Advanced technology
While up to the 3rd century BC, "pedestal" furnaces — in which the vessels containing the salt were placed at the top of a terracotta column — were predominant throughout Celtic Europe, as early as the 6th century BC, the saltworkers of the Seille valley developed a technology based on furnaces with removable grates, on which the loads of salt moulds were placed.
Everywhere else, the technique of using furnaces with grates — at first removable, then fixed — was not adopted until the end of the Gallic period, from the 3rd century BC onwards, then during the Augustan period.
An industrial production process
The switch from horizontal salt mould to vertical types of vessel was accompanied by the disappearance of the basins used to heat the brine to saturation point. The salt crystals were then likely extracted by means that did not require the use of furnaces, saving the energy supplied by the fuel.
At the same time, the technical components of the grates used to load the salt moulds into the furnaces became simplified and miniaturised, and their dimensions also changed. The aim of this transformation was probably to save energy, i.e. fuel. It also facilitated the mass production of these parts by reducing the volume of raw material used to make the bars by more than 80%, thereby optimising manufacturing time.
An unprecedented impact on the environment
The extraction of salt generates large quantities of production waste, which is discharged into the environment. The Celtic, and later Gallic, saltworkers left behind the waste products from combustion, as well as terracotta furnace components — such as basins, bars, connecting pieces and salt moulds. This debris accumulated in the palaeochannels of the Seille, contributing to the clogging of the alluvial plain, in which the water was finding it increasingly difficult to flow normally.
Alluvial deposition in the valley began as early as the Gallic period. In less than a thousand years, the valley had become filled with around two metres of clay sediment, reducing its gradient to less than one metre over a length of 12 kilometres. This section of the valley thus became a vast marshland area, which reached its peak in modern times, around the 17th century.
