How to you know if a microcrystalline glaze has crazed?

[Marilyn T]

Background:  I use the MC6 High Calcium Semi-matt glaze and have put them through the oven to ice water stress test.  With transparent glazes I see craze lines after using felt pen and alcohol, or hear some crackling or sighs when immersing the ware in the ice water to indicate that it has crazed, but with the microcrystalline glaze I don't see or hear any of those classic crazing signs.  

How do you tell if a microcrystalline glaze has crazed?   Do you have a picture of it so I know what I am looking for.  I have seen images of crazing in crystalline glazes where there is larger chrystals so the crazing is evident  but not in microcrystalline glazes.  

Edited April 5, 2023 by Marilyn T

[Min]

One from Glazy below. I have found that with dark glazes it is hard to see crazing, there is a thread here where @Pieter Mostert suggests "Fill the vase with a mixture of vinegar and Whiting (it will bubble) and let it stand overnight. The Whiting and vinegar react to form calcium acetate, which is soluble, so will migrate into the body if it allows water to seep through. After you empty the vase, the calcium acetate will slowly migrate to the surface as the body dries out, and will concentrate along any craze lines." If you try this and it works it would be great if you share your findings here as there isn't much info on this. 

I've also read about Ron Roy accidentally highlighting crazing with steam. Apparently he had a bowl of hot mashed potatoes and the steam from the potatoes showed in the craze lines.  Don't know if it was a matte glaze or not. 

Another thing that helps with testing for crazing on glazes which aren't dark is to use Sumi or calligraphy ink instead of a felt pen. It seems to show crazing better than a felt pen. In the image below the author states they used ink.

[Marilyn T]

Min asked:   If you try this and it works it would be great if you share your findings here as there isn't much info on this.

The jury is still out on whether it worked but here are my results:

See the first image below as to procedure.  I filled two cups with the solution and put two other cups in the solution.  One in each solution had been oven heated and put in ice water; and the other had been frozen and had boiling water poured in it (stress tests).  

You can see from the second image that there appears to be a white ring on the inside bottom of the test cup (I assume this is the whiting/vinegar solution).   Black felt pen and alcohol didn't reveal this stress line.  This stress line is on the Oven/Ice Water stress test cup which didn't have any visible crazing.  The freezer/boiling water test cups didn't show this ring, but they both had a single ping when I filled them with boiling water.  

The third image is from a thesis entitled Drink Up:  A study of the Food Safe Quality of Ceramic Glazes with the Addition of Rutile.   Their study used the Spearmint High Calcium Matt that I was testing and which showed visible crazing on reclaim clay.   I didn't get these same craze lines.  

Any suggestions on what I could do next to determine if the cups are crazed or are liable to craze in the future are appreciated.  Right now I've made a set of dinnerware to give hands-on testing to this glaze in my kitchen.  In the future I'm switching to a different stoneware but I've still got 6 boxes of Laguna #65 to use up -- perhaps with non-functional ware. 

 

[Min]

Thanks for posting your results Marilyn.

I'm not convinced the calcium carb + vinegar test did show anything, perhaps the body is too close to zero absorption to soak up any of the solution or perhaps it didn't craze or maybe that test isn't effective.

Have you tried testing the glaze without the copper (so it is lighter in colour) and see if it crazes? We know copper reduces crazing so if testing without it doesn't show crazing then I doubt it would craze with it. The last images really don't mean anything re crazing as it's a different claybody than what you are using. Any chance you can get some ink instead of using a felt pen?

Only other thing I can think of is to deliberately smash some pots with this glaze on it and see how they break. This seems like a last ditch approach though as you would need something to compare the results with. https://digitalfire.com/glossary/fired+strength

[Marilyn T]

Thanks for your suggestions Min.  I tried the smashing and it was revealing and messy (and somewhat therapeutic).   All of the Laguna 65 clay glazed with Spearmint shattered into many shards of different sizes, so it obviously wasn't very strong.  I smashed ware that hadn't gone through stress tests, as well as those that had.  They all shattered about the same degree so crazing may or may not have had a hand in that.  I knew the glaze was under tension so the result was not unexpected.  As a comparison I tried smashing cups that I'd made with different clays, but with the same glaze, and they broke in fewer pieces or they just bounced off the cement without breaking.  I'd previously tried several glazes on this same clay and this HCSM glaze was the only survivor and now, it too, is destined for the discard pile.  All in all, a great learning opportunity but I'm glad it's behind me.