Fig. 4 - Jean Baptiste Adolphe Gibert (1803-1889), painting of the salt marshes of Ostia.

The historical salt works of the coastal plain of Rome

Author:
Tonnie Huijzendveld (Arnoldus)
The coastal plain of Rome is characterized by two wide depressions that were occupied originally by ponds, the Stagno di Maccarese and the Stagno Ostiensis, separated by the Tiber river (figure 1).
Historical salt works have been established in each of the marshes.

Fig. 1 - Simplified topography and geology of the coastal plain of Rome; C, harbour basin of Claudius; T, harbour basin of Trajan; F, “Fiume Morto”, the river course before 1557 A.D.; with a star are marked the known historical salt works.

Fig. 1 – Simplified topography and geology of the coastal plain of Rome; C, harbour basin of Claudius; T, harbour basin of Trajan; F, “Fiume Morto”, the river course before 1557 A.D.; with a star are marked the known historical salt works.

The salt works of Ostia
The Roman age salt works of the Stagno Ostiensis have not been located exactly nor excavated, so we don’t even know their layout. It is only possible to presume that the warehouses and wharfs of the harbour situated in the suburban zone of Ostia, along the later filled in meander of the Tiber, were the hubs from which the salt was transported to Rome.
One of the most ancient salt deposits is recognizable in the late medieval building called Casalone, now incorporated in the modern township and close to the abandoned river course. After the cut off of the meander in the sixteenth century, salt storage occurred in the Magazzino de Sali, which is now the Museum of Ostia Antica.
The exact size of the lagoon of Ostia was partly defined thanks to recent archaeological excavations, which pinpointed most of the banks of the lagoon in the Roman period (Pannuzi, 2013).

The Romans considered Ostia their first colony and they attributed its founding for the purpose of salt production to their fourth king, Ancus Marcius (second half of the 7th century B.C.), but no archaeological proofs have been found for this. In fact, the soil of Ostia (not even the Castrum, which is the first “urban” development), has so far delivered no findings dating back to the time of the Kings.

Fig. 2 The salt works of Ostia indicated on the map of Amenduni 1884; also visible is the oxbow lake, remnant of the river course abandoned during the flood of 1557.

Fig. 2 The salt works of Ostia indicated on the map of Amenduni 1884; also visible is the oxbow lake, remnant of the river course abandoned during the flood of 1557.

The paleo-environmental reconstruction of Bellotti et al. 2011 could clarify the discrepancy between archaeological and historical sources on the origin of Ostia. The Authors state that initially the coastal barrier belt separating the marsh of Ostia from the sea would have been too narrow and insecure against storms to support a permanent human occupation. Therefore, in the 7th century B.C. there would have been only an outpost, with the aim of controlling the strategic river mouth and, eventually, to set up the first salt works in the marsh. In fact, the data of pollen and molluscs of a drilling core indicate around 600 B.C. a sudden intrusion of sea water, which would have allowed salt extraction. It is not clear if this is a natural event or a man induced breakthrough of the barrier belt.

According to this hypothesis, only around 450 B.C., when the cusp had expanded more than 1 km into the sea following a progradation rate of about 5–6 m/year, the sandy substrate was supposedly large and safe enough to set up a fortified camp (the Castrum of Ostia) and to further develop the salt works.

The salt works of Ostia have continued to function until the nineteenth century, and are indicated on a map of 1884 (figure 2) and visible on an aerial photograph taken from a balloon in 1911 (figure 3).

Fig. 3 - Part of the salt works of Ostia visible on an aerial photograph taken from a balloon in 1911; to the left the abandoned river bend of the Fiume Morto; an arrow indicates the original stream direction; modified from Shepherd 2006.

Fig. 3 – Part of the salt works of Ostia visible on an aerial photograph taken from a balloon in 1911; to the left the abandoned river bend of the Fiume Morto; an arrow indicates the original stream direction; modified from Shepherd 2006.

At the time, the decay of the coastal plain was total: it is described in 1831 as an unhealthy marshy environment. The 19th century painting of Jean Baptist Adolphe Gibert sets this atmosphere rather well (figure 4).

Fig. 4 - Jean Baptiste Adolphe Gibert (1803-1889), painting of the salt marshes of Ostia.

Fig. 4 – Jean Baptiste Adolphe Gibert (1803-1889), painting of the salt marshes of Ostia.

It is evident from the images that, at least in later times, these salt works were structured in serial basins, as described by Georgius Agricola in 1556, who recommends to build salt pans by evaporation “near that part of the seashore where there is a quiet pool, and there are wide, level plains which the inundations of the sea do not overflow”.

