The Spanish botijo is an unglazed earthenware water
bottle with spout that cools its contents by evaporation. Drinking from one without the spout touching the lips requires practice.
Some water is absorbed by the unglazed walls of the botijo. On reaching the surface the more energetic molecules escape thus
reducing the average energy level within and, consequently, the temperature.
In an experiment carried out by the Universidad Politécnica de Madrid in 1994, a classical botijo was
filled with 3.2 litres of water at 39ºC and placed in an environment with this same temperature and a relative
humidity of 42%. The total amount of water was measured periodically to ascertain the quantity lost by evaporation
and the water temperature. It was found that the water cooled by 15ºC in 7 hours: from this point it began to heat
up very slowly and after three days when only a few drops of water remained it was almost the same temperature as
the outside surroundings.
This principle has been utilised for more than three thousand years; remains of bottles similar in shape to the
modern ones have been found in ancient Mesopotamia. Nowadays, it has been replaced by the -less environmentally friendly- refrigerator.