Working with tempered glass

Finding it (cheap), detecting it, de-tempering it

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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 notexactlybullseye.com, 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)

.

Disclaimer!

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:

temper-bowl

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.

YOU ARE WEARING GOGGLES AND GLOVES, RIGHT?

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
(dph)
Temp (F) Hold
(min)
Notes
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.

2017-08-02T21:18:24+00:00

19 Comments

  1. Terri Bourdon July 31, 2017 at 12:38 pm - Reply

    Bonjour Cynthia,

    Actuellement je fais des essais avec du verre trempé (panneau de douche). Je viens de lire tes conseils sur tous les additifs (plastiques, vernis, peintures, etc …) qui peuvent endommager le four mais aussi empoisonner l’air de l’atelier. Je te remercie vivement pour ces précautions, car il est vrai qu’il faut faire attention avec les éléments cachés. Ce panneau de douche étant assez ancien n’a pas “bénéficié” des dernières techniques, sans doute.

    Dans le métal aussi, il y a parfois des éléments comme de l’aluminium pris en sandwich entre deux plaques d’acier et si tu fais un moule avec ce genre de composite, la surprise est pour ta plaque de four … hum ….
    En ce qui concerne la coloration du verre trempé, utiliser des feuilles de cuivre donne un beau ton vert. Et utiliser des feuilles d’argent donne un jaune, un peu pisseux, mais bon … c’est moins dangereux que toutes les autres solutions. Je pense. Pour ce faire, je place les échardes de verre sur les feuilles métalliques et je procède au fusing, puis, dans un deuxième temps je ré-arrange les échardes dans des moules avec un contre moule bien lourd. Le tout bien dévitrifié, c’est magnifique.

    j’ai aussi essayé de colorer avec une toute petite quantité de poudre Uroboros System 96, verte et bleue. Je n’ai pas constaté d’incompatibilité mais ce n’est peut-être pas très recommandé.

    Je n’ai pas encore essayé les couleurs vitrifiables … et n’essaierai pas les oxides, c’est trop dangereux.

    MERCI CYNTHIA DE TOUT LE TEMPS QUE TU CONSACRES A TES SUIVEURS.

    • cynthia July 31, 2017 at 2:20 pm - Reply

      Merci pour les mots gentils! J’ai utilisé des feuilles d’argent pour vaporiser et revêtir le verre soufflé, mais je ne l’ai pas essayé avec une fuse au verre. Il serait intéressant de voir si l’or produit les couleurs or-rose classiques. L’as tu essayé?

      (Et excusez mon pauvre français – je ne suis pas allé en France depuis de nombreuses années)

      • Terri Bourdon July 31, 2017 at 2:33 pm - Reply

        Cynthia,
        J’ai essayé l’or en feuille mais l’or se transforme en minuscules paillettes qui remontent dans le verre et restent couleur OR. Celà donne un effet légèrement scintillant mais pas du tout la merveilleuse couleur rose. Dommage, tant pis !
        Ton français va bien. Quand tu reviens en France, tu peux rester chez moi, mais préviens moi à l’avance pour que je nettoie un peu la maison. Pulir la casa !!!

        • cynthia August 1, 2017 at 2:45 pm - Reply

          LOL! Je parie que ma maison est pire, mais si je visite la France, je vais essayer de visiter! Dommage que l’or ne fonctionne pas bien dans le verre; Je soupçonnais que ce pourrait être le cas.

  2. Denise February 14, 2017 at 12:03 pm - Reply

    I’m wondering if you have ever tinted the broken pieces? Like beach colors, different shades of blue and greens translucent colors. If you have what and how did you do it? Thanks

    • cynthia February 16, 2017 at 1:35 pm - Reply

      I’ve never tried that, mostly because I’ve been experimenting with what the plain glass does–I’ve gotten shades ranging from grass green to a really gorgeous aquamarine, and the colors enchanted me so much I didn’t want to change them. (Also, float tends to devitrify to varying degrees, which also influences the overall color, so there’s really a surprising amount of variation just in the clear glass).

