Saltworks, such as the ones at Salins-les-Bains and Arc-et-Senans, operated through “fire-source” production. The brine which had formed in the subsoil was pumped up from wells or through underground drilling. It was then heated, causing the water to evaporate and the salt to crystallise. The crystallised salt was then drained, dried and conditioned for sale.
Step 1: Collecting and pumping
At Salins-les-Bains, the brine was drawn from three underground wells from which come both salt and fresh waters. Separated and channelled, the waters were brought to the surface by means of mechanisms that have varied over time. A balancing pole, a horse-powered water wheel, a river-driven hydraulic pump have succeeded one another in collecting and bringing up the water. Later, drilling allowed water to be brought up from even greater depths, directly from the level of the rock salt strata.
Step 2 : heating and extracting
Once the brine was brought up to the surface, the salt had to be removed. The technique used was evaporation by heating. The brine was placed in large metal vats called “poêles” in French. Initially round, then oval and finally rectangular, their shape, volume and number changed over time. When it closed in 1962, the Grande Saline in Salins-les-Bains counted four rectangular vats, three of which still functioned.
The vats were placed over a hearth, hanging either from beams or supported from below by refractory bricks and iron pins. The hearth, fed with fuel (wood and later oil) ensured the heat necessary to evaporate the brine. This operation, known as “cooking”, took place in several phases. While the vat was being filled, the heat rose progressively, a process called the “ébergemuire”. A more violent fire then brought the brine to a boil in the first hours of cooking. During the next phase the heat was reduced and the salt began to crystallise. In the final phase, the “mettre-prou”, the fire was weak and the salt crystallised. It could then be harvested.
Cooking lasted several hours (from 8 to 18 hours according to the brine’s level of salinity and the efficiency of the installations) and different cooking phases followed each other. After the cooking of 16 different batches of salt the each vat went down for maintenance. Having become fragile under the effects of the salt and heat, they were cleaned, thoroughly examined and repaired.
Step 3: gathering and conditioning
The salt, crystallised in the vats, was gathered by workers using a scraping tool that resembled a large rake. They drew the salt towards the edges of the vat and removed. The most recent vats were equipped with wooden roofs on which the salt was placed for drainage. It was then dried and conditioned in various forms according to the era: salt loaves, in sacks or barrels… and stored.