Hypothesis: A glass framework stabilizes a tack-fused stringer construction, requiring fewer stringers (and likely fewer firings).
Background: I’m having a fair amount of success with tack-fused stringer projects that build on a glass support structure. Since the support framework is tedious to construct, I’m wondering if it’s really contributing enough to be worth doing.
Test: Make a stringer vessel without the framework, and keep adding stringer (and firings) until the vessel is full-sized and stable.
I suppose, in the interests of scientific method or something, I should also add that I had other motivations for doing this project. First, I’m in the midst of two rather intense sculpting projects and I really needed a quickie tack-fuse project to get my head out of the weeds.
Two, there’s an exposed pipe in the corner of my garage that tends to freeze solid if the temperature outside stays below freezing for consecutive nights. That particular pipe is involved in flushing the masterbath toilet, so I have a vested interest in keeping things, er, going. It’s been freezing for several nights now, but when I’m firing the Skutt my garage stays a balmy 65, 70 degrees or so. A fast project, therefore, would save my pipes.
I decided to try my “unsupported” stringer bowl in blues and greens, and picked out an interesting bisqueware bowl at my local ceramics supply house for a mold. Four-sided, rather like an upside-down pyramid with the top cut off, it was also a bit steep and would make for an interesting test. I kilnwashed it thoroughly, let it dry, then began piling in the stringer.
Even thought I wasn’t preconstructing a framework, I did need to give the thin stringer some support. So I started with the thick clear stringer, laid in a criss-cross pattern in the bottom of the mold and then diagonally from one corner to the other. That would provide a base to hold the thinner colored stringer, much like marbles in the bottom of a vase help hold the flowers.
It looks pretty random in this shot, but actually isn’t. This is before the first firing, when I’ve built on the clear stringer base with colored stringer, laid diagonally from the bottom corner of the mold to the upper corner on the opposite side and creating niches for the shorter string that will form the “walls.” I worked this edge-to-opposite-edge pattern all the way around the mold. Then I slid shorter pieces of stringer straight down the sides of the bowl, into the gaps.
As the amount of stringer grew, the piece started looking like a very colorful hedgehog, as above. The more vertical stringer in the center would collapse down, spaghetti-like, and make a sturdy, interwoven base.
On the sides, though, I wanted the stringer to stay up, to form the walls. Stringer’s so thin that it droops quickly even in a candle flame, so the challenge was getting it to stay. I made sure that at least half of the sidewall stringer extended about an inch over the top of the walls. It would be the first to collapse, and I hoped that most of it would head straight down, hook over the rim of the mold, acting as a support to keep the rest of the stringer from sliding down.
BTW, at this point I have the equivalent of about three packages of stringer in the mold.
I placed the mold in my Skutt bathtub kiln, in the center. That’s important, because it meant that on two (opposite) sides, the elements were about half the distance from the piece as the other two and would get more heat, faster. I thought about baffling those sides to even up the heat, but in the end decided to see what happened.
Not surprisingly, after the first firing that the “distant” sides of the mold had stayed up, with the stringer forming the walls I’d hoped. On the “close” sides, much of the stringer had slid about halfway down. In addition, there were some pretty significant gaps in the stringer walls that would make the bowl much too fragile to remove from the mold, so I needed more stringer and at least one more firing.
I decided to speed up the ramp to process temperatures without raising the top temp–this is one case where relaxing the glass slowly into the mold is probably a bad idea. But throughout this project I was VERY conservative on anneal/soak and cooldown, since I now had a mat of tack-fused stringer on the bottom and relatively thin walls thrusting up at near-right angles. My anneal schedule was between 3 and 4 times the recommended schedule for this thickness to compensate.
A lot of the stringer had tack-fused into the bottom, making its own support structure now that held the new stringer easily. This time, I concentrated on filling in the gaps in the sides, and refired.
The schedule modification helped–one side had slid in places, but the rest had stayed upright. I added new stringer (as you see above) and then filled in the gaps around the piece and fired a third time.
So…it took about 6.5 tubes of stringer to get the bowl this far. A lot of it wound up in the bottom of the bowl, which made the bottom much thicker than the sides and added some stress opportunities.
In contrast, the amber basket, which was constructed on a framework of noodles, was considerably larger yet took only 4 packages of stringer to complete in a single firing. Since they’re radically different shapes, in my next casting lull I’ll use the same mold form to create a second piece with the framework, and see how it compares.
Both bowls are reasonably stable and sturdy in the bowl, but the gorgeous, spiderweb tops are extremely fragile, especially since people tend to pick them up by the top. I don’t know that this design would ever be stable enough to let out of the studio.
Overall, though, I think the framework is necessary. Without it, you need more stringer, and more firing cycles, in effect establishing a framework in an expensive way as you go along.
Makes me feel better, anyway…