Help: stress fractures shattering obsidian clay in glaze fire

[kristinanoel]

Hi friends - 

I'm having a new (to me) problem I can't figure out, something is causing my ware to fracture in the glaze fire.

I'm assuming it's from tiny, undetectable fractures, but not sure what's going on - I've been making these low shallow bowls for a while now with no major issues, and nothing has changed, so I'm stumped. I used to have problems with S cracks which I addressed through better compression but these seem different.

Have you had/solved this problem before? What's the diagnosis? More importantly, what's the cure?

I'm using aardvark obsidian, I bisque to 04 using the suggested firing schedule. 
The clay felt fine when throwing, perhaps a little stiff but not abnormally so, it's never been frozen/thawed, and it's fresh from the bag, not reclaim. 

Thanks in advance!

Kristina

Edited July 12, 2022 by kristinanoel

[Bill Kielb]

I think compression is super important for this item. It’s low and flat so to me the bottom compressed from out to inward evenly is super important. Sidewall vertical as well as rim strike me as essential for this form as well. These cracks do look like the stress was generated during the firing as they were shrinking.

So the question, for large flat bottom items like this as well as platters, do you fire on smooth shelves with grog or alumina underneath to prevent the ware from sticking and dragging during the firing as it shrinks? 

My early thoughts, really good even compression (not simply downward) at the time it’s thrown, not later and alumina or grog beneath during firing so it can shrink and grow easily in all directions.

[JohnnyK]

Something HAS changed, Kristina...Are you using clay that you have had for a while or is it from a new batch from your supplier? If your process and other materials haven't changed, I would look at the clay itself...

[Pres]

I am curios about the thickness of the glaze as compared to the thickness of the pot on a plate such as this. I would also wonder about the thickness of the rim as compared to the thickness of the bottom of the pot. Rereading passages in "Hamer" that are attached to an illustration of a crack similar to yours talks about compression of the base vs the thickness of the glaze surface. If your glaze surface in the bottom of this shallow bowl is thicker than the base of the bowl "the fluxing action of the glaze on the thin base makes the base almost vitrified and hence brittle. In cooling, if the body contracts more than the glaze, even by a small amount, it is too brittle and week to compress the glaze. The body ruptures and the crack probably continues up the side to the rim. . ."The Potters Dictionary, Frank and Janet Hamer.

I hope this helps you out. I have made several low bowls over the years, and find a few things helpful.  These steps also are helpful for plates and platters.

• Never let any moisture sit in the bottom of a freshly thrown bowl
• As soon as possible without warping invert the bowl to set it on its rim for stiffening before trimming
• When trimming, make certain to be careful of the area around the inner and outer edges of the foot ring to make certain these areas are not too much thicker/thinner than the rest of the bowl
• When firing, as often as possible box the forms as in stacking them rim to rim in the kiln.
• When glazing, thoroughly wash the bowl with water without soaking the bottoms more than the rest of the bowl
• Pour or dip the bowl quickly, working for an even coat inside and out, without puddling the glaze in the bottom of the bowl. 

best,

Pres

[kristinanoel]

Thanks so much for these suggestions! 

@Bill Kielb Kiln shelves! - I think that may be the culprit. Your explanation makes perfect sense and aligns with what I'm seeing - the way the stress fractures appear leads me to believe that there's some form of stress being introduced after the drying and trimming are complete, so in the bisque fire. I DID scrape, sand, and re-wash my kiln shelves prior to this effort, and the wash has been persnickety, flaking off and generally acting poorly. I've also not done the silica protocol, but will try that in the future. Thank you so much.

@JohnnyK Not super old - less than a year and stored in my cool, damp basement. I've had no problems with other forms, albeit slightly smaller (plates, smaller forms of this bowl) but I will contact them to see if there are any known issues. That would be a bummer, what a waste of $$$$! 

@Pres thanks for this - I don't think the glaze thickness is a problem in this instance, it's certainly not even close to the thickness of the base, but the relative thickness of the rim to the base is always an issue, I've always had slightly thicker bases than the perfect ideal, but I've never seen this error rate! 
And your suggestions are spot on - I already practice all of them but the boxing of the forms, so I'll try that.  And a question - when I invert the forms for drying, I usually support them from the interior with foam, is that your practice as well? Any cons that I'm not aware of on that? 

Any other ideas? Keep 'em coming! 

Of course, this lesson learned has to happen on a 'big to me' order with a lot of pressure to get it right.
Totally failed, have had to miss the deadline, ask for an extension. 
When you hear people recount stories of "failure and resilience", it's usually from the after position, when they've made a comeback.
I'm here to tell you that in the midst of it, man is it painful. I have a much greater appreciation for the Icarus story. 

Thank you all again for rushing to the rescue.
I sincerely appreciate it. 

[neilestrick]

With a wide flat bottom like that I would fire them on a waster slab. The slab can be very thin, and can even be cracked in pieces. No need to bisque fire the slab, but put alumina wax on it to prevent the pot from sticking to it.

[kristinanoel]

@neilestrick can waster slabs be reused? and do you use the waster in both the bisque and glaze firings? If I don't have alumnia wax, would a little bit of silica sand do the same thing? 

