A pot-in-pot refrigerator or zeer (Arabic: الزير) is an evaporative cooling refrigeration device which does not use electricity. It uses a porous outer earthenware pot, lined with wet sand, contains an inner pot (which can be glazed to prevent penetration by the liquid) within which the food is placed - the evaporation of the outer liquid draws heat from the inner pot. The device can be used to cool any substance.
There is some evidence that evaporative cooling was used as early as the Old Kingdom of Egypt, around 2500 B.C. Frescos show slaves fanning water jars, which would increase air flow around the porous jars and aid evaporation, cooling the contents. These jars exist even today and are called "zeer", hence the name of the pot cooler. Many earthenware pots were discovered in Indus Valley Civilization around 3000 BC which were probably used for storing as well as cooling water similar to present days ghara or matki used in India and Pakistan.
Despite being developed in Northern Africa, the technology appears to have been forgotten with the advent of modern electrical refrigerators. However in the Indian Subcontinent, ghara, matka and surahi, which are different types of earthenware water pots are used to cool water. In Spain, botijos are popular. A botijo is a porous clay container used to keep and to cool water; they have been in use for centuries, and are still relatively widespread. Botijos are favored most by the low Mediterranean climate; locally, the cooling effect is known as "botijo effect".
A zeer is constructed by placing a clay pot within a larger clay pot with wet sand in between the pots and a wet cloth on top.
The device cools as the water evaporates, allowing refrigeration in hot, dry climate. It must be placed in a dry, ventilated space for the water to evaporate effectively towards the outside. Evaporative coolers tend to perform poorly or not at all in climates with high ambient humidity, since the water is not able to evaporate well under these conditions.
If there is an impermeable separation layer between the food and the porous pots, undrinkable water such as seawater can be used to drive the cooling process, without contaminating the food. This is useful in arid locations near the ocean where drinkable water is a limited commodity, and can be accomplished by using a pot that has waterproof glaze applied to the inner wall where the food is stored.
Extended operation is possible if the pots are able to draw water from a storage container, such as an inverted airtight jar, or if the pots are placed in a shallow pool of water. A strap can be used to tie the inner pot down instead of using sand to prevent it from floating.
Pot-in-pot refrigeration has had multiple positive impacts on the population that uses them beyond the simple ability to keep food fresh for longer periods of time and decreasing instances of food-related disease.
- Increased profits from food sales: As there is no rush to sell food to avoid spoilage, farmers are able to sell their produce on demand and can command higher prices.
- Increased opportunities for women: Women can sell food directly from their homes, decreasing their dependence on their husbands as sole providers. Also, because girls traditionally take food to market to sell, and because food in the zeer stays fresh long enough that they can go to market once a week rather than once a day, there is more time for them to attend school.
- Rural employment opportunities: Farmers are able to support themselves with their increased profits at market, slowing the move into cities. Also, the creation of the pots themselves generates job opportunities.
- Increased diet variety because food is available for longer into the year.
A zeer costs about 150 naira (approx. US$1.00 in 2011) to make, and they sell for 180-200 naira (US$1.20 to US$1.30 in 2011).
- Evans, Lisa. "The Advent of Mechanical Refrigeration Alters Daily Life and National Economies Throughout the World".
- George F. Dales, Jonathan M. Kenoyer, Leslie Alcock. Excavations at Mohenjo Daro, Pakistan: the pottery
- prkhitman (2009-06-27). "Cold water in rural India : matka(earthen ware) |". fuel efficiency.org. Retrieved 2012-02-09.
- "The Origina of the Botijo". Universidad de Valladolid. Retrieved 2012-02-09.
- "How a zeer pot fridge makes food last longer". Practical Action website. Retrieved 2010-12-24.