It strikes me that I’m generally posting news of glass problems here instead of glass victories. I do have plenty of the latter, but for some reason I’m finding the problems more interesting. Maybe as part of my New Years resolutions I’ll start posting the successes, too…
Anyway.
One of the things I like most about glassblowing is the ability to attach “bits” and “cookies” and such, i.e., auxilliary hunks of glass that become part of the work. It’s harder to do that with kilnforming, and most people who try use “cold fusion,” i.e., glue, to make it happen.
I decided to see if I could do one of the most basic operations–putting a foot on a piece–during the slumping process. So I chose Bullseye’s square glass bowl, a fairly deep slumping mold, built a foot at the bottom of it, and slumped a blank over it.
Didn’t work. The bowl looks exactly the way I anticipated…but it cracked on three of four corners (where I’ve drawn in the red lines in the picture below). Drat.
As you can see from the photos, the cracks are occurring pretty much where the base contacts the bowl part of the piece. That’s certainly where I’d expect the worst stress on the piece; certainly the cracking pattern hints at stress relief. They go all the way through to the top and you can feel them scraping against a fingernail, making this a bowl that will go any minute.
Of course, the thickness of the foot adds considerably to the thickness of the bowl. At the foot corners (above), with base and bowl glass at a little more than 1.125 inches, the annealing time is deceptively long.
According to Bullseye’s “Annealing Chart for Thick Slabs,” 1.125 inches of glass needs between 4 and 6 hours of annealing time. I gave it a bit more than 4, which obviously wasn’t enough…I hope. I’m going to check on warmglass.com to see if anyone thinks it’s something else. But I’ll at least try a 6-hour anneal on the next test and see if that helps.
Oh well. I wasn’t too thrilled about the bowl pattern on this one, but I did absolutely love the base color, Bullseye Almond Striker. It’s the prettiest, purest warm creme and I want to use a lot more of it.
So..back to testing and we’ll see how it comes out.
Update: Success! Slowing down and dramatically increasing the anneal soak and then cooldown worked. Did a clear glass test bowl so there’d be no ambiguity about the results in the polarizer, in one of my biggest molds so that I’d see any issues magnified.
This is one sheet of BE Tekta 6mm, cut in a circle, drew squiggles in CMC on one side, sifted BE White powder over top and blew it off (shades of my kindergarten art-with-school-glue class), fused flat, flipped it over and did the same thing again. Now I had a decorated round of glass with a lipped edge. Set that aside.
Cut 1/8th inch fibre paper to fit the bottom of my mold (I believe it’s Bullseye’s 8724) in a sort of Chinese-inspired reverse tripod. Then cut four layers of clear 3mm scrap glass to fill in wherever the fibre paper wasn’t (essentially half-inch wide segments of a circle), filled the center with fibre paper and topped it with a circle of glass cut precisely to fit the contours of the mold at that point (which made the fifth layer of 3mm glass for a total of 15mm, btw).
Fused that solid in the mold with a conservative anneal soak and cooldown. Then lifted it out, checked for stress–none–and did a little coldworking to smooth it out. Now I laid the foot back into the mold with its fibre paper supports, set the 6mm decorated blank on the mold, and did a very slow, controlled slump with a 45-minute “bubble squeeze” (which in this case should really be called a “relax soak.” The blank settled down into the mold to become the “bowl” part, met up with the foot in the bottom of the mold and they tack-fused together nicely.
I know it was probably too conservative, but I started with the assumption 15mm foot+6mm bowl = 21mm of glass, even though given the angles, it’s probably quite a bit less. The fact that it’s tack-fused, with some sharpish angles ready to become stresspoints, made me double that for annealing purposes, to 42mm (1 2/3 inches, roughly), or an anneal soak of about 7 hours. Threw in a few extra hours for good measure, to 12 on the anneal soak.
That was undoubtedly overkill, but it worked.. absolutely no stress on the polarizer. I can be reasonably sure that any stress that did show up would have been annealing issues since the entire work was cut from the same sheet of glass. (Somebody tell me if they see a flaw in that logic, please!)
(If you want to know where I’m getting this stuff from, check out Bullseye’s very good annealing tables on their website. From what I read in Graham Stone’s book, I probably don’t need to be quite this conservative and could actually test down to the real sweetspot which I’d imagine is closer to 6 hours but hey–an extra couple hours of annealing is quicker than 3-4 rounds of testing when you’ve only got one kiln.)
In any case, I suspect the symmetry of the round mold is nowhere near as challenging as the square bowl I started out with, so all I’ve really done is establish the baseline to go after the larger problem. And at the moment, I’m more interested in getting back to casting the Emergents series and this makes a great fruit bowl, so that’s an end to this one for awhile.
Comments welcome! (thanks)