“New approaches to old issues: the application of predictive maps in archaeology.
A case study: modelling the location of Grosseto’s salt works from 900 BC to AD 1200.”
Medieval Settlement Research, 26.
Published in 2012
Carlo Citter, Antonia-Arnoldus-Huyzendveld (University of Siena – Italy)
As yet, we have no archaeological data about the salt works near Grosseto (Tuscany), but they must have been one of the main reasons for the town’s foundation and growth. However, the position of the salt works through time, and even their chronology, is still a matter of conjectures (see also the page Le saline tirreniche).
Grosseto was a small village around AD 700, that became a bishop-town (AD 1138) and, later, a Comune (13th century AD). The development of the Etruscan town of Rusellae in the 6th century BC, which is very close by on the hills to the north, could have some relationship to the exploitation of this precious resource.
The first unquestionable mention of salt works is in a chart of 1152 (terraticum salinarum). However, the main document is the charta libertatis of 1204, where the Aldobrandeschi are forced to grant the community of Grosseto half of all the salt revenues. This document informs us that the production of 1203 was 840 tons of salt. The 13th century seems a crucial period of intensive production. The salt of Grosseto was sold in Siena, Florence and Genua, and it was so famous to deserve a literary mention by medieval poets. Well known is the interest of Pisa in the salt revenues of the lagoon of Grosseto.
The salt masters of Siena declared in 1386 that the former salt works near Grosseto could not be used anymore, because the lagoonal environment had changed to freshwater. From the 15th century on, the new salt works were shifted south-west of Grosseto, along the Ombrone river, in a place close to La Trappola. Despite the small extension (1 ha), more than 800 tons of salt were produced here each year, until its abandonment in 1758.
The Grosseto plain has been reclaimed in the 19th and early 20th century by elevating the topsoil of the lower areas in the order of meters, making it impossible to field walk with success. We chose to evaluate the most promising location of the salt-works from the Roman period, or even earlier, until the high Middle Ages, through GIS spatial analysis with the use of hydrology tools. These can give us some indications for reducing a vast plain to small areas (no more than a few hectares each) whose potential is higher than anywhere else. These can be surveyed with geophysics, trenches and, in case of success, extended excavations.
The whole data set and the modeling procedures are explained in the publication. In order to model the possible location of the salt works, as was the goal of these operations, we had to consider the altitudes and the extension of the former lagoon, and moreover the feasibility of creating a channel to the sea, since salt water must flow into the area. The final results for the Roman and late medieval periods are shown in the figures.
According to the modeled results, for the Roman period the effective potential extension for salt winning is around 5 hectares, whereas for the late medieval period it is ca. 3 hectares. According to our calculations, this implies for the Roman period a maximal production of about 460 tons of salt per production cycle, and for the Middle Ages of about 240 tons. We don’t know how much production cycles there were in a year, since we don’t know the salt winning technique that was used at the time, but presumably there were several.