Test tiles are out of the kiln. The one made from Alexis’ frit 3124 is fine, and the one made from the studio’s whatever-is-in-the-frit-3124-tub is milky and pinholey where thick. (But the application was very, very thin, which makes a difference, and someone actually had the balls to say, “But it turned out nothing like your other firings”, which is slightly angry-making, but ANYWAY. If it’d been applied thickly, you can clearly see the result would have been the same as in the catastrophe firings.)
So. I know for sure the problem is where I thought it was all along. And the bag of new frit I’ve ordered will not have been ordered in vain. And as soon as it effin’ gets here I can glaze the small mountain of bisqued Robots I have accumulating on my shelves.
So. That glaze firing was a disaster. About once a year I seem to have a big batch of evil pots, and this was the doozy. (At least, I sure hope it’s the doozy. If there’s a worse one coming I’m quitting to make macrame or something.) See these mugs? Them and two bowls are the only things to come out of the whole kiln load non-fubared.
When I opened the lid, I saw this:
The first issue is the colour of the background glaze. I lost my glaze notebook, and mixed up the latest bath of cephaloglaze from memory. Clearly, 2% iron is not enough. But the texture and opacity and all that is spot-on, so I can add some more to the batch, throw a few new plates and bowls, and all will be good. And the colours in the underglaze painting were perfect. The big problem is the clear; it’s gone sort of muddy, and the underglaze underneath it has blurred. At first I thought it was the way I’d applied it–maybe sponging wasn’t the way to go after all–but when I excavated down to the next layer, the full horror started to hit me.
My latest batch of clear–which was on every piece but two, and one of those developed a handle crack–is evil. Eeeeevil. It’s not clear, for a start. And it pinholed worse than anything I’ve ever seen. And it ran so far I spent two hours in the Craft Council parking lot this morning with an angle grinder, repairing the damaged kiln shelves as much as is possible.
Now, I love that clear glaze. It’s the bestest clear in the universe. It has all of four ingredients, which are all cheap and all kicking around any studio anyways. It’s always clear, it runs only if you lay it on stupendously thick, it rarely crazes, and it’s very forgiving of temperature variation. After firing, it’s gorgeously glossy and superlatively durable.
Clearly it’s not the clear, or even the temperature. It’s me. I screwed something up in the recipe.
Too little silica wouldn’t make it milky. Too much silica would cloud it over, but wouldn’t let it run. Too much bentonite could cloud it and/or make it run, but as a measly two percent additive, I’d have to get it REALLY wrong, and I’d hope I would at least clue in at the measuring stage. Besides, I don’t think it would pinhole like that. Messing up the EPK is more or less the same story. That leaves the frit, which at 75% also happens to be the most important ingredient. There’s a frit 3124 and a frit 3134, and I strongly suspect I got them mixed up.
After calming down, I did notice one small sliver of a silver lining: it didn’t pinhole over areas of underglaze. It came out really pretty over blue and purple shades in particular.
If I can figure out what’s in the underglaze that’s making it smooth, I could have a lovely variegated glaze on my hands.
 This is actually my biggest long-term problem. The disastrous kiln load I can recover from within two or three weeks, but losing five years of glaze notes? That’s bad.
I… actually was aiming for a turquoisey shade of blue-green. Will (reluctantly… I love this shade of green) switch to a blue for the big bowl; that would fit in better with the sky/wave vibe the curved bits give off.
The white feathery bits are only visible if you look very closely, so a thicker layer of white would be a good thing for the big bowl. If I end up layering the clear and white glazes often, I may even end up switching back to cream clay so the contrast is more visible.
There are two spots where the glaze pulled away from the clay, and even a little crazing; I blame it on the very thick layer of clear. Must go thinner next time.
 Or black or red clay! Ooooh, throw lines and carving and stamps and highlights… I could play around a lot with this…
Thrown and carved cream stoneware, with cranberry glaze poured inside. Dipped upside down in a good layer of clear, then dipped again for a quick, thin layer of white. The white has a feathery, runny look I really like when I layer it like that, and it pooled marvelously in the ridges, highlighting the carving.
I have plans for a large, porcelain bowl on similar lines, with a different colour inside and more elaborate carving. Due to the size, I’ll probably have to spray the glazes instead of dipping them, which could be tricky… I don’t have much experience with spraying glazes (mostly because my only spraying equipment is a dinky little mouth-blown doohickey, and I get dizzy using it). I’ve got a small-scale bowl bisqued already, which I’ll be using as a glaze test. If it works out well, onwards and bigwards.
I wound up pouring clear glaze inside, as a liner, and spraying the aqua glaze on the outside. The lid of the tin can teapot also had a little piece of glass (the kind you get at the dollar store for the bottoms of aquariums) left on top of the knob. That’s the crackly blue stuff. I was expecting it to bubble over and pool around the base of the knob, but some serious surface tension kept it contained.
As I don’t have a lot of experience with spraying, and didn’t quite get the glaze even, there are some almost-bare spots around some of the nooks and crannies in the handles. I’m thinking of reglazing those areas (and maybe adding some drippy bits going from the rims down the sides of each pot while I’m at it). Otherwise, though, I’m pleased as punch.
Loooots of blue in the bottom. A bit too much, actually; the spider who made the web is so covered up you can’t see it. And while a little blue is nice, quite that much can be really distracting combined with the put-in-on-purpose design. But hey, the customer liked it, so that’s okay.
I also had a large cephalopot bowl bowl in that firing, but it didn’t turn out nearly so well:
Something terrible has been happening at the bottom of my glaze bucket. From the look of it, and the state of this bowl, I’m guessing some of the refractory ingredients are separating out of the solution and settling at the bottom. You can’t really make it out in the picture, but the inside of that bowl is so rough I don’t even think it’s foodsafe. And since there aren’t as many refractories on the outside section (where the glaze was poured over, instead of poured in and swished around), the melt there is drippier, and the colour is a lot darker.
I’m going to re-sieve the glaze, and add some epsom salts, and hopefully that’ll fix things. If it doesn’t, something else is happening in there, and I’ll have to figure out what.
In the meantime, at least now my Christmas cactus has a big new pot…
Glazed some big serving bowls, and some pate dishes. One mug. (Must remember to glaze one mug from the next batch Cthulhu Green for a friend’s Christmas present.) Tried some glycerine and glaze on a test tile, to see if it’s all it’s hyped up to be.
Big kilnload on tonight… Should be able to see my test tiles tomorrow. I’m on tenterhooks!