Tempered glass

What is tempered glass?

Tempered glass starts as ordinary annealed float, then gets reheated and cooled again in a tightly controlled way. That second trip through the heat builds a powerful stress profile into the glass, the surfaces squeezed in compression and the core pulled in tension, and that hidden balance is what makes it so much stronger and completely changes how it breaks.

That is why it counts as a safety glazing material under US building codes. When tempered glass finally gives, it crumbles into small, blunt little cubes (the trade calls them "dice") instead of the long jagged shards annealed glass throws. Far fewer stitches, which is exactly why the code calls for it in dozens of hazard spots.

How thermal tempering works

It all starts with fully finished annealed glass. Every cut, hole, notch, and edge has to be done before tempering, because once the glass leaves the furnace you cannot cut it, drill it, or change it in any meaningful way. Why that is non-negotiable is coming up in a moment.

The prepped glass rides into the furnace and is heated evenly to about 620–650°C (1,150–1,200°F), just shy of the point where it would start to sag. At that heat it goes slightly soft and any leftover internal stress relaxes away.

Then it slides out into the quench, where banks of high-pressure air jets blast both faces at once. The surfaces cool almost instantly; the core, buried in the middle, takes much longer. That mismatch is the whole point.

Here is what that mismatch does. The surfaces cool and set first and try to shrink, but the still-warm core holds them back. Later, when the core finally cools and wants to shrink, the surfaces are already rigid and refuse to move. The core ends up stretched in tension and the surfaces squeezed in compression, a permanent tug-of-war locked into the glass.

Why does that make it stronger? Because a crack now has to fight through that compressed surface layer before it can even get going. ASTM C1048 says fully tempered glass must carry at least 10,000 psi (69 MPa) of surface compression, and that is where the roughly 4–5× jump in strength over plain annealed comes from.

Why it breaks into cubes

That locked-in stress is also a loaded spring. The instant a crack gets going, from a hard knock or a flaw deep inside, all that stored tension in the core lets go at once and the crack races outward in every direction at the same time.

That instant, radiating web of cracks chops the pane into hundreds of little pieces, and the compressed surfaces cap how big any one of them can get. What you are left with is the signature "dice" pattern: small, roughly cubic bits with no long edges or sharp points to slice you.

Where tempered glass is required

The US model codes (IBC for commercial, IRC for residential), along with ANSI Z97.1, spell out exactly where safety glazing (tempered or laminated) is legally required. Always check your local jurisdiction too, since amendments are common.

Common required locations include:

• Doors: Any glass in swinging, sliding, or bifold doors must be safety glazing
• Sidelites: Glass within 24 inches of a door and extending below 60 inches from the floor
• Windows near floor: Glazing whose bottom edge is less than 18 inches from the finished floor and top edge is more than 36 inches from the floor
• Wet areas: Tub and shower enclosures, steam rooms, pool areas
• Stair and floor proximity: Any glazing adjacent to stairways within certain dimensional limits
• Overhead glazing: Skylights and sloped glazing where breakage could fall on occupants
• Balustrades and guards: Railings and guards using glass infill panels
• Gymnasiums, courts, and play areas

Wherever safety glazing is required, the glass has to carry a permanent label: the maker's name, the standard (ANSI Z97.1 or CPSC 16 CFR 1201), the safety category, and a certification mark. And permanent means etched or sandblasted into the corner, not a peel-off sticker.

What you cannot do with tempered glass

Remember that locked-in tug-of-war? It never lets go. Nick the surface hard enough, or cut through to the tensile core, and the whole thing unloads at once. The pane bursts into dice on the spot.

After tempering, you cannot:

• Cut or score the glass
• Drill holes
• Make notches or cutouts
• Grind edges significantly
• Re-heat to bend (it would lose its temper)

So every hole, notch, cutout, and edge has to be fully worked out and cut on annealed glass before it ever sees the furnace. There are no edits afterward. Put a hole in the wrong spot or cut a piece short and it goes straight in the cullet bin; you start over from a fresh sheet.

Tempered vs. heat strengthened

People mix up tempered and heat strengthened glass constantly, and it is easy to see why: they run through nearly the same furnace. The difference is how hard you quench them, and that single variable changes almost everything downstream:

Heat strengthened is stronger than annealed, but it is not a safety glazing material, because it breaks into larger pieces that fail the safety test. What it gives you in return is real: almost no spontaneous-breakage risk, better thermal-shock resistance, and less optical distortion than fully tempered. That makes it the go-to for spandrel panels, some curtainwall, and laminated safety glass, where the interlayer is doing the safety job anyway.

Nickel sulfide and spontaneous breakage

Tempered glass carries one rare but genuinely unsettling risk: spontaneous breakage from nickel sulfide (NiS) inclusions. NiS is a tiny contaminant that can sneak into the batch from raw materials or equipment. Most specks are harmless, but NiS slowly changes phase over months or even years, and as it does the particle swells just a little.

If that swelling happens to sit in the pane's tensile core, it can tip the local stress past the limit and the glass simply lets go, no impact, no warning. The odds on any single pane are tiny (industry estimates run from about 1 in 10,000 to 1 in 500,000), but scale that across a tower with thousands of tempered lites and a handful of events over the building's life become fairly likely.

The standard defense is heat soak testing: bake the tempered glass at around 290°C (554°F) for a set time to fast-forward that NiS phase change, so any at-risk panes fail in the oven instead of years later up on the facade. It is required or recommended on a lot of curtainwall and overhead work. It does not wipe out the risk entirely, but it takes a big bite out of it.

Optical effects: roller wave and anisotropy

Tempering also leaves two optical fingerprints you will never find in annealed glass:

Roller wave: the glass rides through the furnace on rollers, and while it is soft those rollers press in the faintest ripple. You catch it as a gentle wave in reflections seen at a glancing angle. It is baked into the process, held inside limits by ASTM C1048, and shows up most on thinner glass and big panes.

Anisotropy (iridescence): those built-in stress patterns bend polarized light unevenly. Look through polarized sunglasses and you will catch faint rainbow blotches, the so-called "leopard spots." Every thermally tempered pane does it; it is a fingerprint of the process, not a defect, and it is easiest to spot in reflective glass or against a bright sky at the right angle.