Mass production
The clay extracted for making salt furnaces and domestic ceramics has the same geochemical signature, matching that of local soils extracted from the marl clay formations of the valley bedrock. The Celtic salt-making communities of the Seille valley therefore also included potters.
The technical components used in the construction of the furnaces — such as the load support bars and the salt moulds — were made in large batches, while their outer surface was covered with a coating of organic matter. This covering, designed to prevent the parts from sticking together, made mass production easier. The organic matter used for this coating was largely obtained from cereal crop processing waste, including hulled barley, wheat and spelt. The saltworkers were therefore probably multi-skilled, and at least worked closely with the farmers.
Processing the brine
The brine in the Seille valley is loaded with elements from the sedimentary layers crossed by the downward flow of surface water, then the upward flow of salt water. These are mainly calcium carbonates and sulphates from the limestone and dolomite formations that cover the rock salt layer. These elements are the first to precipitate when the brine is evaporated or heated, and must therefore be discarded.
Salt crystals only precipitate when the brine has been reduced by a factor of 9. It is at this point that they must be extracted, before the reduction of the brine precipitates the magnesium salts, which are unfit for consumption.
Salt production therefore involved four or five successive stages:
- A stage in which the brine is drawn, probably from cased wells;
- A stage in which the brine is concentrated to bring it to saturation point (300 grams/litre);
- A stage in which the concentrated brine is degreased to remove precipitates from the limestone;
- A stage in which the crystals are extracted;
- A stage in which the crystals are packed into salt bars.
For each basin, which has a capacity of around 30 litres, a quantity of around 9 kilos of salt could be obtained.
Work in the furnaces
The brine furnaces were fuelled by firewood, mainly hornbeam, beech and oak. Other species found in open spaces such as field maple, ash and hazel were also used. The saltworkers also burned apple and pear trees, as well as alder, willow and elm, as they gathered wood around their workshops.
Once lit, the brine furnaces could operate continuously for several days, or even weeks. As the salt water evaporated, their basins were regularly fed with brine brought to saturation point from nearby clay-lined tanks.
Geochemical analysis of the brine furnaces at "Pransieu"’ in Marsal has shown that they operated in a reducing atmosphere at a very stable temperature (over 600°C) for long periods of time. Archaeomagnetic analysis of the furnace walls confirms that the hearths of the furnaces were heated to over 550°C.
Extracting salt from brine
The heat was lower, however, when the furnaces were loaded. The brine basins were heated to between 100°C and 550°C, ensuring that the salt water evaporated. The salt moulds in which the salt blocks were packed were only heated to temperatures of around 250°C: this was confirmed by archaeomagnetic analysis of the grates of terracotta bars that supported the loads of salt moulds.
During the heating of the brine, the salt contained in the salt water caused vitrified formations to appear on the terracotta components. Sodium glass was formed by the reaction of clay and salt on the upper parts of the equipment. On the other hand, soda-lime glass, which is richer in lime, formed when salt came into contact with the ashes on the walls of the lower part of the furnaces.
After a certain period of use, the brine basins had to be replaced. Their inner walls are frequently encrusted with a whitish limestone deposit, which is the result of the precipitation of the calcium carbonate contained in the brine. There are also increased levels of phosphorus, sodium and magnesium, which precipitate out when the salt water is heated. In general, the briquetage components – and in particular the basins – were enriched with strontium, which is present in high concentrations in the Seille brine.
