Evolution of a glaze

Staring into the glaze bucket all last week got me thinking about my workhorse base glaze, Third (Fourth? Fifth?) Best Possible White.


It started as a lovely, simple formula:

Best Possible White
cone 10, single-fire

50 Nepheline Syenite
10 Gerstley Borate
10 Talc
10 Kaolin
10 Ball Clay
1.5 Zinc Oxide

I brought it back from Tuscarora Pottery School along with half-a-dozen other recipes, including BP Blue, Yellow, Black, and a cool faux ash glaze that started by mixing a batch of cement and putting it into the freezer. It sat in my notebooks for two years in Wisconsin, three years of grad school (where I was firing unglazed sculpture at cone 6), and a couple more at the Craft Center, until our shop white glaze, Coleman White, started shivering.

It wasn't the glaze's fault. Obviously, something had changed in the clay body itself, causing greater thermal expansion and shrinkage than before, and little bits of glaze started popping off of sharp edges and rims. Had the glaze been more exciting, we would have tweaked the formula to match the clay, but it was a dull grey-white with little in the way of iron-spotting, so we decided to try some other recipes.

Right away, I saw that BP White would need some work. It's more than 20% raw clay, necessary in a glaze that goes on unfired pots. Clay shrinks as it dries, and this glaze shrinks with it. Bisque pots don't shrink, so I needed to substitute calcined (pre-fired) clay for some of the raw stuff. Since calcined kaolin is commercially available, that was my first choice. Compensating for chemically combined water and organic matter lost in firing, that's 8.5 parts calcined kaolin.

Then there's the matter of suspension. Glazes tend to settle to the bottom of the bucket unless you add something to keep them floating. Dennis Parks at Tuscarora favored organic suspenders: gum arabic, gum tragacanth. They worked well, but quickly began to decay. He liked to claim his glazes had the bouquet of French cheeses. I thought they smelled like cowshit. I used 2% bentonite.

I also discovered that the glaze wasn't very white over our buff-grey body, so had to add an opacifier. Tin oxide is the best ceramic opacifier, but it was incredibly expensive (something to do with armed conflict in Bolivia, I think). Twice as much zircopax would do the job just as well.

We used this for a while, then discovered that we had huge crawling issues. When applied by students--very thickly, because that's how they do--the glaze would shrink and crack like a mud puddle in the sun, then not smooth out again when it melted. It was too viscous when melted, so individual bits pulled back on themselves rather than spreading out. If you remembered to rub out the cracks in the glaze before your fired, you'd be fine, but remember, students…

Ball clay has much more wet shrinkage than kaolin, so was definitely the culprit. By calcining the ball clay instead of the kaolin, we got the shrinkage on the drying glaze to a manageable level. Back to 10% kaolin, 8.65% calcined ball clay. Since that's not a commercially available product, every now and then we'd have to run a big bowl of powdered ball clay through a bisque firing.

Second Best Possible White
cone 10, reduction

50 Nepheline Syenite
10 Gerstley Borate
10 Talc
10 Kaolin
8.65 Calcined Ball Clay
1.5 Zinc Oxide
6 Zircopax
2 Bentonite

I used this glaze for years, after leaving the Craft Center and starting my own pottery business at Club Mud. The only major headache with it was the zinc.

You probably know zinc oxide as the white stuff you put on your nose to prevent sunburn. In pottery, it's used in small amounts to give a "buttery" surface to glazes. Problem is, it's hygroscopic (holds on to water). Even if you calcine and grind it fine, exposure to ordinary Oregon humidity causes it to form clumps that don't dissolve in water, and don't go through the glaze screen. It's hateful to use, because you never know exactly how much is in the glaze, and how much you had to dump in the trash.

Fortunately for me, one of my colleagues at Club Mud is a retired engineer, and an experimentalist. We're firing in reduction, he reasoned, so well before the glaze melts, the zinc oxide is reduced to metallic zinc, which has a very low melting point, not to mention evaporation point. He figured putting zinc in a reduction glaze was tantamount to plating it on the inside of the chimney. So I tested BP White without zinc, and couldn't see any difference. Hurray!

Then came the Great Gerstley Scare of January 2000. US Borax announced it would no longer be mining Gerstley Borate, and panicked potters had a run on the mineral, bought out suppliers almost overnight. I bought a couple of bags, myself, to be sure I had a supply for the short run. And I sat down to recalculate the recipe using commercially available glaze frit for the long term.

Having done glaze substitutions longhand in the past (systems of simultaneous equations! Yikes!), I was happy to avail myself of a studio-mate's glaze calculation software. With a little fiddling, I came up with a pretty close match, chemically speaking, using Ferro Frit 3195 with a little whiting to make up the extra calcium. And I rationalized the clay content a little, making it a little easier to measure out a batch.

Of course, right after that, they announced that they'd found a ten-year supply in a warehouse somewhere, and the stuff is still available now in 2014, though I don't know how well it sells, because we're all still using up our panic purchases. I expect mine to last, well, forever, because I'm only using it a teaspoon at a time.

You see, changing out the Gerstley fixed another problem I'd been having, thumbprint lift. That's what happens if you pick up a pot before the glaze is dry enough to handle--it sticks to your finger or thumb and pulls off the pot. There's no good way to fix it, short of cleaning off the glaze and starting over. Since Gerstley Borate is hygroscopic too, it takes forever to dry, especially in an Oregon winter. Using a frit fixed that. On the flip side, though, my decorating time was greatly reduced. The new glaze version got dry and powdery very fast, making it hard to paint on. I was considering using organic binders, smell or no smell, when another potter at my studio made a suggestion. Just add another percent of bentonite, he said, and that will solve it. And it did.

Third Best Possible White
cone 10, reduction

50.21 Nepheline Syenite
13.21 Frit 3195
10.76 Talc
1.4 Whiting
7.7 Kaolin
7.7 Calcined Ball Clay
6 Zircopax
3 Bentonite

The final change is also a result of the frit substitution. Without the suspension properties of Gerstley Borate, the glaze settles like a rock to the bottom of the bucket. A rounded teaspoon of epsom salts per 7000 gram batch solved the problem.

It's been a long time since we talked terms…

Thermal expansion. Clay and glazes both expand when heated, contract when cooled. While the glaze is molten in the kiln, any difference in expansion between the two doesn't matter. On cooling, though, the glaze solidifies into glass, and any difference in shrinkage between glaze and clay body can be a major issue.

Crazing. If a glaze shrinks more than the clay body it's applied to, a network of fine cracks can appear. This is called crazing. In stonewares, it's not a huge problem, but porous clay like earthenware will leak liquids if the glaze is crazed, and I've seen it happen on porcelain occasionally. Sometimes used decoratively, when it's called a crackle glaze.

Shivering. When the clay shrinks more than the glaze coat, bits push away from the pot, especially where there's extra pressure--rims and edges, mostly.

Crawling. When melted glaze beads up, leaving patches of bare clay. Has a variety of causes--dusty pots, high surface tension, too much raw clay.

Calcine. To expose to high heat, generally bisque temperatures. Calcining zinc oxide drives off moisture. Calcining is also used to turn gypsum into plaster and limestone into cement. Calcining clay drives off moisture and organic impurities and changes the structure so the clay will no longer expand or contract on exposure to water.

Kaolin. China clay. A white, primary clay used in porcelain.

Ball clay. A secondary clay--one that's moved by water from the feldspar deposit where it originated. This means it has finer particles, more plasticity, also a little more organic impurities. Usually still fires white. Shrinks more in drying.