The Zeer Pot

Picture by KJ Photography

& is used with permission.

The Zeer Pot, also known as the clay pot cooler or the pot-in-pot refrigerator, is a low-tech way to keep food, medicine, and other perishables cool.

Having been invented in North Africa and the Middle East, the zeer pot uses evaporative cooling to keep objects cooler than the environment in a manner similar to a swamp cooler, and like a swamp cooler it requires an environment of low humidity (10-30%) such as the desert, badlands, or other dry and windy places. In areas of medium humidity or above, it will work poorly or not at all.

Making a Zeer Pot

Materials

• Two unglazed terra cotta pots
• One pot needs to be smaller than the other such that it can sit inside the first with a 2-3 inch gap around all sides
• Sand
• Duct tape
• A cloth able to fit over the top of both pots
• A lid (optional)

Construction

1. Use the duct tape to close off the hole at the bottom of the largest pot. 
2. Place a layer of sand inside the large pot until the lip of the smaller pot is equal to (or slightly higher than) the lip of the larger one. 
3. Place the smaller pot inside and carefully layer sand around the sides, making sure the smaller pot doesn't move and an equal circumference is achieved. Using strips of tape to secure the lip of the smaller pot to the larger will help in this regard. 
4. Fill and pack with sand until the inner pot is tight. 
5. Place zeer pot on a stand that exposes the bottom (such as with three stones) in a place out of direct sunlight but where wind can blow around it. 

Use

1. Slowly pour water into the sand between the pots. If you pour too quickly, you risk disturbing the sand or washing it away. 
2. Wait until the outer pot is dark with absorbed water. 
3. Check the inner pot. If it is cool, place the items you wish to refrigerate inside. 
4. Add more water to the sand. 
5. Cover inner pot with lid, if available. 
6. Wet cloth and cover pot. 
7. Add water to both sand and cloth as necessary. 

Principles
The sand serves as both a storage medium for the water and as a cooling material. When moistened, it will cool down as it evaporates. With the surface area of the sand against the outer pot, the majority of the moisture will be drawn through the terra cotta of the outer pot, which will also cool as it evaporates. This  evaporative cooling will lower the ambient temperature inside the pot.

The zeer pot works on the same principles as your body does to cool itself with sweat; just replace the food with your internal organs, the sand with your blood, and the outer pot with your skin. This serves as a handy guide for where best to place the pot: put it wherever you are most comfortable.

https://commons.wikimedia.org/wiki/File:Clay_pot_cooler_-_Canari_Frigo_-_Tonkrugk%C3%BChler.PNG

Important Note!
Due to the ratio of surface area to volume, many smaller pots work better than one large pot (I would recommend no outer pot larger than 18" across). The zeer pot needs a large moist surface area to cool its inner volume, but as you increase the volume of the pot the surface area requirements increase drastically. (In math speak: its volume will increase proportional to the cube of its linear dimensions, but its surface area will only increase in proportion to the square of its linear dimensions.)