kristinanoel

So glad you solved this problem!
I had it a while back, mine was also a sieving problem. In looking for the cause, I also came across this great post that talked about the importance of blending. Enjoy!
https://glazy.org/posts/173081

So glad you solved this problem!
I had it a while back, mine was also a sieving problem. In looking for the cause, I also came across this great post that talked about the importance of blending. Enjoy!
https://glazy.org/posts/173081

Hi all, just wanted to follow up for posterity and with gratitude.  I followed the advice to Dremel the chunks and and dab to fill and it worked pretty nicely.  Difficult to reapply a consistent amount of glaze, but it's good enough.
As to the root of the problem, I re-sieved and did something I've never done before: cleaned out the glaze bucket.  I always sieve back and forth, ending up in the old bucket that has 5 years of crust all along the sides.  This time I cleaned all the crusted glaze off.  I suppose I could have scraped and mixed it in well, breaking up chunks and I may do that in the future.
Cheers!

just saw this for the first time.  kristinanoel,  i have been using silica sand under my stuff for many years.  i use whole shelves and pour some, maybe a cup, not a mug full in the center.  this is not inside the kiln yet.   i then use a tool with teeth, like a tile setter would to spread the sand evenly across the kiln washed shelf.  i do not go as far as the edge of the shelf, maybe 3 inches inside the edge.  i have also used medium sized grog the same way.  the results are just a little different.
silica sand will eventually stick and stay where it is put.  but, because it is so lightweight, it tends to float in the air during the first firing and might land on something i have on a different shelf.  not good.  so i switched to medium grog.  maybe larger grog is better, i happened to have the medium at hand.  it does not move in the first or any firings afterward.  i am careful to stay at least 3 inches from the edge of the full sized shelf so that when i put the shelf into the kiln, grog does not roll off the edge onto whatever is below.  it tilts as it goes past the thermocouples but i control the tilt to keep the grog from rolling off.
since i make many large, flat pieces, i fire the grog laden shelves often without changing the grog,  sometimes it moves as i put things in and take them out of the kiln.  i refresh the area with a dry foam sponge sweeping in from the edges and run the toothed tool across the center again.    never inside or on the edge of the kiln, that is asking for stuff to be swept into the element grooves.
hope this helps.

Smooth even distribution of grog or alumina is common. I think @Pres used to use a salt shaker for thin even application. Think of it as little ball bearings to roll over rather than a bed of sand so to speak. So a fairly small sparse amount evenly supporting the ware. No high spots, no large empty spaces in my experience.

congrats!
and I agree with BAM2015, a steady $1k a month is nothing to sneeze at! 

Ah, well I hope you don’t have to abandon the glaze! As for the sand wads, simply place the piece where you want it loaded in the kiln and trace a light pencil mark to show the footprint. I’ve been taught to use a very small scoop to place quarter to half dollar sized piles of sand that can be up to about 1/2” tall as needed. Place as many piles as needed to prevent slumping, and you can sweep it off the shelves and reuse it forever. You may need to sieve out small kiln wash chunks over time. Good luck!

It depends on which one. There’s a few possible replacements being tested, and they have different COE’s. Some manufacturers are also opting to eliminate talc from their recipes because it’s become harder to obtain a version that doesn’t occur naturally with asbestos and/or doesn’t fire some shade of amber from iron contamination.
Talc is a source of magnesium and silica, and has been most often added to glazes to create mattes in larger amounts, or control crazing in smaller ones. if you add too much, you might get dunting under the right conditions. If the right structural stresses are in play, a change in talc source that your supplier might not have mentioned could  have tipped the balance from a combo that was working into one that wasn’t. 
Talc has also been added to light coloured low and mid fire clay bodies to adjust thermal expansion rates, and some other things.  It’s not usually found in darker clays like your stated one, so that wasn’t a likely source in this case.
That’s the Cole’s notes version, and if you’d like an expanded rabbit hole with all the delightfully nerdy extra links, Digitalfire does provide! The notes on Tony’s efforts to reformulate a low fire casting body and associated glazes might also be interesting, even if you’re at cone 6. The principles are the same.
 
 
 
Another firing mitigation tactic you can try is to place test tiles, other small pieces or even lay down extra kiln posts around the rim of the piece. They’ll create a heat sink so the rim/lip cools at a rate closer to the bottom of the plates. If you’ve been making this design in smaller versions, sometimes the larger one will require extra babying. I can vouch for it working on my pieces, and I notice kiln pack can affect things you wouldn’t think are a big deal. In any case, it’s easy to try.

+1

@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. 
 
 
 

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 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? 

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. 

@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! 

@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! 

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. 

@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! 

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. 

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.

@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! 

Just a heads up if you use Tin buy some NOW
Distributor Tin cost has just doubled 
Most Tin is from Mymar and its a mess now politically
I'm not much of Tin user myself but if you are may want to double down on it now

Thanks! 
The container - i'm thinking of that as a big low, flat tray, like an open top sagar, a sandbox, essentially, just to contain the silica for easier cleanup. 
Like a kiln shelf with a raised lip? 
 

My studio is packed up as I'm moving soon otherwise I would take some photos of my kilnbrick spacers/blocks. Bill van Gilder images below of his made from the mix I mentioned in a post above. 
 

Min - 
do you happen to have a photo of what you're describing?
I'm having trouble picturing it but would like to try it out. 
 

Late to the party but I say rutile - I call it 'interesting powder', with RIO a close second.