Sea Salt or "white gold", as it has often been referred to, was the reason the island first became prosperous, but it was also the reason it was fought over. From time immemorial salt has played an essential role in preserving food and northern European herring fishermen came here for their supplies. An indication of its former importance is that the Island boasts consulates from Belgium, Denmark, England and Norway. The 10,000 tonnes of salt produced annually at the beginning of the 20th century has dropped to nearer 800 tonnes today, though there is currently a resurgence in production due to both the quality of the salt and tourist interest. The island is dotted with salt marshes and the square evaporating pans called "oeillets", are separated from one another by mud walls known as "bossis". The sea water is brought into the pans via canals or "etiers", which pass through reservoirs that get progressively shallower until arriving at the pans where the final evaporation causes the salt to crystallise, it is then carefully raked off, and left in piles to sun dry.
There are about 80 establishments still producing salt on the island and many have demonstrations of the process and a shop where you can buy their produce.
The salterns of Guérande is a swamp of salt water about 1 700 hectares in size. The current saltmarshes began before the 9th century and lasted for several centuries. Around the year 1500, the marshes reached 80% of the current surface. The latest were built around 1800. In the middle of the 19th century, a gradual decline started for different reasons : competition from a salt mine, lower consumption of salt as a product of conservation and improvement of transport by land. The salt of Guérande used to be traded throughout Brittany, tax free until Emperor Napoleon Bonaparte decided to tax it resulting in the beginning of a decline of salt activity.
Sea salt has been produced in Brittany and western France for a long time. It seems to be something people do wherever there is a coastline and salt marshes. And if there are no salt marshes you make some of your own.