As France endures another summer of punishing temperatures, an unlikely item has become one of the country's hottest commodities. Not air conditioners or cooling fans, but crushed chalk.
Across parts of the country, shelves are reportedly emptying of Blanc de Meudon, a powdered form of chalk traditionally used for cleaning and paint preparation. Mixed with water and brushed onto windows, it leaves behind a thin white film that reflects sunlight before it can heat indoor spaces, offering households a low-cost way to cope with soaring temperatures. French media outlets, including Ouest-France , have reported shortages of the product as demand surged following widespread attention on social media.
Also read: A 400-year-old English forest could be sacrificed for stone used in Britain's historic landmarks
The trend may sound unconventional, but scientists say the physics behind it is remarkably straightforward.
A centuries-old material meets modern climate challenges
Unlike dark surfaces that absorb solar energy, white coatings bounce much of it away. The chalk's primary ingredient, calcium carbonate, reflects visible, ultraviolet and much of the near-infrared portion of sunlight, the wavelengths responsible for carrying heat.
"Chalk is mostly calcium carbonate. It absorbs very little sunlight, including the visible range, making it white. It also absorbs very little ultraviolet and near-infrared light, making it highly effective at reflecting sunlight," Xiangyu Li, a researcher at Purdue University's Cooling Technologies Research Center, told the BBC .
The same mineral has attracted researchers developing next-generation cooling paints. Jiashuo Wang, a researcher at the University of New South Wales, said calcium carbonate particles are already widely used in advanced radiative cooling coatings because of their reflective properties, according to the BBC .
Unlike conventional white paint, some of these engineered coatings not only reflect incoming sunlight but also release heat back into the atmosphere through a process known as radiative cooling.
Research from Purdue University found one ultra-white paint capable of reflecting up to 98.1% of sunlight, substantially more than the roughly 85% reflected by ordinary white paint. Another study found that pairing such coatings with specialised underlayers reduced daytime temperatures by as much as 7.6 degrees Celsius.
Scientists have also explored calcium carbonate coatings for fabrics that help keep wearers cooler by reflecting heat-carrying wavelengths of sunlight.
Also read: AI's hunger for data is now consuming rare books and booksellers are cashing in
Can a coat of white replace an air conditioner?
Experts caution that chalk-coated windows are far from a cure-all. Better insulation, reflective roofs and thoughtful building design remain far more effective long-term solutions.
Still, coating windows may offer meaningful relief.
Li told the BBC that painting windows white could reduce indoor temperatures more effectively than applying similar coatings to walls because windows admit a large share of incoming solar heat.
The approach is also attracting attention because it avoids the drawbacks of air conditioning. While cooling systems offer immediate comfort, they consume electricity and release waste heat outdoors, adding to urban warming and greenhouse gas emissions. Chalk coatings require energy only during their production.
France has also seen growing interest in reflective white roofs, popularly known as "cool roofing", as cities search for ways to adapt to intensifying heatwaves. A separate study found that widespread adoption of cool roofs in London could have lowered average temperatures during a heatwave by around 0.8 degrees Celsius while preventing an estimated 249 heat-related deaths.
The renewed interest in reflective coatings echoes architectural traditions seen across southern Europe, particularly in Greece, where whitewashed buildings have long helped keep interiors cooler under intense Mediterranean sunshine.
There are even more unusual experiments underway. Researchers in the United Kingdom found that applying a thin coating of yoghurt to windows reduced indoor temperatures by an average of 0.6 degrees Celsius, with cooling reaching as much as 3.5 degrees Celsius during hot, sunny conditions. Although the coating initially carried an odour, researchers found it disappeared quickly once dried.
As climate change pushes heatwaves to become longer, more frequent and more intense, France's embrace of powdered chalk points to a broader shift. Instead of relying solely on energy-intensive cooling, people are rediscovering simple materials and age-old ideas that may help make homes more resilient in a warming world.
