or until frozen through and then back into the boiling water repeating many times, and I cannot make them crack/craze/shiver. 

I do a much harsher stress test to check for too much compression of a glaze (too low a CTE/COE). My method is to make quite a few very thin walled cylinders. Mugs without handles or whatever cylinder shaped mold you have would be fine. Cast them very thinly then bisque. Now glaze the insides only and glaze as thickly as possible, you are trying to create "the perfect storm" and force the tests to dunt if it's possible. Fire them with your usual program. Put the fired pots in the coldest part of your freezer overnight. Bring a kettle of water to the boil and put the test cylinders in the sink then fill them up with boiling water. Repeat this several times. Yes, this is extreme and no this isn't something that any normal person would do but what it's doing is stressing the clay to show if there is a glaze/clay fit mismatch. 

4 hours ago, FelicityPalumbo said:

I feel like my glaze is close to a good fit, but how do I know if it may be a crack that is closer to a crazing crack due to the glaze being in tension or a crack that may be due to my glaze under too much compression, understanding that slight glaze compression is positive. 

Run a progression blend. Find where the crazing starts then take a step back to slightly reduce the CTE/COE. Easiest way to do that would be to reduce the silica and kaolin by a ratio of 1.25:1

A glaze that crazes won't put the clay under compression. Crazing is the result of a glaze in tension, in effect the glaze is too small for the pot and is being stretched apart to fit the larger clay. Compression is the opposite, the glaze is bigger than the clay, as the pot cools the glaze is pushing in on the clay. A little compression is good, makes the pot stronger, too much and there are problems such as shivering and thermal dunting that is exacerbated by too great a mismatch of glaze/clay  fit. 

4 hours ago, FelicityPalumbo said:

Am I right that if my glaze was under too much compression and that was the reason for the delayed crack, the wear would shatter more when hit with a hammer as pieces with too much glaze compression sometimes do?

I had a load of soda fired mugs that suffered the same type of cracks as you posted. Mugs were very thinly thrown, glazed on the inside only and didn't receive enough soda in the firing so in essence they were glazed inside and only very lightly glazed outside. I still used them but over time they cracked. If the tension in the pot has been relieved by the crack my thought would be they won't shatter as spectacularly as a pot glazed all over with too low an expansion glaze (therefore clay under compression from a sizeable mismatch of glaze and clay CTE's/COE's.

Are you glazing the undersides of your plates inside the footring or are those unglazed also?

Possible simple solution would be to glaze the entire inside and outside if that's an aesthetic you are okay with.

[neilestrick]

17 hours ago, Min said:

Possible simple solution would be to glaze the entire inside and outside if that's an aesthetic you are okay with.

Or I was thinking use a satin clear for the bottom half or a matte glaze with the same stain in it so they still get the two-tone effect.

[Pyewackette]

So all this talk of only glazing partly down the outside potentially weakening the piece has me a bit concerned.  I've gotten a taste for chattering.  I'm not GOOD at it yet but I'm working on it.  You can't really glaze over chattering or it will be hidden by the glaze.  Hsin-Chuen Lin solves this by using only underglazes over chattered areas and only glazing the insides and outside only partway down from the tops of his chattered pieces (porcelain).

Would this strategy put the piece at danger of cracking somewhere down the line?

My pieces are stoneware and considerably thicker at the moment. I have a bunch of small sake/tea cups from working on re-learning how to throw off the hump, which I have also been using as chattering practice and I had intended to follow Lin's method.  There is no danger of any of these pieces ever being sold - but down the road, am I asking for trouble?

[Bill Kielb]

1 hour ago, Pyewackette said:

You can't really glaze over chattering or it will be hidden by the glaze. 

Some glazes work just fine depending on the desired look. As far as stress and partial glazing, lots of folks do it successfully and enough do fail that it is an added risk that potters should be aware of.

