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Mixing it up
Feb. 6th, 2017 04:58 pmI've had the weirdest problems with my glaze over the years.
Don't get me wrong, I love the glaze. When it's right, it's a beautiful cream color with iron speckles that reacts wonderfully with the overglaze colors. It's tough and durable on functional pottery, cleans up easily with just a little soak.
In the application… well, there's been problems.
It was originally designed for single firing, going on bone dry un-bisqued pots, so it had a lot of clay in it, nearly 20%. This allowed it to expand slightly with the raw clay under it, shrink again as it dried. When I started bisque firing, it'd crack and peel off with too thick an application, drying up like a mud puddle in the sun.
Calcining (pre-firing) half of the clay solved that problem. I now load a big bisque bowl of powdered ball clay into the kiln every few months, then use it when I mix my glaze.
The next problem wasn't with the glaze itself, but with the glaze suppliers. The original recipe was about 20% Gerstley Borate, but around 1998, Hammill & Gillespie, the suppliers, announced that they'd mined out the source and it would no longer be available. Every potter in the world promptly stocked up on it (myself included), after which they say Oh, my bad, here's another warehouse full, and apparently, they're still finding warehouses, because it's still on the market.
I didn't know this at the time, so I fired up my glaze-calculating software and re-jiggered the glaze to use Ferro frit 3195, which is a more reliable composition, and also isn't hygroscopic (water-retaining), which meant that I no longer had to worry about stray patches of not-quite-dry glaze peeling off on my fingertips when I picked a pot up. The surface was a little powdery, harder to paint on, but adding another percent of bentonite fixed that.
I still had a settling problem, though. Nearly half of the glaze is a sodium-bearing mineral called Nepheline Syenite, and it likes to settle like a rock on the bottom of the bucket. Digging it out and crumbling it back into the liquid part of the mix didn't help. Adding about a teaspoon of epsom salts to every bucket did.
But not completely. It tended to settle, though not as fast nor as hard. But I could still wind up with brown pots, thinly glazed because the glaze minerals were in a mud on the bottom of the basin and not in suspension where they belonged. And mixing before each pot gets dipped gets tedious and time-consuming.
I found the solutions more-or-less by accident. I normally mix a new batch of glaze at the beginning of the glazing/firing cycle, two 7000 gram (dry weight) buckets. This time, though, I knew I wouldn't have time; too many shows in too short a period. Since I'm always looking for something to do while firing the glaze kiln, I mixed up glaze for my next cycle, five or six weeks out.
Starting the next glazing was lovely: walk in, stir the bucket up and start dipping. After a while, I realized something was different. The glaze wasn't settling! I'd still give it a stir every once and a while to keep from developing a sheen of water at the surface, but the bottom of the bucket stayed clear, all the glaze materials suspended.
I'm not sure of the chemistry or physics involved, which component needed the extra time for hydration, but I'm not gonna question it. The kiln is firing, I've mopped the floors and put away my stains and brushes. Time to mix more glaze.
Don't get me wrong, I love the glaze. When it's right, it's a beautiful cream color with iron speckles that reacts wonderfully with the overglaze colors. It's tough and durable on functional pottery, cleans up easily with just a little soak.
In the application… well, there's been problems.
It was originally designed for single firing, going on bone dry un-bisqued pots, so it had a lot of clay in it, nearly 20%. This allowed it to expand slightly with the raw clay under it, shrink again as it dried. When I started bisque firing, it'd crack and peel off with too thick an application, drying up like a mud puddle in the sun.
Calcining (pre-firing) half of the clay solved that problem. I now load a big bisque bowl of powdered ball clay into the kiln every few months, then use it when I mix my glaze.
The next problem wasn't with the glaze itself, but with the glaze suppliers. The original recipe was about 20% Gerstley Borate, but around 1998, Hammill & Gillespie, the suppliers, announced that they'd mined out the source and it would no longer be available. Every potter in the world promptly stocked up on it (myself included), after which they say Oh, my bad, here's another warehouse full, and apparently, they're still finding warehouses, because it's still on the market.
I didn't know this at the time, so I fired up my glaze-calculating software and re-jiggered the glaze to use Ferro frit 3195, which is a more reliable composition, and also isn't hygroscopic (water-retaining), which meant that I no longer had to worry about stray patches of not-quite-dry glaze peeling off on my fingertips when I picked a pot up. The surface was a little powdery, harder to paint on, but adding another percent of bentonite fixed that.
I still had a settling problem, though. Nearly half of the glaze is a sodium-bearing mineral called Nepheline Syenite, and it likes to settle like a rock on the bottom of the bucket. Digging it out and crumbling it back into the liquid part of the mix didn't help. Adding about a teaspoon of epsom salts to every bucket did.
But not completely. It tended to settle, though not as fast nor as hard. But I could still wind up with brown pots, thinly glazed because the glaze minerals were in a mud on the bottom of the basin and not in suspension where they belonged. And mixing before each pot gets dipped gets tedious and time-consuming.
I found the solutions more-or-less by accident. I normally mix a new batch of glaze at the beginning of the glazing/firing cycle, two 7000 gram (dry weight) buckets. This time, though, I knew I wouldn't have time; too many shows in too short a period. Since I'm always looking for something to do while firing the glaze kiln, I mixed up glaze for my next cycle, five or six weeks out.
Starting the next glazing was lovely: walk in, stir the bucket up and start dipping. After a while, I realized something was different. The glaze wasn't settling! I'd still give it a stir every once and a while to keep from developing a sheen of water at the surface, but the bottom of the bucket stayed clear, all the glaze materials suspended.
I'm not sure of the chemistry or physics involved, which component needed the extra time for hydration, but I'm not gonna question it. The kiln is firing, I've mopped the floors and put away my stains and brushes. Time to mix more glaze.