An industrial site, a place of production

Above all, the Royal Saltworks of Arc-et-Senans was a factory, destined to produce 60,000 quintals (27,000 kgs) of salt per year.  As at Salins-les-Bains, it functioned on the basis of evaporation.  The brine was heated, the water evaporated and the salt crystallised. In spite of the modernity of the saltworks, the systems employed were traditional.  The only innovation was in the use of what was called the “graduation technique” a means for condensing the brine to increase its salt content.


The technique known as « graduation »

Salt production begins by furnishing the factory with its initial resource, saltwater brine. As there were no saltwater springs in Arc-et-Senans, the brine arrived from Salins-les-Bains by a specially built pipeline 21.25 kilometres long.

On arrival in Arc-et-Senans, the brine transited through a building nearly 500 metres long that was called the “graduation” building to undergo the “graduation technique”, so named because it “graduated” the percentage of salt in the brine, to reduce heating time and the need for fuel and to thus improve efficiency. This was one of the advantages of the new factory. It served to improve efficiency.

Bâtiment de graduation - Saline Royale d'Arc-et-Senans


The graduation technique functioned on the basis of the natural evaporation of the brine. As it emerged from the pipeline, the brine was pumped to the top of the “graduation” building by means of six pumps powered by two large water wheels that were themselves driven by water brought from the Loue River. The brine then flowed through pierced pipes that spread it about the entire building.  From there salty water was made to flow over bundles of thorns.  The building was equipped with mobile shutters that allowed the wind to blow in and naturally evaporate the water. Salt not being volatile, the brine thus obtained was much more concentrated.

The water could be routed through the building again and again to obtain an even more highly concentrated brine. These successive operations could multiply the salt content by five, going thus from 30 to 140 grammes of salt per litre.  Following the graduation phase, the brine was stored in an underground tank of 200,000 litres to await the heating phase.

Today the graduation building stands no longer, having been destroyed around 1920. Only its foundation and a few traces of hydraulic equipment remain.

To see: Vestiges of former installations: the two-wheeled casing and the masonry of the canal embankment. Found on the Gabelous Trail at what was once known as the "Bâtiment de graduation" (locale devoted to the "graduation" technique for increasing the salt content in the brines). 


Evaporation and crystallisation

After passing through the graduation building, the brine was sent, via horse-driven pumps, from the storage tank to be heated in another building.  There the water was stored in wooden recipients called “bessoires” before being put into large pans or vats for heating.  Four square pans, plus a trapezoidal pan for pre-heating the brine were suspended over a large stove.  Heating took place in four phases.

The entire operation takes 24 to 72 hours and is repeated 18 times in a row so as to avoid losing heat.  After this cycle (called a “remandure”) the operation was stopped for a period of six days in order to clean and repair the pans.

The salt was then packed in barrels or formed into loaves in the work room at the far end of the building.

Bâtiment de fabrication des sels - Saline Royale d'Arc-et-Senans

These operations require a great quantity of wood. 14,000 square metres per year were harvested in the Forest of Chaux in the immediate vicinity of the saltworks. This consumption, added to that of other local industries (glass, nails, forges…) generated a lively competition for wood and gave rise to numerous conflicts with the local population.


An unprofitable factory

Salt production in Arc-et-Senans began in 1778, even before the construction of the saltworks ended in 1779. Between 30,000 and 40,000 quintals (3 to 4 million tonnes) of salt were thus produced every year. This was, however, too little in comparison to the annual 60,000 quintals (6 million tonnes) initially planned for.  From the outset, and throughout its period of activity, the factory’s output was insufficient.

What is more, the Royal Saltworks was quickly outmoded.  A few years after opening, its competitive advantages of being near the forest and using the graduation technique were outstripped by the technical advances of the era:

This lack of profitability would lead to the closing of the saltworks.  A termination of its activity was envisaged as early as 1840, though efforts at modernisation would keep it open for another fifty years. All production finally ceased in 1894, after a leak in the brine pipeline had once again polluted the village drinking water, leading to a trial and condemnation.