      I do know that people have used Thompson enamels, lustres, and mason stains to color float, however. You could start there…

  3. MM August 10, 2016 at 2:02 pm - Reply

    Thank you for posting that-I work with a glass shop and he has a huge stack of tempered Starfire that for one reason or another didn’t work in shower installs-He just gave me a 4′ x 4′ piece and I’m going to clean it and smash it.

    A couple questions if you have a moment.

    This glass is float of course so it’s going to have a tin side-does that matter when I fuse it?

    I see you separate the broken pieces into different shapes according to how they broke-why, do they behave differently?

    Is your recommendation to add ~80-100 degrees mean for fusing, fire polishing and annealing?

    I’ve been working with leaded glass for almost 50 years, make my own sheet glass, used to have a large studio with a dozen employees and have had kilns for many years, using them for casting glass my bevel work-I just recently have been fusing, it’s a kick. Your work is beautiful, very inspirational.

    Thank you
    Mattei

    • cynthia August 10, 2016 at 3:07 pm - Reply

      Thank you, Mattei!

      Technically, the tin side DOES matter, because it changes the way the glass sticks to itself under fusing. With shattered tempered glass, though, it’s so difficult to manage that I don’t worry about it. It hasn’t seem to have been a problem so far.

      Yes, on the temperature–I increase the whole schedule by 80-100 degrees. I have found that different kinds of float fuse differently, however, so there’s usually an element of surprise.

  4. Bert Weiss January 4, 2016 at 7:19 am - Reply

    Bullseye schedules for thick glass simply need an addition of 80 – 100ºF. My testing always told me that float slumps 80ºF hotter than Bullseye or Spectrum. Typically, I use 1000ºF for a long anneal soak, and take that same amount of time to drop to 900. (other strategies (like Stone) often soak shorter at a higher temp and take about the same time getting down to 900. Comparing the Bullseye schedules to the Stone schedules, I think you will find them taking about the same amount of time, but spending it in slightly different temp zones. As you get thicker, the Bullseye schedules are less conservative.

    I believe that fused broken tempered is difficult to anneal. When I did a round of bowls, I’d cast the circles and then slump them. The slumping often (not always) resulted in thermal shocking, so I slowed down, and down, and down. Most, but not all, of the bowls that came out good stayed intact. I think tack fusing is the key to what is going on. That is simply a difficult state for glass, in general.

    The other really big deal with float glass is that the tin side does not stick to kiln wash or molds, but the air side does get rough. It is much less kiln wash friendly than fusing glasses, while the tin side is much friendlier than fusing glasses. So, when working with tempered glass, most surfaces are not tin bearing.

    The other constant with float glass is that compressing the tin side results in tin bloom that looks like devit. I don’t think this is much of a problem with refusing broken tempered as each piece stretches or compresses less than in a sheet.

    • cynthia January 4, 2016 at 11:07 am - Reply

      Hi, Bert;

      Yup. about 100 degrees above is what I said, too, so sounds like we agree. I’ve gotten differing results on the actual softening point with different types of salvaged glass, so I don’t really lump it all in a “float” category; unless I have evidence to the contrary I assume that each pane is unique and that’s kept me in pretty good stead. Typically, I’ll never mix glasses from multiple panes.

      That said, I do assume the 100+ schedule to begin with. I’d say I have to adjust it in about 30 percent of the glass I use. Right now I’m playing with a new kiln and just fired a piece made with some of the tempered glass I worked with in this post; 100+ underfired it but it’s hard to tell if that was a function of the glass being harder or my not zeroing in on the kiln characteristics yet.

      I use the polarizing filters almost constantly when working with tempered glass (or any unknown glass that I can get a light through/around), and I haven’t had that much of a problem with annealing the glass. It’s probably because I do so much tack-fusing to begin with so I’m extremely conservative; I assume the glass has problems and work my way backwards until I come to an annealed state. I had a couple of thermal shock issues very early on (which prompted me to get happy with the polarizing filters), but haven’t had problems since.

