Because it is free, facet-nating, versatile and found in so many places I tend to frequent, I use a fair amount of Tempered Glass — or "TG" — in my work. I have mostly used auto glass salvaged from the auto wrecking yards and am familiar with it's wide range of peculiarities and challenges. It comes in a variety of shades of blue, gray, green and occasionally black. It is many different thicknesses and some brands shatter in different patterns than others. Front windshields tend to be tempered AND laminated, which is not generally conducive to mosaic-making, and it's best to avoid windows with metallic automatic defroster filaments embedded in them unless you plan on making them part of the overall "look" of the finished piece. I can tell you from first hand experience, nipping those filaments out of the glass bits is t-i-m-e consuming. Shower doors are not too hard to find and can give a nice effect depending on what type of surface treatment the glass was originally given as a matter of "privacy decor". A few times I've been lucky enough to end up with someone's fabulous old TG plate glass window: they are clear and nearly colorless, usually break cleanly, are all one thickness and almost always have the advantage of NOT being covered in dirt, grease or soap scum.
Because they were there (being there is half the game) and they were colorless (and colorless is not always easy for me to come by) and they were free (my favorite kind) and I thought they would be fun (fun is the other half of the game), I scavenged a few shelves out abandoned refrigerators in the discarded appliance yard at the recycling center. Part of the fun of using tempered glass, if you are fortunate enough to be getting an entire intact piece (like I had just scored with those shelves), rather than scraping it up off the ground or out of the back seats of junked cars, is the act of breaking it. There is something delicious about setting everything up just the way you want it then ceremoniously and oh-so-gently tapping the side of the glass with a hammer and watching it start to crackle and then spread, with a subtly satisfying little sound, from one side to the other until what was once a solid piece of transparent material with a strong specular reflection develops into randomly shaped and sized gem-like bits of pure sparkling goodness.
However, this is NOT what happened with my refrigerator shelves. I tried tapping them gently, then harder, then even harder — then so hard that they flew across the studio floor and landed, intact, under the big work table. I tried wrapping the whole thing up in a towel, putting it on my concrete studio floor and whacking it with a sledge hammer — the hammer bounced! The harder I hit it with the hammer, the harder the hammer bounced. I was flummoxed, I was fascinated...and I was growing frustrated. I climbed on my table and dropped the shelves. Stil wrapped in the towel, they bounced. I climbed onto the counter (a whole foot higher) and threw the shelves. They bounced and skittered, although one lost a sharp sliver of the corner — and it did NOT resemble safety glass. I gave up at that point, set them aside and began working on the parts of the piece pictured above, PERCH ON A BIRCH, that did not involve TG.
A couple of weeks passed (I am, by nature, slow, and it was a tedious piece!) and then I really needed clear, colorless TG in order to finish that little cardinal's snowy realm. So I got out the refrigerator shelves and considered: a.) trying to use my big, wet scary tile saw on them; b.) put them on the concrete garage floor and run over them with the car; and c.) call my husband. But first I wanted to have one more go. I was going to video the awesome bounce of this TG for posterity's sake — who would believe it? And then I was going to double check to make sure the shelves really were composed of TG and not some sort of modern miracle plasti-glass product. To get warmed up, I wrapped one of the shelves in a towel again, put it on the concrete studio floor, grabbed my sledge hammer, stabilized my cell phone, started the little video camera on it and swung half-heartedly. Suddenly, the glass gave way! I could hear and feel it, and sure enough when I opened the towel, it revealed that the fridge shelves had cracked into REALLY tiny, absolutely separate pieces. No clinging clumps or larger bits to be laid onto the mosaic in interesting patterns, just teensy, tiny, nearly microscopic little pieces fit only for tweezers and magnifying glasses.