Why the French are covering their windows with chalk powder in the heat

As record-breaking heat sweeps over France, some shops are running out of a simple, cheap and unexpected product – crushed chalk, the BBC writes.

Known as Blanc de Meudon, or Meudon whiting, it is normally used to make paints or as a cleaning product. But faced with punishing temperatures, there are reports that ingenious people have been using the chalky material as a home remedy against the heat, covering windows in schools and private homes.

Mixed with water, then painted on glass, the result is a milky, whitish coating that lets in some light but reflects the heat.

White paint – on walls and roofs, usually – is widely known to have a cooling effect. Generally speaking, white surfaces reflect sunlight and heat, while dark surfaces absorb it.

The chalk, as well as white paint more generally, is cheap. And unlike air conditioning, which worsens the overall heat effect and emissions problem by consuming energy and releasing heat outside, paint only uses energy when it’s produced.