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People who salvage old glass for kilnforming frequently tell you to avoid tempered glass like the plague because:

  1. It’s impossible to cut or break (unless you don’t want it to break and then it explodes all over you like a bad Clive Barker movie)

  2. It’s dangerous; tap it in the wrong spot and–blammo!–here comes good ol’ Clive

  3. When it does break (shatter, really), it only produces these useless little sharp-edged glass cubes

  4. Devits like crazy if your kiln so much as breathes on the stuff

  5. It’s more likely to be coated or laminated with some kind of plastic that will ruin your kiln, rot your lungs and eliminate all possibility of offspring

All of that is true. None of that is true. Sometimes.

Here’s the deal: Tempered glass can produce remarkably beautiful work…if you know how to handle it.

Unlike standard fusible glass, tempered glass wasn’t meant to be used by kilnformers, so its manufacturers (who might be dead for all we know, this being old, salvaged glass) never tested it for stability through multiple firings. And let’s face it: Are people who make glass for store displays gonna maintain, an educational float glass website with tipsheets, technotes, video lessons and whatnot?

They are not. Which is why I’m doing a little bit of that here.



In this series

Learn more

  • Shards of tempered glass
  • Make your own polariscope (not yet)
  • Making ice sculptures (not yet)
  • Casting with tempered glass (not yet)

What you’ll need

  • Salvaged sheets of glass
  • Heavy-duty gloves and eye protection
  • Glass cutter
  • Razor blades
  • Glass cleaner and paper towels
  • Small bottle each of acetone, mineral spirits, paint stripper, and denatured alcohol
  • Hammer w/nail or ice pick
  • Heavy-duty drawstring trashbags
  • Bins for sorting
  • Permanent marker (Sharpie)
  • Polariscope of some kind (learn more)



This is how *I* work with tempered glass. I’m by no means the best or brightest at it and I’m constantly learning as I go along. Please, please, PLEASE: If you’re an expert at this stuff, correct me, add your own expertise, give me more ideas in the comments below! Thanks!

What is tempered glass?

Tempered glass is also called “toughened” glass, because its surface has been put under so much compressive stress it’s now 4-5 times stronger than annealed glass….along those stress vectors.

Apply the force against the direction of stress, and the glass shatters almost instantly, much like the famous Prince Rupert’s drop that hotshop managers love to show off.

If you haven’t seen Prince Rupert’s drop demonstrated: Get a bucket of cold water and scooper of molten glass, drop one into the other (I’ll leave you to guess which). Some of the glass will form what looks like an elongated raindrop that, when cooled, could withstand a direct blow from Thor’s hammer.

Twist the tail of that raindrop, however, and the whole thing instantly shivers into powder. Corning has a cool video demonstration.

There are different ways to temper glass, but the oldest methods rapidly cool, or quench, the glass so that the exterior forcefully contracts while the interior is still hot and expanding.  The tension created between those two forces puts the glass under tremendous stress; chips or scores that would typically “run” and allow the glass to fracture will instead be pushed closed and heal.

The danger with tempered glass, is that astonishingly little is needed to disrupt the system. Even after years of working with tempered glass, I can still be caught off guard. A few weeks ago I tapped the corner of a 12×48 inch glass shelf against the studio doorknob and it exploded in my hands; I’ll be digging out tempered crumbles for weeks.

Tip: That tendency to explode means that tempered glass is never used in weight-bearing applications such as floors, stair treads, or automotive windshields…without a protective coating to hold the glass together in case of breakage. If you know that was the purpose of the toughened glass you’re salvaging, be extra careful about checking for coatings.

There are actually two types of toughened glass you’ll encounter in salvage, and they can be tough to distinguish: Heat-strengthened and tempered. Tempered (what I’m discussing here, and the more common) tends to break into the familiar cubical crumbles (although not always, as you’ll see). Heat-strengthened isn’t as strong as tempered, and its breakage pattern is more shark-toothy, closer to regular annealed glass.

If you’re interested in learning more about how these glasses are used, PPG has a nice tutorial on the subject.

Finding free (or cheap) tempered glass

Cheap, readily available supplies of old glass are easy to find. RELIABLE, cheap, readily available supplies are a bit harder.

I got interested in tempered glass when my parents bought a crazy…er, eccentric lady’s house and needed to remove a bunch of tempered glass shelves and doors. I happily inherited many lawn-sized garbage bags of tempered glass shards, and spent weeks experimenting with fusing them.