[Pres]

I don't use the foam to turn the pots onto as I believe that causes the area to dry differently than the rest of the pot. I let them set on the rim, thus drying the outside enough to stiffen them, and then trim. I like to trim most forms as soon as I can.

 

best,

Pres 

[kristinanoel]

@Pres 

excellent point. I've had trouble with the center slumping, which is why I've resorted to foam support.
Perhaps I am inverting too quickly. 

I should perhaps more consistently cover the rim and leave the center open, something I do periodically, so that the center dries enough to invert without slumping but without overdrying the rim. Thanks for the input! 

[Bill Kielb]

19 minutes ago, kristinanoel said:

excellent point. I've had trouble with the center slumping, which is why I've resorted to foam support.
Perhaps I am inverting too quickly. 

I should perhaps more consistently cover the rim and leave the center open, something I do periodically, so that the center dries enough to invert without slumping but without overdrying the rim. Thanks for the input! 

I can suggest something to try, it has worked well for us, sort of an economical plaster batt so to speak. For items we want to dry evenly, keep their shape, and not have to risk flipping them, when possible we simply set them on a paper towel right side up.

The paper towel wicks the water away and the bottom dries pretty much the same speed as the rest of the pot without flipping or handling.

Started doing this with the cheapest of cfold recycled paper hand towels and never looked back since. Works great for anything you can remove from the wheel and place directly on a paper towel on a ware board.  Doesn’t stick to the board, drys evenly, don’t have to flip it. Lightly cover to slow down the drying as needed.

you might find a use, we never flip anything on its rim anymore until time to trim.

[neilestrick]

2 hours ago, kristinanoel said:

@neilestrick can waster slabs be reused? and do you use the waster in both the bisque and glaze firings? If I don't have alumnia wax, would a little bit of silica sand do the same thing? 

Waster slabs cannot be reused. They need to shrink with the clay, and once they are fired in a glaze firing they won't shrink any more. They are not usually necessary in a bisque firing because very little shrinkage occurs in the bisque, however if you're getting cracking in the bisque then it wouldn't hurt to try it. About half the total shrinkage occurs in drying, the other half when fired to vitrification.

Silica would probably work, but alumina would be better. Buy 1/4 pound of alumina hydrate, or whatever the smallest amount you can get is, and mix a little in some wax resist. The wax makes it easy to apply, and alumina is very refractory so the piece won't stick to the waster slab. A very thin layer of kiln wash would work, too.

[Roberta12]

@kristinanoel Your glaze is gorgeous!  Is it the same glaze you have been using?  And do you have the same glaze on the outside of the pot?  I ask this because of an experience I had last fall.  I put one glaze on the interior and a different one on the exterior.  A combo I had never used before and what happened looked just like your photo.  

Roberta

[Min]

It's hard to tell from the pictures but if you look closely at the edge of the cracks is the glaze cut sharply across and not rounded into the cracks? Reason I ask is it looks like a cooling dunt to me. If the edges of the crack are sharp and not rounded over it means the crack happened while the kiln cooled, not on the heating up. Where in the kiln were the pots? top or bottom shelf pots worse? how fast did you cool the kiln? Is the outside of the pot glazed? Could you break one of those open and show the profile of the pot up close?

+1 for what Roberta said, gorgeous glaze!

Edited July 12, 2022 by Min

[Rae Reich]

I think @Minmay be right. I had similar breaking when my kiln was cooling draftily and unevenly. It happens if the pieces are on top with the lid cracked, or beside an open peephole early in the cooling. The difference between the dense hotter shelf and the cooling plate can be a factor in uneven cooling. 

[Babs]

When are you opening the kiln?

Looks like dunting.

Waster slabswill help with this too.

A photo of the profile of broken piece would help re thicknesses of pieces

Edited July 13, 2022 by Babs

[kristinanoel]

More great lines of inquiry - thank you all. 

@Roberta12thank you! I love this glaze. There is actually no glaze on the exterior, which is my normal way of making, so no change there. 

@Min can I break one - absolutely. they're already broken! Here's one, I'd say this is on the thinner side for me, but well within the normal range of thickness. as for the break - it' sharp, in some cases it's not even separated, just obviously fractured, and it happened all up and down my kiln. I fired 9 of these. 8 of them fractured. I did have a vent fan going, which I do not use consistently so that could be a problem, although I've not noticed it causing such trouble in the past. I think i will eliminate that variable, however, and just stop using the noisy thing entirely. 

@Rae Reich oh, peepholes for cooling, yes, I have done this forever, it hasn't been a problem before but that doesn't mean it's not one now! great suggestion. At what temperature do we consider it safe to remove peepholes? I generally don't crack the lid before 500 degrees, and then only if i'm in a hurry, but I usually DO remove peepholes early, I never knew that was a problem. Something else to eliminate. 

@Babs I usually crack the kiln between 400 - 500 degrees if i need to get it going again soon, otherwise i wait till i can touch everything, about 150. I do not remember what temp i opened this kiln, but either scenario would have been normal for me. 