According to the indications, the brine is collected into shallow ponds and allowed to evaporate in the sun. A stepped process along a series of interconnected basins separates the undesirable substances from the fine salt. The basin should be moderately deep depressions, surrounded by embankments and separated by ditches with adjustable openings. The gentle drop down applied to the complex would allow the water to flow from one basin to another. The low tidal range of only 30-40 cm, common to all coastal plans along the Tyrrhenian Sea, should make it impractical, but not impossible, to use the high tide for letting the marine waters enter the salt basins.

The Maccarese saltworks
An important environmental event that occurred, before or during the Etruscan period, with an almost stable sea level, is the transformation of the water of the Maccarese lagoon from fresh to salt/brackish (naturally and/or artificially promoted?), enough to allow the construction of salt works. This change is recorded to have occurred after 910-800 cal B.C. (calibrated 14C dating) by Giraudi 2004. The cause of this transformation should have been the re-opening of the connection with the sea.

Following this “environmental revolution”, the area of Maccarese became an important centre of salt production under both Etruscan and Roman domination. Although mentioned by the ancient sources, the Etruscan salt pans have never been found. After the conquest of Veio in 396 B.C., the salt works came under Roman control and were used under the name Campus salinarum romanarum throughout the Republican and Imperial period.

The salt pans of Maccarese were exploited at long, and are mentioned in several documents at least until the end of the fifteenth century, under the names Campus Maior, Campus salinarius or Campus Salinus Maior (Morelli & Forte 2014).
Recent trench prospection and excavation campaigns (Grossi et al. 2015) have brought to light Roman imperial salt works, composed of a complex of canals dug into the earth, used to channel and distribute salt water from the Maccarese Pond. Connected to the salt extraction systems was a structure composed of a one-kilometre string of 1439 amphorae inserted upright into the muddy substrate (fig. 5).

Fig. 5. Principal features of the Maccarese hydraulic system (1st century A.D.) overlain on the map of Amenduni 1884; a dotted line marks the historical lagoon shore. D, amphorae dam; W, ground canals departing eastward from the main walled channels; yellow squares mark the position of the masonry canals with sluices. Modified from Grossi et al. 2015.

Fig. 5. Principal features of the Maccarese hydraulic system (1st century A.D.) overlain on the map of Amenduni 1884; a dotted line marks the historical lagoon shore. D, amphorae dam; W, ground canals departing eastward from the main walled channels; yellow squares mark the position of the masonry canals with sluices. Modified from Grossi et al. 2015.

The row of amphorae, datable between the Augustan period and the middle of the first century A.D., composed the framework of an earthen dam and was crossed by two canals in cement with opus reticulatum facing, each about 25 m long and provided with a sequence of two sluices. These canals have a characteristic funnel shape, with the opening to the west, which confirms their function as collectors of water from the lagoon (figure 6).

Fig. 6 - Aerial view of one of the funnel-shaped masonry channels crossing the earthen dam with the string of amphorae. Standing out is the white travertine of the two sluices (from Morelli & Forte 2014).

Fig. 6 – Aerial view of one of the funnel-shaped masonry channels crossing the earthen dam with the string of amphorae. Standing out is the white travertine of the two sluices; West is on the right hand side of the image (from Morelli & Forte 2014).

From the brick channels branch off to the east two long canals dug into the earth. The system is completed by two ground channels or “basins” reinforced with rows of wooden poles (guaranteeing walkways), located parallel on both sides of the amphorae dam, and probably intended as water collectors. Their depth is shallow, max. 0,40-0,50 m, which coincides with the tidal range of these coastal plains.

The layout of the system suggests that the dam and the two masonry canals with sluices allowed to control the salt water flowing from the Maccarese Pond, which was then distributed over the vast territory behind the dam through canals dug into the earth. One or more times a year, from the start of the dry season, the salt water would have been let in during high tide. Then the sluices were closed, thus isolating, together with the amphorae dam, the system temporarily from the main body of the lagoon. Behind the dam, a shallow water surface would spread out over the flat areas between the canals, where salt concentrated and could be harvested.

Such a single-cycle process, without the use of interconnected basins to gradually purify the brine, would imply that these salt works produced unrefined marine salt. These salt works might be similar to the system is described by Rutilio Namaziano (I, 475-486) in the fifth century AD near Volterra.

For service and salt storage activities, two building complexes attributable to the late-Republican and Imperial age, have been identified along the Via Portuensis. In one of them an epigraph was found datable to 135 A.D., of a dedication to Neptune made by two men identified as conductores campi salinarum romanarum, i.e. Roman salt work contractors (Morelli & Forte 2014).

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