When my supply finally ran out, it seemed silly to just buy new float glass, so that’s when I got into the glass salvage business. These are some of the places I check.

temperedglass-portlandstorefixturesUsed equipment stores

Glassland (AKA Portland) has several places that buy up old store or restaurant fixtures. Most will either give you cracked glass shelves and doors, or sell them for almost nothing. I’ve found glass up to four inches thick in those places, and I’ve rarely paid more than $2 or $3 for 6-12mm thick shelves up to 12×48 inches.

temperedglass-portlandstorefixturesstackPortland Store Fixtures, for example, gets in stacks and stacks of chipped 10″x16″ glass storage cube panels. I can usually pick up 20 or 30 pieces for 50 cents to $1.50 each, depending on how much they need to get rid of.

Every so often my timing is perfect, and they’ve simply let me carry away as many trashed glass shelves and doors as my car will hold. (yum)

Glass fabrication shops

You’d think the recycling piles of fabrication and auto glass stores would be obvious first stops, but I haven’t had much success. Generally these guys have insurance coverage to worry about, so they’re leery about letting anyone plow through their broken glass piles. And in Glassland they’ve gotten a bit tired of ALL the glass artists looking for free practice glass.

Cherish the ones who say yes anyway, because they’re a great source of specialty glasses that are hard to come by but make beautiful music in your kiln. They can also warn you when a really gorgeous tinted piece is actually coated with something.

Renos and demos

Wandering around restaurants and old buildings in a gentrification area, hotel, or apartment complex can sometimes pay off. I once got a free stack of really nice plate glass shelves, all 12 x 36 x 6mm, in a gorgeous aqua color I’ve never been able to replicate. Early in my recycled glass experiments I made a series of thick glass bowls that I sold to a collector; if I’d known how rare that aqua color is, I would have kept them.

Rebuild centers

Habitat for Humanity and a few other non-profits run “rebuild centers,” i.e., used building materials stores. People donate their old doors, sinks, electrical fixtures, and whatnot; some may go into the organization’s home building/repair work but most is resold to the public and the proceeds go to those programs. These places usually have piles of scratched up or broken glass, and it can be free for the taking or very low-cost.

The kinds of glass you’ll find depend on what people are bringing in, and it’s much less reliable than the tempered shelves and doors you’ll usually find at store fixture places. It’s still worth checking out, though.

How can I tell if glass is tempered?

temperedglass-labelOften, you can tell because it says so. Tempered glass frequently is labeled with a maker’s mark, etched or stippled into a corner.

It’s usually small whitish block letters that include a safety designation such as “ANZ97.1-{date}…” as in the mark I found on these 12×24″ quarter-inch thick glass shelves.

Don’t count on it, though. My first float glass mentor gave me four hard and fast rules for telling at a glance if glass is tempered; our first exception to those rules occured an hour later. The old (probably 10+ years) 3/8″ thick tempered glass shelves shown below break all four:

  • Any tempered glass thicker than 1/4″ always carries a tempered glass label. Nope. Nada.
  • Tempered glass never shows scratches, pits, or nicks on the flat planes (surface). Wrong again; these shelves had plenty of scratches.
  • Tempered glass won’t exhibit conchoidal (shell-shaped) fractures. Folks think that because tempered glass breaks into roughly cubic chunks, it will never exhibit “dishing” or chipouts. Now, I’ve not seen tempered glass shelves with massive, shell-sized gouges, true. But these shelves contained many small, roundish divots and shell-shaped fractures, especially at the edges.
  • Tempered glass generally has intact corners. People think if the corner of tempered glass is nicked, the whole panel will shatter, so a nicked corner means the glass can’t be tempered. On these shelves, though, every single corner that wasn’t rounded over with a grinder was knocked off.

This is how badly beaten up tempered glass can be (razor blade for scale):temper-conchoidal-edge-scratches

I was going to film this shelf when I shattered it, just for proof, but before I could grab the hammer, a corner touched down on concrete and –blammo– it turned into this:


Oooops. Yep. Tempered glass, alright.

So how DO you tell if it’s tempered (without a polariscope)?

Click to view full-sized image

Standard score on regular glass

Polariscope is the most reliable if the glass is unmarked. If you don’t have one handy, get out your glass cutter and score the glass.

If you’re an experienced glass cutter, you’ll feel the difference right away. Tempered glass just doesn’t score like annealed glass. It usually feels like you’re trying to score plastic, or as if maybe there’s something stuck in your cutter wheel. It feels dead.

Click to view full-sized image

Tempered glass score and the “rice krispies” effect

I have to work twice as hard to get a nice, crisp score line and that wonderful sccrriiiiiitch sound we all know and love. Try that, then wait about 60 seconds and watch the score you just made.


If the glass is tempered, the glass on either side of the score will start to lift, so that it appears you’ve almost scored a double line. A few seconds later, you’ll start hearing what I call the “rice krispies effect,” i.e., snap, crackle, and pop, as microshards fling into the air, often several inches off the glass.

