The slow transition of a liquid to a gaseous, vaporized state is called evaporation. This is in contrast to the rapid transition of a liquid to a gas, better known as boiling. Water has a high boiling point relative to Earth’s climate, so while all bodies of water are subject to evaporation, unless a substantial amount of heat energy is absorbed from sunlight, the process is very slow indeed.
Seawater is a natural and easily accessed source of salt. It has salt at an average of 35 parts per thousand. That means that a gallon of seawater has roughly 4.5 oz. of salt in it.
The process for extracting salt from sea water is an ancient technology and involves evaporation ponds. Shallow, water-proof ponds are dug out and connected to the sea by means of a short canal. A broad area and shallow depth allow a given volume of water to absorb more sunlight. The pond is flooded, and then the canal is closed. The sun evaporates the water. As the water vaporizes, the salt remains behind, creating ever-more saline water. Eventually enough of the water evaporates to leave behind a layer of sea salt crystals that can be harvested. Modern sea salt extraction operations typically have a number of these ponds concentrated in one location, separated by levees. Time is usually of the essence in evaporating sea water to extract salt, because an untimely rain can ruin days of evaporation. Therefore, some salt evaporation operations use very large indoor ponds called pans, which are protected from the weather.
Small Salt Pans
The same technology used in evaporation ponds is also employed for smaller sea salt extraction works using salt pans. A salt pan is a shallow pan made of metal, ceramics, or pottery, and uses the same principles as a large outdoor or indoor evaporation pond. A jug is not used, since the surface area exposed to sunlight and the open air is very small relative to the water volume, and that slows the evaporation process markedly. These small, personal pans are very useful for turning out salt for private use: evaporating 3 or 4 gallons of sea water can yield roughly 1 lb. of salt.