These are the same glaze, fired in the same kiln under similar conditions, and I’m still not entirely sure what made the difference here; but either way it is clear that the CTE (Coefficient of Thermal Expansion) of the glaze is too low. I like the look of that first tile, though, so I decided to see whether I could achieve a glaze with similar appearance, but without the bad behavior. There was no guarantee that this would be possible, but I thought I had a fair chance: I’ve made other glazes that were along these lines. In fact, I have fired over a hundred Rutile Blue tests during the past ~16 years, and I have lots of test tiles (and recipes), with varied characters. I looked through them and decided that this one —
— would be a good candidate to combine with the problem glaze. There is some blue in it, and although it slid a little on the tile, which indicates a melt with low viscosity, it is neither badly pinholed nor crazed.
I wrote out two new recipes. Version 0.2 is 2 parts of the problem glaze to 1 part of this slightly older glaze, and v0.3 is 3:1. The pure averages were a bit messy, so I tweaked them until they felt right (I think I’m not going to try to explain that here and now), and then I mixed up a test batch of each. Here they are, after a firing to cone 10.5 or so in the gas kiln, the night before last:
[The color balance is a bit off here; the background should be almost a neutral gray, and the glazes perhaps a bit more purply.]
Version 0.2 [2:1] is just beginning to slide where I double-dipped it, and is badly pinholed (this is a common problem with glazes that contain Rutile), but otherwise I like the way it looks. Version 0.3 [3:1] is quite acceptable, though perhaps a bit less exciting. I averaged the two recipes and tweaked the average to increase its thermal expansion just a bit more; I will fire a test when time & tide permit. Once I’m sure I have a recipe that is good, I will convert the existing bucket of v0.1 into a larger batch of the improved version. With any luck, I will have an example in the Art Show at SFContario, in November.