Laminated Safety Glass

Every break is done inside a closed transparent bag, on a bench, with heavy gloves and eye protection. Use thin picture glass or, better, acrylic. Never handle broken glass bare-handed.

Bénédictus claims a celluloid core interposed between the sheets. This is not a surface coating: the layer is trapped in the middle.

Inside the bag, break a bare sheet with one sharp blow. Count the fragments and note their size. This is the benchmark.

Without touching them, examine the fragment edges. They are sharp and irregular — glass breaks by following surface flaws, not by deforming.

Prepare two sheets of the same size, about 150 × 150 mm. They must be flat and clean — the smallest speck of dust becomes a bubble later.

Cut a sheet of clear flexible plastic to the same size. The original celluloid was replaced long ago; here a clear adhesive film plays the same role.

Glass, film, glass. Press from the centre outward to drive out the air. A bubble is a place where the sheet will hold nothing.

Clamp the assembly between two boards for an hour. Adhesion across the entire area is what separates a laminate from a stack.

Look through it. A windscreen must stay perfectly readable — a cloudy laminate is unusable, however safe it is.

Strike the centre with the same force as in step 3. The glass breaks — but nothing leaves. The pane stays in one piece.

Set the fragments from step 3 beside the cracked pane. Note carefully: the glass did not resist any better, it failed differently.

Look at it edge on. The cracks in the two faces do not coincide and stop at the core. The film interrupts propagation from one layer to the next.

Turn the broken pane over and shake it gently, still inside the bag. The pieces hold. That is precisely the effect wanted in a motor car.

The patent. FR 405,881, "Wired glass by interposition of a celluloid core", filed 10 August 1909 and published 15 January 1910, by Édouard Bénédictus (1878-1930), a French chemist and decorative artist. ⚠ Many sources give 25 November 1909 as the filing date; the document says 10 August. A second patent, FR 418,257, follows in 1910.

The discovery preceded the application by years. Around 1903 a laboratory flask fell from a shelf. It cracked but did not scatter: it had held a cellulose nitrate solution, and a dried celluloid film lined the inside. Bénédictus noted the fact and did nothing with it. Only in 1909, after reading an account of a motor accident in which two women were badly injured by windscreen fragments, did he connect the observation to the problem. The idea sat unused for six years for want of an application.

Changing the failure mode rather than the strength. This is the point almost always summarised wrongly. Laminated glass is not "unbreakable glass": it breaks at the same stress as bare glass. What changes is what happens next. Glass is brittle because a crack, once started, propagates freely; the flexible core absorbs crack energy, stops the two faces cracking together, and above all mechanically retains every fragment. Steps 10 to 13 show this better than any explanation.

Triplex, and very slow adoption. Bénédictus named the product Triplex — three layers — and founded the Société du Verre Triplex in 1911. Car makers did not rush: the process was expensive, and celluloid yellowed and delaminated with age. Laminated glass first saw service as protective lenses in First World War gas masks, then reached cars through the 1920s and 1930s as celluloid gave way to more stable interlayers, today polyvinyl butyral (PVB).

Two different glasses in the same car. A modern windscreen is laminated, per this patent: it retains fragments and keeps its integrity, which also stops occupants being ejected through it. The side windows are usually tempered glass, which takes the opposite strategy — it disintegrates into small blunt granules, and is designed to break so that people can get out. Two answers to the same hazard, chosen according to what each pane has to do.