It is made by putting a clay pot inside a larger clay pot with wet sand in between the pots and a wet cloth on top. As the water evaporates it cools the inside, letting food stored in the inner pot to be kept fresh for some time. It must be placed in a dry, ventilated place for the water to evaporate to the outside.
Evaporative coolers perform poorly or not at all in climates with high humidity.
Mohammed Bah Abba of Nigeria patented the device in 1995 and was awarded a Rolex prize in 2000 for developing the “pot-in-pot preservation/cooling system”. However, the device was known at least as early as the Middle Kingdom of Egypt, as seen by the hieroglyphic kbb, "to cool".
Wine coolers[change | change source]
Earthenware wine coolers work on the same principle. They are soaked in cold water for a while, then emptied and put on the table. The pores of the earthenware are now full of water. Evaporation from the earthenware keeps white wine cool enough for several hours.