Only a test
Aug. 9th, 2019 03:29 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
offcntrI've been struggling a little with my black stain, the last two firings. For years, I've been using a chrome-free black stain, Mason 6616, mixed with Gerstley Borate, 50:50 by volume. It was a little fluid on vertical surfaces, but overall reliable. Which is why I was so unhappy when I learned last spring that Mason had stopped making the stuff.
The thing is, chrome is a refractory--it resists melting. Because I want the stains to melt into the surface of my glaze, using as little chrome as possible is a good idea. I had to use it for green, but at only a quarter of the volume, it was unlikely to be a problem. Problem being defined as bubbling, blistering, refusing to melt smoothly.
Which is a problem I have with my new black stain. I ordered the one with the least chrome content, but it's still around 25-30% chromium oxide.
So I changed the proportions. I no longer mix with Gerstely Borate half-and-half. Now I use two parts Gerstley to one part stain. Even so, I have to be careful not to apply too thickly, else blisters appear. I can grind them down, but can't really sell the resulting pot as a first.
I considered refiring them to smooth them out, but I've had problems with that in the past. Glazes get more fluid on refire, more likely to run. I also tend to see them get darker brown, as reduction brings more iron out of the clay. If only I had some way to fire them cooler, say, cone 9, in oxidation.
Then last week I had an inspiration. As I mentioned, after having to rewire my studio, the new electric kiln is a production model, a Skutt 1227 PK. What this means, besides it requiring a bigger circuit breaker and heavier wiring, is that it can fire to cone 10. In oxidation, because an electric kiln doesn't burn fuel.
So I boxed up a dozen unsuccessful pots, brought them home, ground down the blisters. Dabbed on a little straight Gerstley, to make sure they smoothed out. I also had a couple of pots that had gotten scratched in handling, so I put them in as well. Set the kiln Fast Cone Fire mode, going to cone 9, turned on the vent and pressed the Big Red Button.
The next morning, the kiln flashed Complete. It'd taken a bit over 5 and a half hours to reach temperature, was already back down to 665° (from a high of 2300°). It took the rest of the day to cool it safely down to open, but the results are gorgeous.
Blisters and bubbles gone, glaze still creamy, with iron speckles and warmish tone. And one scratched casserole, which had been a little over-brown, brightened up nicely. I'd rehabilitated over $350 worth of pottery, for about 80 kilowatt-hours of electricity.
Wow. Just wow.
I'm gonna have to reign in the tendency to refire everything that comes out of the big kiln with problems, but for a specific problem set, I've got a new solution.
The thing is, chrome is a refractory--it resists melting. Because I want the stains to melt into the surface of my glaze, using as little chrome as possible is a good idea. I had to use it for green, but at only a quarter of the volume, it was unlikely to be a problem. Problem being defined as bubbling, blistering, refusing to melt smoothly.
Which is a problem I have with my new black stain. I ordered the one with the least chrome content, but it's still around 25-30% chromium oxide.
So I changed the proportions. I no longer mix with Gerstely Borate half-and-half. Now I use two parts Gerstley to one part stain. Even so, I have to be careful not to apply too thickly, else blisters appear. I can grind them down, but can't really sell the resulting pot as a first.
I considered refiring them to smooth them out, but I've had problems with that in the past. Glazes get more fluid on refire, more likely to run. I also tend to see them get darker brown, as reduction brings more iron out of the clay. If only I had some way to fire them cooler, say, cone 9, in oxidation.
Then last week I had an inspiration. As I mentioned, after having to rewire my studio, the new electric kiln is a production model, a Skutt 1227 PK. What this means, besides it requiring a bigger circuit breaker and heavier wiring, is that it can fire to cone 10. In oxidation, because an electric kiln doesn't burn fuel.
So I boxed up a dozen unsuccessful pots, brought them home, ground down the blisters. Dabbed on a little straight Gerstley, to make sure they smoothed out. I also had a couple of pots that had gotten scratched in handling, so I put them in as well. Set the kiln Fast Cone Fire mode, going to cone 9, turned on the vent and pressed the Big Red Button.
The next morning, the kiln flashed Complete. It'd taken a bit over 5 and a half hours to reach temperature, was already back down to 665° (from a high of 2300°). It took the rest of the day to cool it safely down to open, but the results are gorgeous.
Blisters and bubbles gone, glaze still creamy, with iron speckles and warmish tone. And one scratched casserole, which had been a little over-brown, brightened up nicely. I'd rehabilitated over $350 worth of pottery, for about 80 kilowatt-hours of electricity.
Wow. Just wow.
I'm gonna have to reign in the tendency to refire everything that comes out of the big kiln with problems, but for a specific problem set, I've got a new solution.