A Minor Victory, But Sweet None-the-less

Problems in ceramics tend not to surface until your full-on into production. Case in point: my slate matte glaze.

I love this glaze. I hung onto if for a year, working out how to use it in my work. When my glossy sky-blue glaze struck a chord with the slate matte, it was full steam ahead. Or so I thought. Most pieces were fine. Made it from start to finish with no problems. But anything taller, with straight-ish sides was a whole different ball of wax. I was losing anywhere from 50 to 95% of these pieces in each kiln load.
That's a lot of garbage to be making, and honestly, I can't afford to make trash.

The problem? Large pinholes, or what looks like blisters in the glaze that had popped, the edges healed, but the center remained bare clay. On the smaller of these pieces, like my tall cups, there would be one or two of these blisters. Enough to ruin the piece. Larger pieces, like my large vases, would be covered. COVERED in these blisters. Not a pretty site.

So where to begin?

There's any number of reasons why a glaze would do this, so I just started moving down the checklist trying to figure out what the hell was happening.

Perhaps some organics left in the bisque were causing the problems, but a higher bisque didn't help. Still got blisters.
Moving on, perhaps it was gases swirling in the kiln? I checked my kiln vent, adjusted it accordingly but that didn't seem to help either.

Okay.....

Next thought: water vapour. Thinking wet glazes were casuing problems, I let them dry before loading my kiln.
Nope. that didn't work either.

Okay. Maybe it's from overfiring the glaze? It only seemed to happen on these taller pieces, which also occupy the top shelf of my kiln. Cones indicated my kiln was firing evenly so I tried going from cone 6 down to cone 5. Boy did that ever make the glaze ugly and flat. And the blisters persisted.

With firing things ruled out, I moved on to the glaze recipe iteself. Was there something in there that could be causing this? I sent copies of it off to trusted glaze experts, but they didn't notice anything out of the ordinary. The EPK was a bit high, in the crawling range, but I wasn't experiencing any crawling, and even substituting some calcined kaolin for the some of the epk didn't solve my blister problem.

At this point I was beginning to believe it was an application problem. And I'm not set up for spraying glazes, so that wasn't even an option.

I moved on to glaze additives. First up was some CMC gum. Perhaps the glaze was drying too fast on the ware?
Nope. That didn't help and just made things messy.
Ugh.
Next was some Darvan and sodium silicate. I thought, what the hell, but that didn't work either.

Now it's important to point out that by this time, my husband just cringed whenever it was time for me to open the kiln. It's hard making work when you have orders and deadlines, and you have to throw most of it away. Many kiln openings were accompanied with cursing and/or tears. But I'm stubborn and I persisted.

Chatting with a fellow potter at a show, I asked him if he added anything to his bucket of matte glaze. The answer? Epsom salts. The same epsom salts that ruined my clay body. I tsp per 10kg batch of glaze.
And bloody hell, if it didn't work like a charm!

Epsom Salts, or magnesium sulfate, is a flocculant. These help prevent glazes from settling, which I didn't think my glaze needed since it had so much clay in it. But apparently they also help with glazes used for dipping or pouring. I had come to believe that air bubbles were being trapped against the surface of my bisqued pot when I dipped it, and that they couldn't escape until the glaze was melted. But being a matte, the glaze didn't flow enough to heal the surface like a glossy glaze would. The epsom salts create a porous surface in the raw glaze so perhaps its easier for those air bubbles to escape before the glaze melts?

At any rate, I guess there's a couple of lessons I take away here.
- Don't put out a line of pottery to my wholesale clients until I have worked with it thoroughly first, to try and spot any problems.
-Ask for help. I have no problems with this. Many times I'll get answers that I have already tried, but there's bound to be someone, somewhere, who's done something I haven't thought of.
- and most importantly, never give up! If you want to solve those problems, you have to keep plugging away at them. Now, it's better to do this if you don't have the pressure of HAVING to get things done by a certain date (see the first point) but perserverence pays off!