Clear Glaze Crazing On ^6 Laguna Frost

[angela_w 4]

Hi, I am a total newbie to making glazes and I just ran some tests at ^6 on Laguna Frost. My objective was to find a clear glaze that didn't craze, and I tried a few glossy and a few satin/matte options hoping to find some success. 

 

To test for crazing, I used a procedure described in Mastering ^6 Glazes, where the sample is heated in an oven at 300 degrees, then quenched in water. Then I used a blue sharpie marker to reveal any cracks that formed. All of my samples crazed after this test, so I am hoping to get some advice on how to interpret the results. I've attached a couple images of the results.

 

The tiles are arranged in order of the expansion coefficient (as generated in GlazeMaster), with the lowest on the left (6.29) and the highest on the right (7.16). The tiles labeled 9, 6, and 11 are satin/matte and the rest are glossy. Tile 15, on the left, is the low expansion recipe from Mastering ^6 Glazes, which I had been optimistic about since the book suggests it won't craze on the majority of Cone 6 bodies. I've read that adding silica will help solve crazing issues, but since even the low expansion glaze crazed, it leads me to wonder if I am doing something else wrong that is causing problems with the clay body itself? 

 

I bisqued the tiles to ^04, but did not do any sort of controlled cooling on the bisque. Also, the cracking seems to extend pretty deep into the porcelain body, as shown in the detail image. I added more ink to the sample on the unglazed portion to reveal the cracking. Is that a normal byproduct of crazing, or is it possible the body itself is cracking and the glaze defect is actually appearing as a result of that?

 

A few more details: The witness cones on the same shelf as the test tiles show the 6 cone bent properly and the 7 cone just starting to lean. I tried to do a controlled cooling using a portable pyrometer (it's a manual electric kiln) but suspect I went a bit faster than is recommended in M^6G. When I opened the kiln to take everything out, I got a reading of 185F on the pyrometer, so it seems that should have been a safe temperature.

 

Any advice on next steps I should take would be appreciated. Adding silica to the glazes is on my list, but are there other things I should be considering? Thanks in advance for any help.

[Min 777]

Some people think that the oven and water test does stress the body so they prefer the boiling water then ice water test. If you have more glazed test tiles you could try that instead. However, when I recently tried the boiling/ice test I didn't have crazing so I went ahead and glazed some pots which then proceeded to craze so I have gone back to the oven/water test.  I subject each test to 3 oven/water cycles, oven up to 320. (I found Sumi ink to be easier than felt pens for the staining)

 

The tiles with the crazing the furthest apart will be the glazes that are the simplest to fix, silica additions alone might be enough there.

If an increase in silica doesn't fix it then you need to swap out the high expansion fluxes for low expansion ones. This is where glaze software is a huge help.

 

You don't need to slow cool bisque so no problem there. Fast cooling (just shutting of the kiln) wouldn't cause your glazes to craze so for the clears the crazing isn't coming from not cooling exactly as in MC6G. Your satin glaze looks nice so I think you did slow cool enough.

[Biglou13 202]

See COE.

 

Porcelain is special. It needs special glazes designed for porcelain.......

 

Read above post .

[Marcia Selsor 1,301]

I use frost and had a crazing problem with one glaze. I fixed it by adjusting the silica. my notes are in the studio and I am in bed? I can tell you tomorrow how much I used? I am thinking it was 2%but let me check in the morning.

 

Marcia

[angela_w 4]

Thanks so much for all your comments. The point about fixing the tiles where the crazing is furthest apart is really helpful, it makes perfect sense when I think about it… but I hadn't thought of it!

 

I am going to focus on trying to fix tiles 12 (glossy), 11 (satin), and 6 (matte) and will post results back here when I have them. Also, I am going to fire some test tiles without any glaze on them, to see what happens when they're subjected to the oven/water test.  

[Biglou13 202]

Curse u Angela......(in a friendly manner)

Many of my glaze and clay combos are crazed, knowingly.

I even got a sharpie out.

Now you have me rethinking my glazes.

To correct craze.

Correct me if I'm wrong but a crazed surface is technically not food safe? From the perspective that you cannot properly sanitize the crazed areas.

[angela_w 4]

Wait, Norm, I'm a bit confused now... wouldn't that mean that glazes would be more likely to shiver or dunt on Frost, not more likely to craze?