Shards of tempered glass (the Shards series)

There’s been some interest on warmglass lately about doing chunky, frosted glass vessels like the one above (and also the one at the top of this blog), something I’ve been experimenting with ever since my parents’ big glass display doors came down. I take pretty extensive notes (and pictures) of my experiments, so I thought I’d post a longer explanation here…

My parents bought a home formerly occupied by a collector of antique dolls and old china. She’d housed about 300 dolls inside a massive mirrored shelving system with huge and dangerously unstable tempered glass sliding doors.

The only way to get them out was to shatter them, which resulted in about 10 garbage bags full of broken tempered glass. What was nice about it was that there was a LOT of self-compatible glass there (you can’t be sure that two separate float glass sheets are compatible). What was bad about it was that there was a LOT of it, and I did a lot of size-sorting and washing to get it into a usable state. After that, however, the experimenting (and the fun) started:

I’d never fused with float glass before, and quickly found that it has very (VERY) different characteristics than glass especially made for hot/warm working. For one thing, it’s a lot stiffer, it doesn’t tend to soften and spread as much, and even though its working temps are only 50-100 degrees above, say, Bullseye glass, it takes a lot more heatwork to make it do stuff. It has a (much) stronger tendency to devitrify, ie, begin to crystallize for a frosted look, and there’s also that little compatibility issue–float glass can come from anywhere, and manufacturers do not maintain a single compatibility standard (well, heck, they don’t do that for stained glass, either… .

Still, it’s inexpensive (in this case free), clear and smooth, and if it’s tempered, breaking it results in cubes, squares and what I call shark tooth-shapes that catch and bounce light better than the usual conchoidal fractures of regular glass. I tried to preserve that with the Shards series, which kicked off a whole bunch of frit experiments that are turning out rather well.

Multiple layers of float shards fuse together nicely and in the first firing either stay transparent or acquire only enough devitrification to be interesting. Cleaning the glass really, really well with Dawn and/or denatured alcohol removes potential nucleation sites from the glass surface, reducing the level of devit.

To keep the glass as transparent as possible, I tend to do these pieces in a single firing, which means I lay the shards directly on the mold and fire in place.

I lay up the glass so that every piece has at least three (preferably four) contact points with adjacent pieces. I’ve found that pieces with one or two contacts tend to break off with use. The smaller cube/crumbles can be piled into a mold and leveled, which makes a nice, faceted look, but I tend to prefer combining smaller and larger shards in a kind of mosaic-and-grout style. I’ll often build “bridges” of rectangular pieces underneath the larger shards, which helps make the piece more stable.

A more sharply vertical mold either needs to be done in two firings (fuse and slump) or I use dual molds. The cup at the top of this post, for example, has an inner and an outer mold (thanks to my mother the ceramicist). I carefully assemble the shards inside the outer mold, something like building a jigsaw puzzle, put a second layer in the bottom for stability, then place the inner mold in the glass.

In the picture, I’ve got a piece of eighth-inch fiber paper between the inner mold and the glass, to take care of expansion issues. I still had a fairly high rate of cracking on these, though, so I’m now using a thinfire liner with a sand/plaster core. It still presses the glass in place, but isn’t as abusive on the cooling tackfuses. Big deal with that technique is keeping sand from getting trapped between the shards.

The single-layer cups are my favorite in this series–they remind me of ancient Roman glass.

The alternative with this method is to use multiple layers of glass all the way up the cup. It produces a different look, heavier, icier and less delicate, but you could probably also bounce this on the floor with relatively little damage (except to the floor).

I did this with nested stainless steel mixing bowls, kilnwashed, with about an inch in diameter between the inner and outer bowls. Since the stainless is so much lighter than the ceramic, I filled the inner bowl with sand to keep the glass compressed against the outer mold.

Taking this to extremes, I’ve also been making what I call “glassnests,” like the bowl at the top of this blog, which can be 5-6 layers of shards in between the inner and outer molds. The technique produces a very cool crushed ice look with lots of edge detail, as much as two inches thick on the sides. These probably aren’t to everyone’s tastes, but there’s something very seductive about the solid mass of glass.

The Shards have been really popular with friends and relatives; the smaller trays make nice thank you gifts and the big, 20-inch round trays are a hit as holiday presents. I’m just starting to mess around with adding colors to this style, and figuring out how to get a similar look with Bullseye glass.

On firing schedules
Many people have asked me to post firing schedules, but I’ve found there’s a lot of “your mileage may vary” in tack-fusing tempered float. Since I can’t control what kind of float you’re using, how big the tack-fuse components might be (it matters), how your kiln heats, the texture you’re looking for, or the degree of clarity/frost you want in the end result, it’s a little hard to provide a working schedule.

I tend to prefer slow ramps with lower process temps, and prefer the “frosted” look that results, i.e., I get some devit. If you go faster through the devit zone your glass will be more transparent, but I’ve found that you also wind up with less component adhesion if you want to preserve the cubed/faceted look of the original glass. Letting things cook at a lower temperature for awhile seems to help the components settle and fit into each other better, so that you need less melting-in to get things to stick.

My top process temps range from about 1380F to 1480F. 1380F keeps the pieces very sharp (you could cut yourself on the edges of big pieces) and barely tacked together. It requires a long hold with a slow upramp, and works best with smaller components. 1480F will produce a lot of softening and a very stable piece that’s relatively clear, but I don’t like the overmelty look. Beyond 1480F I don’t get a whole lot of difference in the look, maybe a little softer; this glass doesn’t seem to move much.

My ideal working range (at the moment) is about 1425F-1440F, depending on the size of pieces I’m using. I tend to ramp initially at about 500dph, initiate a much slower “settle cycle” (in regular fusing you’d call this a bubble squeeze) across 1200F-1300F, and then move 50-200dph up to my process temp.

Hope that helps, but as I said, your mileage will vary. I guarantee it.

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