Low vs Mid fire: Crazing and Warping

[Cfeng]

I recently joined a studio that is mid fire, I didn’t think much of the switch from my low fire work, as the owner makes her own glazes to fit the 5 clay bodies she sells. 

Fast forward, all of my hand built work has warped or crazed significantly. The owner uses the same clear gloss on her thrown pieces with no issues. From what I’ve always been told, crazing is simply an issue of a glaze not fitting your clay body. Now I’m starting to believe hand built vs thrown changes the clay body or the rate at which it expands/shrinks - has anyone found this to be true? 

I’ve never had anything warp in low fire, I presume thats human error and that the level of precision with slab work needs to increase with firing temp. My pinch pots and mugs warp significantly less. NO warping occurs in bisque, only in glaze at cone 6. 

what I thought was interesting, is when I use a matte glaze, my work doesn’t warp. When I use the owners clear (dipped) my work tightens up (it’s like putting on Spanx as a human). Could all of my troubles just be said clear glaze?

[Callie Beller Diesel]

Hi and welcome to the forum!

The difference isn’t just attributable to the one clear glaze, I’m afraid. While it’s possible in a universe of infinite possibilities for a glaze to cause a piece to warp, it’s not something I’ve heard of or experienced.

As far as warping goes, earthenware is soooo nice to work with, because it resists things like that so much. You can really put it through some abuse that other clays won’t take, and it isn’t pyroplastic at all. When you start working with mid and high fire clays, especially any whiter ones that have more frit/feldspar/other melty things, things can move in the kiln. If it’s moving in the kiln (as opposed to resuming a shape via plastic memory as it dries), it could be your design that causes the piece to heat or cool somewhat unevenly, or it could be the kiln is packed in such a way that there’s more heat sink materials like kiln furniture or other pots that cause the same. It’s usually more noticeable with large pieces, or really thin ones.

Crazing is definitely a mismatch of clay/glaze COE’s. That part isn’t wrong.

There’s a few possibilities that could be happening here. 

A lot of potters unfortunately don’t make a practice of shock testing their glazes in order to bring out any delayed crazing.  Your teacher may not have done this, and it’s an extra step that not everyone thinks is necessary. It can be hard enough to find a clear that fits, so when folks find one that seems to work, they don’t try to fix it until it’s obviously broken.

If your pieces are a different thickness than your teacher’s, it could mean that you’ve set up the right conditions to bring out that crazing. If  one of you glazes all over and the other doesn’t, that could also be a factor.

Another issue could be related to a talc supply issue that’s affecting potters. Basically, everyone was using one kind of talc that isn’t being sold to potters anymore, and there’s been a rush to reformulate a bunch of clays and glazes with substitutes that have different COE’s. If you’re working with a new batch of some form of cone 6 white stoneware or porcelain and the glaze batch is not as recent, it could account for crazing on your work but not your instructors.

[Min]

Another couple things that could explain why the clear crazed for you but not your instructor could be yours might have been in cooler parts of the kiln and/or your glaze application was thicker. A thinner glaze can be uncrazed when out of the kiln but it’s likely to craze with use or over time if the same glaze applied thicker crazes sooner. Under firing a glaze can also cause crazing.

[Cfeng]

Wow, thank you for the information! 

When I keep bringing this up at the studio, it’s just dismissed as being too thick even with a 1 second dip. My mom has been working with ceramics for many years and has her own kiln - I’m very weird about crowding and placement when I can use hers and it makes a big difference  - but in this studio there is one kiln and it gets packed! It makes me extremely nervous - I’ve lost pieces to this issue. 

I agree with the shock test. My pieces take a day or two to start crazing and if I do ice cold or heat, it’s immediate. I was thinking about buying one of the owners mugs and testing it. 
 

But in conclusion, I’m finding a new clear glaze  
 

Thank you!!

[Callie Beller Diesel]

@Rockhopper cool!

I mean the emphasis to be “I believe it’s possible” when I make statements like that. I’m trying to get away from making too many absolute statements when answering technical questions online. If there is a rule in ceramics, there is very likely an exception to it. 

@Cfeng If your mom’s kiln isn’t as tightly packed as your instructor’s, the instructors kiln is probably cooling more slowly due to thermal mass. That will definitely slow crazing down. A tightly packed kiln is more efficient to fire, and makes a lot of other glazes turn out better. It’s frustrating to loose pieces to kiln kisses though. 

It might be impolitic to buy one of your instructors pieces to test it for crazing. If you want to clarify something, I’d suggest making 2 test cups and firing one in each kiln to compare the difference .