Second test: When you apply breaking pliers to the glass to run the score, nothing happens. Tap the back of the glass, and usually nothing happens there, either. Tap it hard enough, and you might get enough force to shatter the glass. (If you think the glass is going to shatter, put the glass inside a plastic garbage bag or old bedsheet first, and save yourself hours of sweeping glass crumbles out of unimaginable places. Not kidding.)

Now, you’ll get some of that double-line flake-off stuff with annealed glass, too, especially if it’s not especially well-annealed. But it won’t be nearly as much as with tempered.

It’s a lot faster and easier, though, to put the suspect glass in front of some kind of polariscope.

How do I use a polariscope on tempered glass?

Anything that supplies or filters polarized light can provide a fast way to identify tempered glass–it will show you the regular stress pattern arrays that are a signature of tempered/toughened glass.

Click to enlarge image

Look at a car windshield through polarized sunglasses and you’ll see the stress points from tempering.

There are lots of places to find polarizing filters, and I’ll go into greater detail on this in a post on making your own. For now, though, the quickest and cheapest is to use what you probably already have, a pair of old polarizing sunglasses.

First, check to make sure your sunglasses have polarizing filters that can show the stresses in glass. Put them on, go out in the sun, and look at a backlit car windshield. You should see a pattern in the glass as shown here. What you’re seeing are stress patterns left by rapidly “quenching” the glass.

In car windshields, they usually are dots or diamonds. In tempered glass shelves, they can also look like tiger stripes or concentric outlines of the shelf shape. They will often be brighter at corners.

Once you have verified that, pop one lens out of the frame. Then find two pieces of glass, one you know to be tempered or poorly annealed, and one that is well-annealed (i.e., good old plain glass).

BTW, don’t go by whether or not the glass is cracked when you’re looking for signs of stress. The cracking relieves the stress (obviously), so it’s not much use.

Then follow these steps:

  1. Hold one lens up to a very strong light
  2. Place the second lens in front of the first and rotate it until there’s no light getting through at all (usually perpendicular to the first lens).
  3. Now take the suspected tempered glass and have someone hold it up to the same light. Put your first lens behind the glass, and your second lens at that darkest angle, right in front, so that you’re looking at the glass through both lenses.
  4. See any repeating dark and light spots? Polka dots, stripes, bars, diamonds? The glass is probably toughened/tempered.

Basically, the more light/dark variations you see, or the more rainbows, the more highly stressed the glass and, supposedly, the stronger the temper. From personal experience, I can tell you that high-contrast stress marks make the most spectacular explosions…

If you don’t have sunglasses, you can buy a couple sheets of polarizing film from any scientific supply house, although it’s kinda pricey. A better option: eBay, where you can purchase a couple of smartphone LCD polarizing film sheets for less than $10. Put them in a strong, light frame, and you have a portable glass tester.

Supposedly, you can also do this with one polarizing filter and your smartphone, tablet or laptop; they’re all equipped with a polarizing filter. You simply set the device to display the brightest, whitest screen possible, then use it as the backlit filter, put the glass in front of that, and the second filter on top.

It’s never really worked that well for me. If it has for you, let me know how you’re doing it.

Does tempered glass really explode?

Well…let’s find out, shall we?

I used my home-made polariscope to verify that this was tempered glass, and then tried to shatter it. As you can see from the video, it took quite a few whacks to get it to shatter but when it did…

Shatter products

So all you get when you shatter tempered glass is these useless little square crumbles, right? Wrong…but it varies. A lot.

temper-razorbladescrapingAs I said, float glass wasn’t made to be fused, so there’s no guarantee that one manufacturer’s float is compatible with another’s in the kiln. Or, to take it a step farther, that one manufacturer’s batch is compatible with the next batch. So when I shatter a piece of tempered glass for later use, I label and store it separately from other similar crumbles.

Doesn’t mean I can’t combine them later if I determine that they’re compatible (through testing). I have my own system for doing this:

  1. Clean the glass, really well. I’ll be cleaning the shards before I use them, but it saves a lot of time to do a thorough cleaning while the glass is in one piece and easy to clean.
  2. Razor the surfaces of the panel. There can be accumulated crud on salvage glass that even scrubbing won’t remove. I get a clean razor blade, hold it nearly flat to the panel, and scrape it along the surface. You’d be surprised at the amount of stuff that comes up.
  3. Clean again and check for signs of coating.
  4. Place the glass in a lawn-sized trash bag (or the smallest bag that fits, or a bedsheet), and let one corner stick out. Hang this corner off the side of the table.
  5. Tap on the exposed corner with your hammer. Usually a few taps is all it takes.
  6. When the glass shatters, the force of the explosion should carry most of it into the bag.

Once you have your glass, you’ll want to sort it by type.

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CAUTION: Freshly shattered tempered glass is the gift that keeps on giving, i.e., it has a tendency to continue to continue popping apart for some time after the initial explosion. Wear eye protection and keep small children, pets, and anyone who freaks out (or might sue) at the thought of a glass cut strictly away.

