How the salt marshes work

The Guérande salt marshes function by gravity.
They are built between the level of the high tide, to fill the reservoirs, and the low tide to empty them. The fresh water of winter needs to be removed before the preparation work can begin.
The soil type is very important - it must be clay to guarantee waterproofing between the different levels, be malleable, and able to keep cool to ensure the best crystallisation.

A good salt works must be "ventilated", ie. well-orientated compared to the prevailing winds, far from obstacles and not too enclosed.
Illustration from Gildas Buron, Source: "Musée des Marais Salants from Batz-sur-Mer"

Along the water flow, we find :
• The "vasière" (primary pond): a water reserve which supplies the salt works between tides (15 days), it is the largest part of the salt production site, horizontal over most of its surface (the "pelloué") with a trough along its edge ("rai") to allow cleaning. Its surface may vary, but represents 40% of the salt works.
• The "cobier" (evaporation ponds) are made up of fairly rough, large ponds, which are designed to increase concentration to a high enough level to remove algae ("limu") and crustaceans ("bigots") - the salt worker's enemies. This area is not mandatory, but very practical, and it appeared fairly late in the history of the salt marshes. It can represent on average 10% of the salt works.
• The "fares" (evaporation ponds): these are smaller ponds, and thus more adapted to the meticulous maintenance along the bends, enabling a regular flow of a fine film of water (1cm). They are as flat as possible, with the exception of a small ditch (fossé) along the bridges (carrière). These are the most efficient ponds for the concentration of sea water. Their surface often increases as the distance from the sea grows, and they represent 30 to 40% of the salt works.
• The "adernes" (reserve ponds): these contain the water reserves for the day, with a concentration close to saturation. Their surface covers 8% of the salt works. The water flows out by a distribution channel (délivre) when the harvest takes place.
• The "oeillets" (crystallisation ponds): these are the final ponds from which the water doesn't leave, and where the harvest takes place. The word "oeillet" comes from a poor translation of the original Breton word, "lagat", which means small pond or pool, but also means eye, or "oeil" in French.
• These rectangular ponds (7m by 10m), are on a human scale to enable harvesting to take place, by hand, of around fifty kilos of salt crystallised in a day. Their number varies depending on the size of the salt works, but they represent around 10%. The greatest care is taken in their preparation. The middle must be very flat (we talk about "galoche" or upper part), whilst the edge is a trough of 1-2 cm by 1.5m, to ensure the required volume of water for harvest. They must never dry out, and are supplied with water every day before the harvest. Small round platforms ("ladures") placed in the middle of the largest bridge allow the daily salt production to be stocked.
• The "fosses"(ditches) : these are mounds of earth which are an integral part of the salt works and enable them to be marked out. They are also sometimes used as roads to transport the salt.

For more information about the specific terms about salt marshes, please see the glossary