Other riches in the Salins subsoil

In Salins-les-Bains, geological processes created a subsoil rich in sedimentary rock. Gypsum, dolomite, marl and limeststone are found alongside strata of rock salt.  Some of these rocks, following the example of salt, have greatly contributed to the town’s economic dynamism. This is especially the case for gypsum.


Gypsum: its formation and nature.

Gypse blanc et gypse fibreuxGypsum is an evaporitic rock, like rock salt. It is formed through the same process, by evaporation of mineral-rich water. At Salins-les-Bains, the lagoon present 215 million years ago during the Upper Triassic offered favourable conditions for the formation of these rocks.  The intense evaporation of water led to the deposit and crystallisation of minerals which formed gypsum and dolomite.

Gypsum is composed of hydrated calcium sulfate: CaSO4, 2H2O. It is a white or yellowish rock, and mate. Very soft, with a rating of 2 on the Mohs scale of mineral hardness, it is easily scratched with a fingernail, thus distinguishing it from limestone.

Numerous varieties of gypsum exist, in function of the nature and arrangement of its component crystals: saccharoid, fibrous, desert pink, spearstone, alabaster.


The uses of gypsum

Gypsum presents one major use: the production of plaster.  Through partial dehydration of the rock through heating, a semi-hydrated calcium sulfate is obtained (CaSO4, ½ H2O), the main (and for a long time the only) component of plaster. To use the plaster, it must be rehydrated by mixing it with water. It is thus transformed into a malleable paste before it solidifies as gypsum once again.

Gypsum is collected in mines or quarries.  It is then sent to plaster factories where it is ground, sifted, pulverised and transformed. At Salins-les-Bains gypsum was extracted from an underground quarry in a place known as “Naples”. In 1895, 15 workers annually produced 8000 cubic metres of gypsum.  In operation until about 1925, the quarry furnished many plaster factories with their primary resource.

In addition to plaster production, gypsum finds other uses, mainly in agriculture.  It is used to complete poor soils, thus increasing their fertility.

Extraction de pierre à plâtre dans une mine à M. Clément Raton - Salins-les-Bains