They’re not so much pinholes as pin craters, really.

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.

I told my cat I made jellyfish mugs the same colour he is. He seemed unimpressed, and slightly baffled.

When I opened the lid, I saw this:

The first issue is the colour of the background glaze. I lost my glaze notebook[1], 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[2], I could have a lovely variegated glaze on my hands.

[1] 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.

[2] Opacifiers. But which ones?

2 thoughts on “They’re not so much pinholes as pin craters, really.”

  1. I actually rather like those bowls. Did they come out as expected? Are they for sale?

    If you re-do the sheep mug, I might need one. Or three. Hell, I’ll even take the blurry one.

    1. They did not come out as expected. They’re also not food-safe (due to the pinholes in the glaze). And the sheep are being re-done for sure.

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