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Glass · Identification

Is this glass toughened or annealed?

Look for the mark first. If there is none, look through the pane at a shallow angle wearing polarised sunglasses: toughened glass shows faint dark spots or stripes, annealed glass shows nothing. Everything below is the same question asked five ways, in the order worth asking them — cheapest and least destructive first.

The five checks, in order

The order is the advice. Most write-ups list these as equals, which is useless standing in front of a pane, because the memorable test is the one that destroys it and the four-second test is the one nobody starts with.

  1. The permanent mark

    Thermally toughened safety glass placed on the European market under EN 12150-1 carries an indelible mark — sandblasted, etched or ceramic-printed, almost always in a corner — naming the standard and the manufacturer. Laminated glass to EN 14449 is marked in the same way. It is deliberately unobtrusive, so it hides behind beads, gaskets and frames; if the pane is installed, look along the very edge of the visible glass with a torch before concluding there is nothing there.

    A mark settles it. No mark does not settle the opposite: it may be covered, it may be an old pane, or it may have come from outside the marking regime. Carry on to check two.

  2. Polarised light — the test worth knowing

    Toughening works by cooling the two surfaces fast while the core is still hot, which leaves the surfaces in compression and the core in tension. That locked-in stress makes the glass birefringent, and the quench is not perfectly even: the rollers the pane rode on leave a regular pattern of stress behind them.

    Put on polarised sunglasses, stand so the pane is reflecting sky or a bright surface, and look through it at a shallow angle. Toughened glass shows faint dark spots, blotches or parallel stripes — often called leopard spots or quench marks — that shift as you move your head. Tilting your head through ninety degrees makes them come and go, which is how you tell them from dirt. A phone or laptop screen works as the light source instead of the sky, because those screens are already polarised.

    Spots or stripes mean toughened, or at least heat-treated. A clean, featureless pane is almost certainly annealed. This is non-destructive, needs nothing you do not already own, and works on installed glass.

  3. The edges and the flatness

    Every edge, hole and cut-out has to be finished before the pane goes into the furnace, so toughened glass has worked edges: arrised, ground or polished, never raw. A sharp, straight-off-the-cutter edge with the score line still readable is annealed. Toughened panes also come out of the furnace very slightly less flat than they went in — a gentle bow, and roller wave running across the pane, which you see as a rippled reflection of a straight line such as a window frame or a fluorescent tube.

    Raw cut edges mean annealed. Worked edges are consistent with toughened but prove nothing on their own — plenty of annealed glass is polished for shelves and table tops.

  4. What has been done to it since

    Toughened glass cannot be cut, drilled or notched. Scoring the surface releases the entire stress balance at once and the pane falls apart on the bench. So the history of the pane is evidence: if somebody has trimmed it to fit, drilled it for a handle, or ground a corner back on site and it survived, it is not toughened.

    It was cut on site, so it is annealed. This is the check that most often explains a pane that "should" be toughened and is not.

  5. The break pattern — certain, and destructive

    Toughened glass breaks into small, roughly cubic pieces with blunt edges, all at once and across the whole pane. Annealed glass breaks into long shards and daggers, which is the entire reason safety glass exists. Laminated glass cracks but holds together, because the plies stay bonded to the interlayer.

    Certain, and it destroys the evidence. Useful when a pane has already broken and you are working out what was fitted; not a test to choose.

Heat-strengthened is the awkward middle case. It is quenched more gently than toughened, so it shows the same polarised-light pattern and the same worked edges, but it is roughly half the strength and it breaks into large pieces rather than dice. It is not a safety glass. Checks one and five separate it from toughened; checks two, three and four will not.

Why it is worth four seconds to check

The places where annealed glass is not acceptable are the places where a person can walk into it, lean on it, or fall through it.

Doors, and the panels beside themImpact safety glass, toughened or laminated, in the zone up to 1500 mm from floor level in a door and 300 mm either side of it
Low-level glazingSafety glass below 800 mm from floor level in walls and partitions
Balustrades and barriersLaminated, because what matters is what the pane does after it breaks, not before
Shower screens and bath enclosuresToughened, and in practice always marked
Overhead and roof glazingLaminated on the underside, for the same reason as a balustrade

Those zones are the British convention, from Approved Document K and BS 6262-4, and the impact classes themselves come from EN 12600, which is where the 1B1 and 2B2 markings on a specification sheet come from. Most European countries set the same idea at slightly different heights, so check the national rule rather than these numbers before you specify anything. What does not vary is the principle: in a critical location the question is not whether the glass is strong enough, it is what it does when it fails.

When toughened glass breaks with nobody near it

Worth knowing before you go looking for the impact point, because there often is not one.

A toughened pane that cracks on a still night with no one in the room is usually a nickel sulphide inclusion. A microscopic particle left in the melt sits in the tension core and slowly changes volume in service; when it grows enough, it cracks the pane from the inside. It needs no impact, it can happen years after installation, and the dice pattern radiates from a point somewhere in the body of the glass rather than from an edge or a strike mark.

It is rare, and it is not something an installer did wrong. Heat-soak testing to EN 14179 is the accepted mitigation: the toughened panes are held hot for hours so that most of the affected ones fail in the oven instead of on the building. It reduces the risk; it does not abolish it. For overhead glazing, and for anything where a failure would drop glass on people, it is worth specifying, and worth checking whether it was.

Edge damage is the other common answer. A nick or a shell chip on the edge of a toughened pane, from handling or from being levered into a frame, sits right where the tension is highest. Those failures do have a visible origin, at the edge, and they are worth ruling out before reaching for nickel sulphide.
Specification sheetPut the glass make-up, the standard and the impact class on one page for the client. Thickness for wind loadHow thick the pane has to be before any of this matters. Balustrade thicknessWhere laminated stops being a preference and starts being the point. Low iron, or clear float?The other identification question — the one about looks and about cost, not safety. Weight and stock sizesWhat the pane weighs, and what a full sheet weighs, on one printable page.