How they break
Toughened glass is heated to around 620–650 °C and then rapidly cooled. The surface is left in compression and the core in tension. That makes it up to three times stronger in bending than ordinary float, and able to take a much larger temperature difference. When it breaks, the whole pane shatters into thousands of small fragments. There are no sharp shards, but there is no pane either: the opening is empty. Nothing can be cut or drilled after toughening; all holes and cut-outs are done beforehand.
Laminated glass is two or more panes bonded together with an interlayer, usually PVB. The code 4.4.2 means two 4 mm panes and two 0.38 mm interlayers. When it breaks, the fragments stay on the interlayer, the pane stays in the frame and keeps the opening closed for a while. The interlayer also blocks almost all UV. Laminated float can be cut; laminated toughened glass cannot.
EN 12600 classes
The standard uses a pendulum impact test. The impactor strikes the glass from three drop heights: 190, 450 and 1200 mm, giving classes 3, 2 and 1. The result is written as α(β)φ. The first figure is the highest class at which the glass passed (did not break, or broke safely), the letter in brackets is the mode of breakage, and the last figure is the highest class at which it did not break at all.
- A: breaks like ordinary glass, large sharp shards.
- B: breaks like laminated glass, cracks but stays on the interlayer.
- C: breaks like toughened glass, small fragments.
Typical classifications: 6 mm toughened 1(C)1, 4.4.2 laminated 1(B)1, 6.6.2 laminated 1(B)1. The exact class for a particular glass comes from the manufacturer’s test report, not from a catalogue.
Which goes where
Human impact (doors, partitions, shower enclosures, floor-to-ceiling glass): toughened.
Falling through the glass, or glass falling on people (balustrades, rooflights, canopies, overhead glazing): laminated. Glass that breaks must stay up and stay in the frame. Toughened glass would come down as a shower of fragments.
Break-ins: laminated, and thicker. Toughened glass breaks with a single hammer blow and the way is clear. Laminated glass takes time, noise and repeated blows. Resistance to manual attack is tested to EN 356 (classes P1A to P8B); for a house, P4A or higher is usually specified. Toughened glass does not help here.
Thermal stress (partly shaded glass, glass next to a radiator, dark glass in the sun): toughened, or at least heat-strengthened. Ordinary float and laminated float crack at a temperature difference of a few tens of degrees.
| Application | Recommendation | Why |
|---|---|---|
| Shower enclosure, glass doors | Toughened 8–10 mm | Body impact, breaks into small fragments |
| Balcony balustrade, staircase | Toughened laminated (8.8.4, 10.10.4) | Stays in the frame after breakage |
| Canopy, rooflight | Laminated, preferably heat-strengthened (HSG) | Must not fall on people |
| Shop window, street-level ground floor | Laminated P4A or higher | Break-ins |
| Façade spandrel, dark glass | Toughened, heat-soak tested | Thermal stress, sun |
| Floor-to-ceiling glass in a flat | Toughened or 4.4.2 | Impact; laminated if there is a drop behind it |
When you need both
Toughened laminated glass is a laminate made from two toughened panes. It carries load like toughened glass and, once broken, holds together like laminated glass. It is the standard for frameless balustrades, canopies and load-bearing glass.
One catch: when both toughened panes break, you are left with a sack of small fragments on an interlayer that sags. That is why overhead glazing more often calls for laminated heat-strengthened glass (HSG). It breaks into large pieces that lock into each other, so the pane stays stiff. Its strength is lower than toughened glass, but its post-breakage residual strength is better.
The other catch with toughened glass: spontaneous breakage caused by nickel sulphide inclusions. Rare, but real. Where glass is above people or on a façade, a heat soak test to EN 14179 is specified, which breaks most such panes in the oven rather than on the building.