Edited July 13, 2022 by kristinanoel

[Bill Kielb]

58 minutes ago, kristinanoel said:

There is actually no glaze on the exterior, which is my normal way of making, so no change there. 

Hmm, glazed inside but not out will often stress the piece more. That could definitely make this more fragile and eventually even for the initial survivors cause sudden breakage months to years in the future. It’s not uncommon for a mug only glazed inside to one day reach its limit after a bit of use and shatter. Better if you could find an aesthetic acceptable partial exterior glaze or perhaps clear glaze solution to try and minimize this possibility.

Edited July 13, 2022 by Bill Kielb

[kristinanoel]

@Bill Kielb  true - that's a risk I'm willing to take for the design. While I don't have pieces that are decades old yet, I've been making this style for about 3 years and this is the first time I've ever seen this kind of shattering, either immediately or with time. And I even throw these things into the dishwasher with no problems to date.  I'm planning to do a full round of troubleshooting test fires, trying out:

• refinishing the kiln shelves
• firing with silica sand during glaze
• firing with waster slab
• slow cooling - looks like a rate of 150/hour down t0 about 1600 sounds like a place to start
• keep those peepholes CLOSED!
• skip the vent fan (maybe can re-introduce after I've figured out the problem) 

Thanks to everyone for all the generosity - any more ideas are welcome! 

[neilestrick]

8 hours ago, kristinanoel said:

slow cooling - looks like a rate of 150/hour down t0 about 1600 sounds like a place to start

Slow cooling at this portion of the firing probably won't make any difference. If the cracking is caused by slow cooling, it's happening at the lower and of the cooling, either during quartz inversion or from opening the kiln too early.

If the cracking is caused by friction with the shelf, then silica or grog on the shelf or a waster slab should fix it.

[kristinanoel]

@neilestrick Ah ha!  The slow cool schedules I found do seem more aimed at achieving glaze effects than addressing cracks - 

Would a natural cool to 1100, followed by a 150/hr ramp to 1000 make sense? 

[Min]

6 minutes ago, kristinanoel said:

@neilestrick Ah ha!  The slow cool schedules I found do seem more aimed at achieving glaze effects than addressing cracks - 

Would a natural cool to 1100, followed by a 150/hr ramp to 1000 make sense? 

If your kiln cools quickly then yes that would help for the quartz inversion. If by chance there is cristobalite in your clay then let the kiln cool slowly from 500 down to 400. Have to remember that the tc is measuring air temp and not clay temp so there needs to be a bit of leeway in the temps you cool through. 

[Ryan Greene]

Hi Kristina,

As many people have said, this is definitely a crack that occurred in the cooling process. I see this as being the cause of either direct contact with the shelf over a wide area, or more possibly, a glaze fit problem. It seems this form has no foot/flat bottom, so using small sand pile “wads” on the shelves could allow the floor to cool evenly with the walls. Hollow core shelves or advancer shelves cool more evenly, but sometimes, they do not cool fast enough to keep up with the wares of much less mass. Using a thin layer of sand may help prevent it from sticking to the shelf, but may not create enough space between to allow for cooling to occur evenly. I think using waster slabs would only perpetuate the problem because it doesn’t go so far to separate the two masses. If you do try to slow cool, I would look at focusing on temperatures closer to quarts. At 1600, the pieces are still very vulnerable to cooling cracks. If you need to slow cool through quartz to prevent cracks like this, then I would say there’s a bigger problem at hand anyways. This brings me back to my original thought which is glaze fit! Do you mix your own glaze? Do you know if this has lithium? Lithium has one of the lowest thermal expansion rates among ceramic materials and almost always causes dunting cracks when only used as a liner glaze. Have you had this problem with other glazes? It could also be the clay body contributing to a poor glaze fit, so there could be a little testing in that. 

[Min]

6 hours ago, Ryan Greene said:

a glaze fit problem

Totally agree that for this style of glazing the glaze has to be a very good fit. I’ld run some dunting stress tests to see how it does.

[Callie Beller Diesel]

A possibility that hasn’t come up yet is if there’s any talc in your glaze recipe, or if you’ve purchased a new batch of materials or pre-mix, whichever you’re using.

If your glaze contains talc, it has been subject to a material change in the last few months. The talc that was predominantly used for potters  is now unavailable, and manufacturers and suppliers have been scrambling to reformulate around the substitutes that have different COE’s.

[kristinanoel]

@Ryan Greene  I'm intrigued by this sand wadding technique - I agree with everyone that this does seem to be happening in the cooling phase, and you're spot on, I also think it's a shelf/air problem where the sides are cooling much faster than the rest of the piece. How does one do the silica sand wadding? As for the glaze - yep, I mix it myself, no lithium, and I've used this glaze/clay combo on other forms, even low plate forms (albeit smaller and with less height, so no fast cooling sides to worry about) without any problem, so I'm thinking it's that irregular cooling problem. If my mitigation tactics don't  solve the issue, I'll likely finish out this order and then abandon this glaze on this form! 

@Callie Beller Diesel well, no talc in this one, so that's something! how does the new talc behave - does it encourage crazing/crackling? pinholes?  introduce any other problems? I've got another recipe that does have talc, haven't used it in a while, but would be good to know.