Cause of these cracks?

[tomhumf]

I've never had a problem with cracking before. Last firing 3 mugs had similar looking cracks, this being the worst. 

I'm trying to understand what's caused it. Its the same clay and glaze I always use. Firing was same schedule. The only thing I can think was it was a cold night when I turned the kiln off. I close the chimney and peepholes but leave the burner port open. I don't down fire the kiln after final temp and it's small gas kiln - about 3 cubit ft. 

I've not had a problem before but maybe fast cooling is the problem? Any ideas welcome...

[Babs]

Prob 

The crack edges are sharp which indicates a cracking on the way down imo.

Cracking on way up in glaze firing, when the glaze melts it softens the edges of the crack.

But F Hamer says these cracks at right angles to rim petering out is indicative of

Rim too thin,

too rapid drying,

tired or over worked clay, clay too short.

Over wet rim.

Overworked rim

If slip used it was applied when dish too dry.

Readsorption.

Have you access to his book? 

He doesn't mention this type of cracking being associated with rapid cool.

Is your kiln brick or ceramic?

Were the mugs affected randomly placed in kiln?

[neilestrick]

Since the glaze is breaking away from the crack, that crack existed before cooling. You probably flexed the rim at some point when it was too dry to flex and it started the crack. They're often not visible until the glaze firing.

[Babs]

Right so if it was a cooling crack the glaze would be right up to the edge of the crack . 

Thanks Neil.

[Bill Kielb]

I think Neil is spot on but ....
I see it as a crease that was present in the firing  we know likely because of the glaze movement around this area (Glaze breaks along the crease in the ware)  and then the ware fractured along this line on cooling and therefore the jagged not rounded edges of the glaze. Am I missing something?

I think that is what Neil was referring to ?

[Babs]

I thought cooling on way down because of jagged edged  but Neil is saying happened before cooling occurred.

Add to that that the crack was pre bisque for whatever of reasons listed by Hamer, and Neil states a reason above.

Whether these cracks would have occurred in a different firing????

Well probably.

When I had a bunch of handles fail coupled with a wierd firing because I had 2 mugs left that I was able to fire in my normal schedule I was shown that the firing had nothing to do with it as the mugs in 2nd firing had failed handles also.

Still up in the air... crack enlarging cooling or heating because of the edges. 

 

[Bill Kielb]

Hmm, I agree cooling crack because of the jagged edges so  it’s very likely the glaze cracked after it had cooled enough to solidify.

At some point Glaze flowed around the area or accent broke as they say as it melted so while fairly liquid the crease in the Clay was there as a crease and not yet cracked.  This allowed the glaze  to accent it like any other surface irregularity and finally it all cracked in cooling.
I think that’s likely correct.

Hard to tell from the picture but it looks like the clay crack is brittle as well so likely post fire or while cooling................I think

[curt]

What Neil said.   Glaze pulled away from crack just like where it breaks elsewhere on the pot says this crack was there pre-glaze-firing.  Likely from Bisque either fired too soft or Bisque cooled too quick, or both.  Have seen similar cracks in our studio.  Or as Neil suggests possibly there from drying stress for the types of reasons Babs quoted above.

[Marcia Selsor]

I would have said possible stress when trimming. Clay remembers. If the lip is thin compared to the weight of the body, it could have stressed the lip.

Marcia

[Min]

I think that another clue as to when the crack formed is the whiteish thicker looking part of the glaze is not running the length of the crack but it is in between the throwing ridges adjacent to the crack. If it cracked on the cool down I would expect the whiteish part to be uninterrupted between the throwing ridges. @tomhumf, do you have any more of this batch of mugs in the bisque state that you can tap on the rim to see if they all ring? Looks like a crack that formed in the drying stage to me too.

[Bill Kielb]

2 hours ago, curt said:

What Neil said.   Glaze pulled away from crack just like where it breaks elsewhere on the pot says this crack was there pre-glaze-firing.  Likely from Bisque either fired too soft or Bisque cooled too quick, or both.  Have seen similar cracks in our studio.  Or as Neil suggests possibly there from drying stress for the types of reasons Babs quoted above.

Wouldn’t the cracked glazed edges be melted and smooth then?

[Callie Beller Diesel]

If the crack was one of those ultra fine ones you can’t see after bisque, it would have been glazed over. The glaze would have melted, expanded and formed around it, but when the clay shrank in the cooling, the crack would have widened and broken the glaze.

[Bill Kielb]

1 hour ago, Callie Beller Diesel said:

If the crack was one of those ultra fine ones you can’t see after bisque, it would have been glazed over. The glaze would have melted, expanded and formed around it, but when the clay shrank in the cooling, the crack would have widened and broken the glaze.

Got it,

would the clay appear melted inside the crack or if not the weakened location then cracked because it was not fused with enough strength and cracked on cooling.

It seems either way, the defect was there during the firing and most likely it definitely cracked on cooling. To what extent  the damage was present during the firing is speculative so I think most of this is saying the same thing. Construction issue and cracked during cooling........ I think

[neilestrick]

51 minutes ago, Bill Kielb said:

Got it,

would the clay appear melted inside the crack or if not the weakened location then cracked because it was not fused with enough strength and cracked on cooling.

It seems either way, the defect was there during the firing and most likely it definitely cracked on cooling. To what extent  the damage was present during the firing is speculative so I think most of this is saying the same thing. Construction issue and cracked during cooling........ I think

If it had cracked during cooling, the glaze would have remained white right up to the edge of the crack. Because the glaze has pulled away from the crack as it would on a lip or ridge, we can assume it cracked before cooling, while the glaze was still fluid enough to break away from the crack. Usually these cracks exist before the glaze firing, or there's at least a weakening there even if it didn't actually crack through. It cold be that it was bumped or flexed or the form was stretched in such a way during throwing that it weakened the rim. Once the pot heats up and expands, the crack appears and/or widens.