  5. kathryncecelia January 4, 2016 at 12:19 am - Reply

    Cynthia, Again I love that you’re posting. It is timely that you are sharing about tempered glass. In December, before Christmas break, I did a short two day demo and workshop for the kids in the Hill Top Hot Glass program in Tacoma. I have a glass company that supplied me the glass, then I shared much of what your article says. (I’m going to share your post with the people there so they have more info). We made snowflakes with the kids. I made about 12 that were 16-20″ that I sold for indoor decorations. They were well received. I think next year I’ll start earlier. I also made a couple of bowls. That was what lead me to snowflakes. I loved their wintery appearance. I was keeping a diary of things you’ve written about over the last four years. I had it stored on my Mac. I updated my OS to El Capitan and lost all my records that had been stored in the app called Notes. So now I can’t remember what I had and what I didn’t, but I’m going to keep this article for sure. I personally like things written out, as my hearing is not good, even with hearing aides, so I miss understand some words. (Eg. 15 vs 16) So when one is watching a video and listening to the speaker one can miss steps. I think having both is VERY nice, but a ton of work for the blogger. I think write about a topic once. Have YouTube movies about how you use/do a topic. Then viewers can refer to which ever media is most helpful to them. I’d still like to visit you sometime. I’d love to see some of your new toys; hear about how your health is going; learn about how you work. I admire your willingness to share; to give good advise; to make me smile; to advance the art form to new levels. Thank you. May 2016 bring you good health and prosperity.

    • Lyn Feudner January 4, 2016 at 2:19 pm - Reply

      I would love a link to your information. I’ve used float since I started using 10 yrs. ago because it is free! I fuse and slump fish, random wavy things, use glassline to “paint” on float and wire embedded between the layers.

  6. bobjheathBob Heath January 3, 2016 at 10:57 pm - Reply

    Very informative article Cynthia. Thank-you. You asked for comments about the format and I’ll say that I like this format. It took several sittings for me to read the whole thing, but I like having all the info in one place. I have a folder where I keep info about ideas for uses of scrap, float and tempered glass. I put a copy of this article in there so I’ll be able to reference it easily when I want to play.

  7. mira January 3, 2016 at 5:14 pm - Reply

    Very nice write up. I use a lot of tempered glass in my work – I’d be embarrassed to admit how many years it took for me to figure out how much easier it is to whack from the side. I usually wrap my glass with a welding or moving blanket and set it in a plastic tub to catch it. Then just shake the blanket so that the broken glass falls into the bin. All best!

  8. Ellen Abbott January 3, 2016 at 4:50 pm - Reply

    I had finished a long comment about one of my experiences with tempered glass and then my iPad got all squirrelly and crashed. it doesn’t like some formats.

  9. Barbara Cashman January 3, 2016 at 4:18 pm - Reply

    Good info Cynthia, especially on where you can get cheap tempered glass. I use a lot of tempered in my “Maverick Fusing–Float Glass” classes. I would love to send you a few pics, if access is available. I am logged into your site but could not access from this one. (????)

    • cynthia January 3, 2016 at 4:31 pm - Reply

      Barb, I sent you an email privately, thanks!

  10. SB Anthony January 3, 2016 at 9:32 am - Reply

    Thanks for all this info! You are a fountain of knowledge and so generous to share it all! I have done some work with tempered and other salvage float glass, but am now much better informed! 🙂 I DID believe that if a piece of glass had a chip on an edge, it meant it wasn’t tempered! Oops! I also am glad for the coating information, was also not really aware of that (though did know that windshields don’t work, though car side windows often do). And the polariscope — I use one for compatibility but never thought it as a tempered indicator.
    I am lucky enough to live near a university that runs a salvage store and often find quite lovely pieces of thick float and tempered glass from shelves, doors, tables, etc. for just a few dollars.

  11. cathy January 2, 2016 at 11:54 am - Reply

    This was extremely informative and beneficial! Thank you so much for taking the time to compile all the information.
    Happy New Year!

Comments welcome! (thanks)

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