Heatwaves & Glasses: Why a Parked Car Is Their Worst Enemy

Exactly what happens to glasses in the heat

• The frame warps: acetate - the material of most quality frames - is a thermoplastic. Above ~55-60C it softens and "remembers" whatever shape it takes: temples that splay out, a bridge that loosens, a fit that's lost.
• Coatings crack: the lens and its coatings (anti-reflective, mirror, oleophobic) expand at different rates. Sudden temperature swings create the characteristic net of micro-cracks - "crazing" - that won't clean off and can't be polished out: the lens needs replacing.
• Hinges loosen: the metal hinges and tiny screws work inside softened plastic - a play that remains even after cooling down.
• Photochromics suffer: their chemistry is sensitive to extreme temperatures; repeated "baking" speeds up their ageing.

The 4 rules of summer

1. Never on the dashboard or the front windshield ledge. It's the hottest spot in the car - and with the sun coming through the windscreen, it works like an oven with a glass door.
2. Hard case, shady spot. If they must stay in the car, use a closed case in the glovebox or under a seat - never exposed.
3. Don't casually perch them on your hair at the beach. Sand + sweat + sun = scratches and loosened temples. A case here too.
4. Rinse after the sea, always clean properly. Salt crystallises and scratches - see the full cleaning guide in how to clean your glasses properly.

Already warped? What NOT to do

Don't attempt a repair with a hairdryer, boiling water or "bending by eye" - acetate needs controlled, even heating and experience, otherwise the damage becomes permanent (or the lens pops out of the frame). Every optician has the equipment to restore the fit in a few minutes, usually free of charge. And if the lens has crazed or the frame is badly deformed, it may be time to check how often we change glasses.

The sunglasses paradox

Sunglasses are made for the sun - but not for an enclosed space in the sun. On your face, with air around them, they never come close to dangerous temperatures. The problem is always the "greenhouse": a car, a boat with a closed cabin, a bag left for hours on the sand. A little care there - and the frame will stay with you for years.