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.