An olla is a type of ceramic container with relatively thin sides, a round body and a smaller neck. Usually unglazed, ollas can be used for cooking, storage and irrigation. The word "olla" is Spanish, but Native Americans of the American Southwest made and used pottery jars of this type before the arrival of the Spanish.
Ollas make good cooking pots for beans, stews and other foods that require long cooking times. The thin sides allow the pot to respond quickly to changes in temperature while the shape retains moisture in the food. Ollas are traditional cooking vessels in Mexico, where cooking ollas are usually glazed and may be decorated.
In the southwestern U.S., ollas were a traditional way of carrying and storing water and were also used for cooking and irrigation. Traditional potters used the coil method to make ollas rather than throwing them on a pottery wheel. Potters often made the base of the ollas slightly concave so those carrying water could easily rest the pots on their heads. These pots were not glazed but were often painted with decorative designs before firing. Some ollas of the American Southwest were dried in the sun instead of being fired in a kiln.
Unglazed pottery makes a good water storage container, because moisture sweats through the clay. The sweating lowers the temperature inside the vessel and cools the water. This same sweating is what makes ollas suitable for irrigation, and it is as irrigation devices that ollas are increasingly used in contemporary society. Ollas used for irrigation typically have narrower necks than cooking ollas and remain largely undecorated.
An olla buried in soil and filled with water allows the water to slowly seep into the surrounding area. Plant roots near the pot grow toward the source of the water, making it even easier for the olla to keep them supplied with moisture. People setting up a garden irrigated by ollas tend to cluster the plants around each olla. It is best if the plants in each cluster have similar water requirements.
Those using ollas for irrigation choose a pot size suitable for the size of the plants that will grow near it. Larger plants will need a larger pot. The gardener buries the olla in the ground, leaving the top of the neck above the surface so more water can be added when necessary. The visible part of the neck sometimes has painted or incised designs on it.