[Bill Kielb]

1 hour ago, neilestrick said:

If it had cracked during cooling, the glaze would have remained white right up to the edge of the crack. Because the glaze has pulled away from the crack as it would on a lip or ridge, we can assume it cracked before cooling, while the glaze was still fluid enough to break away from the crack. Usually these cracks exist before the glaze firing, or there's at least a weakening there even if it didn't actually crack through. It cold be that it was bumped or flexed or the form was stretched in such a way during throwing that it weakened the rim. Once the pot heats up and expands, the crack appears and/or widens.

@neilestrick

Hmm,

The glaze crack seems brittle not smooth. To me that would indicated it cracked after melting and cooling to a solid brittle state. If it cracked during the melt I would expect the edges of the glaze to be smooth and melted.

[neilestrick]

16 minutes ago, Bill Kielb said:

Hmm,

The glaze crack seems brittle not smooth. To me that would indicated it cracked after melting and cooling to a solid brittle state. If it cracked during the melt I would expect the edges of the glaze to be smooth and melted.

How could the glaze run off the edges of the crack if the crack wasn't there when the glaze was fluid?

[Babs]

These types of cracks made me open larger bowls from top down.

I used to get to the rim of flaring pot and find i didnt have enough clay to keep the form I was going for to eventuate without throwing a rather thin rim and cracks would often be the end result.

Thanks for spelling out the logic of the visuals

[Bill Kielb]

1 hour ago, neilestrick said:

How could the glaze run off the edges of the crack if the crack wasn't there when the glaze was fluid?

Pretty much as I mentioned above. Rough edged cracks generally mean a brittle failure which generally means  after being cooled and solid. I just don’t see a way around this really. As far as the glaze running or breaking, the deformation was obviously there during the fluid phase of the firing. Had it cracked while being fluid we should see melted edges of glaze and likely melted edges of the claybody. I really don’t see a brittle fracture of material ever being likely to have occurred while soft and flexible really.

[Min]

34 minutes ago, Bill Kielb said:

Rough edged

It seems we are all dancing around this part of the issue. Has the glaze broken over a crack or did the crack break the glaze. From looking at the rim and high points of the ridges it looks like the glaze really thins out on these areas. If for the sake of argument we take a test tile or anything with a sharp edge and glaze it with a breaking glaze the edge will be sharp, the glaze doesn't round over the cut edge.  This is what it looks like to me is going on with this mug. If it was from a cooling dunt then I can't see how the glaze would be fluid enough at those temps to form the distinct breaking thin areas that it has around the crack.

[liambesaw]

If it broke on cooling it would be much sharper. Cooling dunts happen when glaze is already hard.  It looks like it cracked, probably through QI/CI on the way up.  There would still be no glaze in the crack because there was no glaze in the crack when it went in. No glaze to heal over, etc.  

[Bill Kielb]

50 minutes ago, liambesaw said:

If it broke on cooling it would be much sharper. Cooling dunts happen when glaze is already hard.  It looks like it cracked, probably through QI/CI on the way up.  There would still be no glaze in the crack because there was no glaze in the crack when it went in. No glaze to heal over, etc.  

I am not seeing that for sure. Would make for a few interesting tests though

[Bill Kielb]

59 minutes ago, Min said:

It seems we are all dancing around this part of the issue. Has the glaze broken over a crack or did the crack break the glaze. From looking at the rim and high points of the ridges it looks like the glaze really thins out on these areas. If for the sake of argument we take a test tile or anything with a sharp edge and glaze it with a breaking glaze the edge will be sharp, the glaze doesn't round over the cut edge.  This is what it looks like to me is going on with this mug. If it was from a cooling dunt then I can't see how the glaze would be fluid enough at those temps to form the distinct breaking thin areas that it has around the crack.

Interesting,

Maybe, In our for arguments sake example I believe the glaze melts up to the edge and doesn’t pull away or delaminate in a sharp fashion it simply matches the profile of the edge.

 Maybe better close up picture in order.

As far as the glaze moving, definitely a defect during firing that made this run down and away. The question seems to be can it crack during the firing and end up fractured and delaminated later. I would think unlikely but worth the test. Rough edge glaze cracks would never default to in fire fluid  occurrences in my mind. Time to test!

[Babs]

Ok we need some great photos of definite dunting to compare:-))

And a very close up shot of this flaw.

Maybe the sharpness that is in question would show the thinned glaze is melted but it is the tearing further of the rough clay body on the second firing which attracts our eye

Add to that it is common practice to glaze fire much more rapidly does aggravate the size of the preexisting crack , invisible to our eye at bisquing.

[liambesaw]

35 minutes ago, Babs said:

Ok we need some great photos of definite dunting to compare:-))

And a very close up shot of this flaw.

Maybe the sharpness that is in question would show the thinned glaze is melted but it is the tearing further of the rough clay body on the second firing which attracts our eye

Add to that it is common practice to glaze fire much more rapidly does aggravate the size of the preexisting crack , invisible to our eye at bisquing.

From Tony Hansen's digitalfire site.

https://digitalfire.com/4sight/glossary/glossary_dunting.html

Usually from cooling too fast through quartz or cristabolite inversion, or mismatched glaze coe.  

[curt]

Yes now that is a cooling dunt.  Very sharp edged, sharp enough to cut you if you run your finger over it with a bit of pressure.  

The earlier crack does not have anything like this.  The glaze knew the crack was there from the very beginning and simply pulled away from it throughout the firing, just like it was pulling away from ridges elsewhere on the pot (since it is pretty clearly a breaking glaze).