[Callie Beller Diesel]

2 hours ago, Pyewackette said:

Would this strategy put the piece at danger

There’s a bunch of qualifiers to this. There’s a bit of a sliding scale where an allover glaze that fits is best practices/ideal strength, but that doesn’t make all other options unworkable.  An allover glaze that fits results in a pot that you have to put some real intention into breaking. But even the best piece is going to meet an end if high velocity and pavement are involved.

That doesn’t mean the partially glazed pieces are totally unusable, it means they’re more susceptible to this kind of cracking. You’re not going to be knocking around decorative/occasional use/gentle use items all that much, so partially glazed it is fine in those conditions.

If you’re using a glaze that does craze, the tension is already broken, so the glaze isn’t trying to pull the piece apart. Maybe not the best for a flower vase with a clay that isn’t fully vitrified, but for a butter dish or covered box of some kind? Why not?

If the wall of the pot is thicker than the glaze or the pot design absorbs tension well, the glaze isn’t going to contribute to the strength of the piece, but it won’t pull it apart either. Sturdy 10 lb serving bowl/platter with a gradual and shallow curve and a trimmed foot? Probably going to be fine. Super thin, flat minimalist platter with an angular floor to wall transition? That’s going to need some babying from both maker and end user.

[Pyewackette]

@Bill Kielb Let me rephrase, *I* haven't found a studio glaze that will break over chattering like that (yet).  The one time I glazed over some fairly deep chattering, the glaze just ran into all the little cracks and crevices.  Just enough edges showed up to make it look like crap.

Can you share with me what glaze that is, or what types of glazes are most likely to have similar effects?

Also they're getting ready to change all the studio glazes, or at least lots of them, so maybe if I have an option or two for that sort of glaze I can lobby for it to be added to the line up.

@Callie Beller Diesel Well yeah, I'm not looking for something to survive being lobbed into a brick wall.  I just would like it NOT to be more likely to crack just all by itself under normal use. I have a lot to learn about glazes and glazing.

Frankly I've been a little iffy about the idea of doing the underglaze thing without covering it at least with a clear glaze.  The one time I tried to glaze chattering turned out so awful I'm a little gun shy. That was on the one bowl I've ever thrown that had the bottom blow out (y-cracked, probably from uneven drying on plastic bats, I have my own hardiebacker ware bats that I schlep around with me now).  I was actually sort of glad it was trashed, given how awful the glaze looked.  I had a definite excuse to throw it away, LOL!

The one person I have talked to about it seemed to think clear glaze over underglaze on chattering was a bad idea.  I can't remember why.  I should probably try it.

Right now I have difficulty imagining me ever throwing anything even approaching "thin", but even when I get better at it, I'm pretty sure minimalistic super thin flat things are not going to be a large part of my aesthetic.   

[Bill Kielb]

26 minutes ago, Pyewackette said:

Can you share with me what glaze that is, or what types of glazes are most likely to have similar effects?

Sorry, just a clear glaze with stains added to taste. We have wiped underglaze into the chattering / carving  and lightly wiped off the surface and have glazed over it to accentuate things. It’s not uncommon to glaze over underglaze, transparent or even semi transparent. Semi transparent and fully transparent examples below

[Hulk]

I love chattering!

The look and feel can vary quite a bit - from barely perceptible to loud and rough; I'm glazing over almost all chatter.

Looking to avoid any crawly spots - where the glaze hadn't "wetted" all the way to the bottom of the mark - I fill the marks with underglaze or glaze (glaze is easier), wipe away the excess, allow to fully dry, then glaze.

Some variables:
  how soft the clay is
  how sharp the tool, how springy, how wide the cutting edge
  the angle to the clay, err, angles, for one may vary the presentation angle in more than one plane, eh?
  how tightly the tool is held, where on the handle the tool is held
  how fast the tool is moved, how it is moved
  how fast the wheel, err, how fast the clay, for a wide piece moves Much Faster at the same rpm
  how the chips are handled, if they stick to the work
  how smoothing is done, if any - when the chip is cut, the ware's profile is disturbed
  what material the chatters are filled with

Here's a few.
The teal is more opaque, and lays flat - the chattering is muted.
The blue and red are more translucent...
The green is somewhere in between