Generally, each panel I shatter gives me about 30 percent big spidery panes, 40 percent irregular-ish crumbles, and then smaller proportions of what I call shark teeth, cubes, edges, and chunks. The proportions can change depending on the way the glass was quenched during manufacture.

I roughly sort the types into ziplock bags, label them with the panel and date, and then store them together.

Coatings, paint, and mirrors

Nothing worse than coughing your lungs out before you can open the kiln, right?

Probably that will never happen. Probably. But if you’re gonna salvage glass, understand that the glass industry has a LOT of coatings and stuff that gets sandwiched in between layers of glass. A lot of it is plastic/resin-based, and some is downright nasty to breathe at glass-fusing temperatures.

In addition, quite a bit can get stuck to your kilnshelf, even with kilnwash (ask me how I know this), where it becomes a nuisance to remove. At the very least it can leave a blobby residue in your nice design, so it’s best to make sure your salvaged glass is free of any kind of that stuff before you fire it.

You can sometimes see it on the surface simply by running your hand down both sides of a smooth glass panel. If your hand catches or drags, or you can feel some texture on one side and not the other…be suspicious.

Also become suspicious if the glass appears to be colored on the facing plane but perfectly clear from another angle. It may be “flashed” glass, i.e., have a layer of colored glass, or it may have a colored adhesive laminating multiple panes together. Or there could be a colored film applied to the surface. (think,  for example, about how difficult it can be to distinguish which side the dichro is on)

I carry a little kit of solvents: Methylated or mineral spirits, paint stripper, acetone, and denatured alcohol. Between them, they’ll dissolve just about anything. I dab a bit on an inconspicuous corner, wait, and then apply a piece of paper towel.

If I can feel the towel stick, something’s on there. I’ll try to get it off with a little more solvent and a razor blade, but unless it’s a piece of glass that I just really need, I’ll usually move on to the next.

Adhesives and other layers can be laminated within the glass as well, and those are nearly impossible to work with; I’ll either offer them to friends who coldwork the glass (stained glass artists and carvers), or move on.

Mirrors and painted/enamels

I’ve got about a 60% success rate here; typically, a bit of paint stripper spread across the silvering and allowed to sit should take care of mirrors and most paints quite nicely. Just let it sit for a bit, then scrape it off with a razor blade.

OF COURSE you are working with heavy-duty protective gloves, with eye protection and in a well-ventilated area…right?

Sometimes the enamel is literally baked onto the glass. I suppose you could remove it but there’s plenty of salvage glass out there, so it’s better to move on.

Can I DE-temper tempered glass?

Sure. Just treat it as you would any badly annealed kilnformed glass: Re-fire it and re-anneal it.

Usually the biggest limitations to de-tempering salvaged glass are equipment-based:

  1. It must fit in your kiln (which pretty much eliminates car windshields and sliding glass doors, unless you’ve got a giant kiln–remember, you can’t cut tempered glass into smaller pieces)
  2. The underside of the glass will take on some shelf texture if you get it a bit too hot and soften the glass, so you won’t have that lovely, perfectly smooth surface on both sides.

Remember that this glass isn’t formulated for kilnforming, so the manufacturer didn’t worry about devitrification from repeated long kiln firings. You’ll want to keep your firings as few, fast, and cool as you can (unless you WANT devit, which can be beautiful in the right piece). That means you’ll keep the de-temper firing fast and cool.

I’ve found that float glass schedules tend to run 75-125 degrees hotter than Bullseye glass schedules, so I routinely add 100 degrees to a standard Bullseye schedule and then adjust if necessary. This is the schedule I typically use to de-temper quarter-inch salvaged glass panels:

Segment Ramp
Temp (F) Hold
1 500 1100 15 fast rise to just past the strain point; float
is tough glass so it shouldn’t thermal-shock
2 AFAP 1000 60 anneal (roughly 100 degrees above Bullseye)
3 OFF —— —– cool naturally

I tend to fire as much as will fit in my kiln and use the same schedule for anything up to about 3/8 inch (9mm) thick. For thicker glass, I follow Graham Stone’s annealing tables for thick float glass, although I’m not all THAT fussy about the downramp. In a pinch (if I’ve misplaced Stone, my computers are down, my memory’s shot, and I dunno, there’s some weird Al Gore-looking dude mumbling klaatu barada nikto from the driveway), I’ll use  Bullseye’s schedules for annealing thick slabs.

As always, what works in my kiln may not be the best in yours, so run a few panes in your own kiln, check with a polarizer and standardize on your own de-tempering schedule. If it still won’t score and break, refire a bit longer and hotter, and test again.

After that, test every so often to ensure that you have indeed created well-annealed, usable glass panels. And start experimenting.