New School, Old kiln and supplies, New Problems

[CoveArt]

Hello Everyone,

 

I'm 100% new to this forum but not new to teaching and firing ceramics.  I just moved to this school however and I'm running into problems I've never run into before when firing some of my students work.  My understanding of the materials I've inherited is that the teacher before me didn't really use them in their few years here but the teacher before them was here for years and used them all the time.  I have 2 ceramics classes this semester (grades 9-12) and so we've been doing the basics.  We've been using a low fire clay body in both red and white and these issues oddly enough have only been appearing on the white.  I will do my bisque firing at cone 04 and then the students are able to use either 05 or 06 glazes and then I'll do 2 glaze firings.  Both are brands I've not had a lot of interaction with before but it's what we've got and as we all know, public schools gotta use what they've got.

 

What I'm noticing is some of our cone 05 glazes are flaking off.  Some a lot, some a little, and (for some students who use the same color in the same firing) not at all.  It doesn't happen to everyone in every firing but it's happening A LOT to some pieces and I feel for the kiddos as they were so looking forward to seeing a completed piece only to have it flake off.  I'm noticing it more with certain colors so I'll be asking those kiddos which ones exactly they used (we've got 3 types of purple in the same brand so I'm not sure which is which on their pottery).  What finally sent me over the edge is when a students piece  practically completely flaked off and then a few days later when I picked it up to move it, it simply broke in half (see attached picture).  I will freely admit, I'm no ceramics expert.  Before this school I was K-12 and was teaching everything from Kinder-basics up to 12th graders on the wheel.  I've been a jack of all trades but master of none at my small town school so I'm out of my depth at this point.  I want to be able to give the kiddos an answer when they ask me what's wrong other than the 4 or 5 guesses I've got.

 

Clay Brand and Type:  Continental Clay, Low Fire Clay (upon googling looks like this says bisque 03 gives best results so this may be my issue right there)

Glaze Brand and Type:  Spectrum Glazes Inc., Low Fire Glaze (says fire to cone 05/04 but we have other 05's so I do them at the same time)

Kiln Brand: Skutt Automatic Kiln

Firing Cycle (bisque): 2 hour preheat, medium speed, cone 04

Firing Cycle (glaze): medium speed, cone 05

[Hulk]

Hi Cove!

Looks like shivering*, classic glaze fit issue - opposite of more common crazing, where the glaze cracks.

I'm guessing you have lots of the clay? You might crank out a couple dozen test cylinders, test each available (well mixed!) glaze, then shelve the glazes that aren't working - try them again on different clays, down the road.

No doubt more forum members will weigh in here; meanwhile, are you verifying heat work with witness cones? ...cones on each shelf indicates the "heat work" achieved, thereby nailing down a variable...

*Shivering (digitalfire.com)

[Bill Kielb]

3 hours ago, CoveArt said:

Clay Brand and Type:  Continental Clay, Low Fire Clay (upon googling looks like this says bisque 03 gives best results so this may be my issue right there)

This is interesting as shivering has a bit of energy in it usually and folks say it pops off their pots. Your description is as if it flakes off so I wondered is this clay fully fired? Looking at continentals site, this clay likely fires to cone 4. It is listed as 04-4.  Firing it only to cone 04 may leave it very soft and the glaze easily falls off because of the mismatch. I would call continental and ask, does this need to be fired to cone 4 to be reasonably sturdy? Judging from the shrinkage chart, I would expect the red to be worse than the white. Neither are great though.

 

 

Edited November 1, 2021 by Bill Kielb

[neilestrick]

It's shivering, and it's not all that uncommon on low fire white bodies. Is this clay right out of the box or recycled?

Definitely try bisque firing a little hotter and see if that helps.

[Candace Jacobs Pottery]

I have been using this same clay for years.  Continental recently had to change their clay recipe.  I have been struggle with the same issue recently.  I have only recently realized after listening to an episode of For Flux Sake that the issue may be the clay body.  During the episode it was mentioned that talc in the clay body will cause this issue. Continental had to change their low fire white recipe because of a mine closure.  They sales person told me the ingredient that cause this change was talc.

I am sure you know that firing your (clay as some one else stated) to cone 4 is not a viable solution to our problem.  I am trying though, to bisque fire to 02 and then glaze fire to 05.  I don't have results yet.  I am also going to try a Laguna Clay to see if that makes a difference in the flaking.  My glaze requires a slip there has been another variable for me to consider. I am using Hirsh Satin Matte glaze.  It is literally just falling off many of the pots.  However there are a small percentage of pots that they clay does not flake off.  I cannot find a consistent factor to success and failure.  

This new clay recipe has also cause a yellow tone under my glazes and I have been battle to fix that as well.  I am hoping as always that I have found the solution, but I won't know this until I have a few more kiln firings.

[Candace Jacobs Pottery]

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Edited October 21, 2022 by Candace Jacobs Pottery

[Kelly in AK]

I definitely see this as shivering. I don’t think it’s just the clay being too soft and not holding the glaze. It could well be both for all I know. The clincher, for me though, is the pot that’s cracked in half. The clay is shrinking more than the glaze.

I’m chiming in because I’ve seen other ways shivering presents and they’re worth sharing because it’s always a nightmare. The O.P. shows what I think of as classic shivering. I’ve had two instances in my own studio that didn’t present this way, but were also shivering. Both were “energetic,” as Bill describes, but not dramatic like a giant flake of glaze falling off. 

Cone 6 soda fired pots were unloaded and sat overnight on a table, in the morning they were surrounded by tiny flakes of glass and dust that had popped off. Some bits were a foot away from their pots.

Two culprits: The worst was a clay body ill suited to soda firing. I learned I can’t use that clay. Those pots sprayed off glassy flakes for days. 

The other was applying a flashing slip (albeit very thinly) to bisque ware. The slip works fine applied to greenware and I still use it, but applied to bisque it shivers. I had a hard time accepting this at first because the slip adheres to bisque just fine after firing. It’s not the slip flaking off the pot, it’s the soda glaze flaking off the slip. This one was more subtle. You would handle a pot and see glitter on your hands. That’s glass, of course. No amount of washing, scrubbing, or sanding could stop it. The pots shed glass. Trash bin. Learn. Keep going.

Finally, I have to give due credit. By complete chance and fortunate timing, it was Julia Galloway who told me, “That’s shivering.” when I described what was happening with the slip. I’m like, “But, but,” and she looks me square in the eye and says, “It’s shivering. Stop putting your slip on bisque ware.” Problem solved. None of us truly manage it alone. Thankful. 

[Callie Beller Diesel]

The thread originally started most of a year ago, so I assume the OP has resolved their issues. Given the timing, it might not have been an issue with talc, and it’s likely they were working with materials using the old stock, especially if they were using inherited materials. But certainly anyone who is encountering this issue with materials purchased in 2022 or later could be affected by changes in the talc supply. It will affect mid and low firing white firing bodies, and many, many commercial glazes. We’re likely going to be seeing adjustments for a few years, as people use up stock, and change over to new formulas. Combining new clay with old glaze batches could result in shivering. We have a post about talc supply changes pinned to the Clay and Glaze Chemistry section if anyone wants to follow along with the info, or if they come across any new info.