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Final Report Summary - SWORM (Stone-working across the ancient Mediterranean. Building techniques, artisans and cultural identities: a view from North Africa)

Executive Summary:

Objectives of the project:

Old and new hypotheses on the origin and diffusion of building techniques: the case of the so-called Opus Africanum

The aim of the SWORM project was to explore the problem of the diffusion of stone-working techniques for ancient construction in the Mediterranean basin, starting from the North African territory of Maghreb, where peoples of indigenous, Phoenician-Punic, Greek-Hellenistic, and Roman origin met and influenced each other.

A combination of archaeological, anthropological, geological, seismological, and archaeometrical methods was used and helped to identify local and imported building traditions as well as the relationship between the Maghreb and other Mediterranean regions, in terms of knowledge exchange, and of the diffusion of building models and techniques.

In particular the research has focused on the study of a specific technique, the so-called opus Africanum, with the aim of identifying its origin, its development in the Maghreb and its diffusion across the Mediterranean, from the late Punic age to the Roman period (2nd c. BC to 5th c. AD).

The term opus Africanum is used in archaeological literature to identify a technique, characterised by walls with vertical frameworks made of squared blocks that create a skeleton filled in with panels made of smaller stones. According to traditional hypotheses, this technique is of Syrian-Palestinian origin and was exported to Phoenician colonies in the western Mediterranean (mainly Carthage and the surrounding territory). From here it eventually spread further in the Mediterranean. During the Roman period it continued to be used intensively in the Maghreb, hence the label opus Africanum. Clearly this definition is based on an ethnic and territorial association derived from 19th to mid-20th c. North-African archaeology of the French colonial period.

In order to test these hypotheses the project had to main objectives: the first was to study the diffusion of the technique in the Mediterranean and the second was a detailed study of opus Africanum in ancient Morocco.

The diffusion of Opus Africanum in the Mediterranean: general observations on its form, function and construction method

From the analysis of attestations of Opus Africanum in the Mediterranean it appeared that it covers three techniques, distinguishable by form, function and construction method.

a) The first has single rectangular blocks set vertically and irregularly spaced; these are used to reinforce wall socles and are considered to be of Phoenician-Punic origin. In fact they occur in many Punic contexts (Tunisia, Sicily, Southern Spain, Sardinia) from the VII to the IV c. BC. In nearly all of these contexts the walling is built with re-used material and it is one of the few techniques that can make good use of the material of varying dimensions that occurs in such situations. But the technique also occurs in different cultural areas: the Etruscans used it from the 6th c. BC, and another example is the Celtic oppidum of Enserune (Southern France), in the 2nd to 1st c. BC., where it is considered a Roman import.
b) The second technique has piers made of superimposed blocks. In cases where the blocks are all vertical, they need to be carefully squared because the piers are structurally independent from the infill between them. Again this is found in Punic contexts, but with a later start date (examples in Tunisia in the middle 2nd c. BC and Spain, Ist c. BC). In the Roman period it is common in the Maghreb but it is found also in Spain in areas outside of earlier Punic influence (examples in the upper Ebro valley).
c) In the third technique the piers are made of alternating vertical and horizontal blocks. Here the piers are tied to the infilling and are not structurally independent; the shaping of the blocks can therefore be less accurate. This is first found in Italy and Western Sicily in the 4th and 3rd c. BC. and the most famous example is that of Pompeii in the houses of the Samnite aristocracy (III c. BC). Here it is used in the side and internal walls while the more symbolic ashlar construction was preferred for the facades. In this sense the Pompeii examples suggest that solid walls could be built from Opus Africanum, but at less cost than ashlar. The first African examples are at Bulla Regia in the 2nd to 1st c. BC, under Numidian reign. The technique spreads further in the Imperial period, particularly at Thugga and Bulla Regia (Tunisia), in both public and residential architecture. As we will see it was used also in Morocco.

From the analysis of attestations of the three techniques a more complicated picture emerges than was previously thought about the origin and diffusion of Opus Africanum. Moreover the building process of these techniques appears to be not so complex as ashlar masonry for example. For this reason we do not need to think about a single tradition diffused from one place to another, particularly in the case of the first type, the most common and easy to build: it could develop independently in different places and periods. As regards the second type, in North Africa it seems to represent more surely a Punic tradition, given the continuity of the technique from the IInd c. BC onwards. The third type is first found outside Africa in the IVth and IIIrd c. BC. In the Maghreb it appears to be rare and confined to limited areas.

Structural properties of Opus Africanum and its use in Roman buildings of Morocco.

The structural properties of Opus Africanum were considered by analysing some Roman buildings of Morocco. This approach allowed clarifying why and how the technique was used.

In Morocco there is a variation which uses alternating horizontal blocks and headers (as opposed to vertical blocks). Apart from some rare exceptions it was used for public architecture. The example of Sala (Rabat) is here discussed in more detail.

The capitolium of Sala was built in the Hadrianic period. The temple is situated on the slope of a hill; it had two levels and the upper platform was supported on the southern side by a row of barrel-vaulted shops. The whole construction is in Opus Africanum with an infill of small irregular blocks. Depending on their position, the piers take on different functions: 1) bonds between perpendicular walls; 2) reinforcement of the walling, as is evident in the massive terrace wall behind the shops; 3) channelling loads coming from the upper level of the structure, in the case of piers positioned in the centre of the walls between the shops.

The pier positions were accurately calculated, and to clarify this it is useful to look at how the barrel vaults were built. The shops were actually covered by a special type of vault: three voussoir arches were built and then concrete was spread onto them. In each of them there were three arches with the central arch corresponding to the pier of Opus Africanum at the centre of the dividing wall. The vaults were reinforced for two reasons: so that the extrados on the upper floor could be walked on, and because the colonnade of the third and now lacking portico of the upper piazza ran on the line of the central arches and piers of Opus Africanum.

The use of ashlar was limited to the facades of the shops, the base of the podium and also probably the facade of the temple. Opus Africanum was therefore used not only for structural reasons but also to keep costs down.

Conclusions and observations on the socio-economic impact of the project:

By analysing the range of cultural traditions, within which Opus Africanum had to fit, it was possible to outline the factors which underlie our interpretations of ancient building techniques. Indeed, a detailed analysis of the techniques can allow a proper consideration of their traditions – Hellenistic, Punic, African, Italian-Roman – reducing the risk of these definitions being just abstract and general categories. Through the analysis of some monuments, of a closer reading of the process of construction, and of the many local adaptations, it could be shown how the diverse cultures of the North African region interacted to create a distinctive architectural language.

This project was carried out by combining different methodologies for the study of building techniques. Thanks to this, the project has an indirect socio-economic impact. In fact, a broader understanding of the built environment based on the study of building materials, techniques and ancient construction technology by means of up-to-date methods will play an important role not only for the circulation of new ideas on ancient architecture in the academic sphere, but will also have an effect on implementation of cultural heritage enhancement politics.

Moreover, for the study of the use of Opus Africanum in the capitolium of Sala, unpublished documentation preserved in the colonial archives of Aix-en-Provence (Centre Camille Jullian) was examined. It will be the subject of a whole publication on colonial archaeology in Morocco and the story of a monument that was excavated more than 50 years ago will be finally told to the wider audience.

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