In India, a 3,000-year-old technology is hitting the shelves—and flying off them—as South Asians struggle with rising summer temperatures.

Terracotta’s porous surface makes it a uniquely timeless passive cooling system. As water soaks into the pores, it permits evaporation to carry the heat of the vessel’s immediate surroundings out and away.

This principle has been cooling Indian homes since the Harappan Civilization that lived in the Indus Valley from ancient times. Yet it still has a role to play in the India of today, where companies are leveraging terracotta’s passive cooling to chill everything from buildings to food.

MittiCool is a company that manufactures terracotta refrigerators—capable of keeping items cool and preserved for 3 to 5 days in optimal conditions—all without power or ice. Currently sold out, it works by placing a tray of water in an upper chamber that seeps through the pores in the terracotta and cools the 50 liter interior space.

Another way that terracotta can replace electric cooling systems is through something called the Venturi effect. It states that as air moves from a large space into and through a narrow space, it must not only cool, but also speed up.

For this reason, Indian homes and verandah have been shaded by terracotta screens known as jaali for centuries. If air is to pass through the jaali it must necessarily speed up, leaving its moisture content behind in the pores of the terracotta. That moisture can then evaporate, repelling heat and cooling the area behind the jaali. 

CoolAnt is a design studio that’s using terracotta materials and designs to skin buildings and homes to mitigate the effects of the harsh summer sun. The same principles at work in a jaali could be applied at scale to a whole building facade.

“We’ve harnessed its hydrophilic properties and observed average temperature drops of [14F°] across more than 30 sites,” in India, CoolAnt studio founder Monish Siripurapu told Scientific American.

Scientific American had reported that just 20% of Indian households can afford to run an air conditioner, while just 35% can afford to run a refrigerator. That’s a substantial home-life challenge when summer temperatures routinely climb above 100°F in most of the country.

If, as some scientists predict, the intensification of the greenhouse effect from carbon emissions will lead to a worldwide increase of 3.6°F on average before the end of the century, there will have to be more adaptations to a warmer climate beyond just continuing to innovate with refrigeration.

Civilizations have dealt with extreme temperatures for millennia, and their best methods shouldn’t be overlooked in urban and suburban planning today.

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