1Among personal female objects from archaeological contexts, one of the most frequent is the hair coil, linked to a particular hair-style that seems to be a constant throughout the entire Iron Age (10th-9th century BC) in the Tyrrhenian area. This style was characterized by two plaits of hair on each side of the head, fixed by spirals made of a simple wire wrapped more or less tightly, with diameters ranging from 2-2.5 to 4 cm. This type of spiral appears in the tombs of the first Iron Age (11th-10th century BC), both in the necropolises of the Latium area, as well as in the Faliscan and Veio areas. The spirals are probably a translation in metal of strings of organic fibres but, in any case, it is not possible to separate their decorative function from the more functional properties of the spring spiral. Specific metal alloys had to be used, and a particular metal treatment had to be chosen as a function of the ‘elastic’ effect of the ornamental object. In general, the spirals were composed of two or more turns of double wires with the terminal part often flat and undulate, with a diameter varying from 2 to 4 cm. In the richest tombs of the 8th and 7th centuries BC, representative of a more differentiated society, these simple coils evolved into more precious objects through the use of noble metals finely decorated by elements in filigree or gold lamina. These last two variants characterize the jewels of the orientalising period (Formigli, 1976).
2The shape of the spirals varies: some exemplars present a double torsion, while others are decorated all along the wire; however, in all cases, they represent a concentration of style, jewellery, technology and science containing important diagnostic information. Previously, two exceptional hair coils were found by Alessandro Bedini in a princely tomb (101) of the Laurentina Necropolis in Castel di Decima (Rome) and dated to the end of the 7th century BC (Bedini, 1977; 1985). In the 7th century BC, found always only in princely tombs, the spirals have a small diameter of about 1 cm, perhaps indicative of a new fashion of hair style, with the two terminals decorated by a horizontal and undulating motif in filigree between two series of horizontal parallel threads, as visible in the exemplar from tomb 74 described in the present work. This object can be compared with other hair coils of silver covered by electrum of the ‘pseudo-camera’ tomb (133) of the same necropolis (Bedini and Cassotta, 2006). This new typology is also present in the Faliscan area (Narce) and in Etruria (Marsiliana and Vetulonia), and is probably of Greek inspiration, as the exemplars from Amendolara (Cosenza) indicate (De la Genière, 1973).
3The analytical study by non-destructive methods is aimed at gathering information on the ancient goldsmiths’ degree of knowledge and ability to optimize the elastic properties of metals through specific treatments. Scanning electron microscopy (SEM) and X-ray energy dispersive microanalysis (EDS) have been employed for the purpose of determining the micro-morphology of the metal surface and the chemical composition of the areas that present some details of interest for the technological definition of processes. From the interpretation of the morphological and analytical data, it is possible to derive information on micro-diagnostic markers for the characterization of the spring metal treatment, to be compared to similar objects.
4The use of scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) has been employed to investigate both the technological peculiarity of the item, as well as the possible transformation of the precious metal into an elastic spring.
5In the year 1678, the physicist Robert Hooke formulated a method useful for the description of the macroscopic action of a spring, by studying the mechanical behaviour of a metallic wire wrapped in a cylindrical-shaped spiral operating by compression or strength. In both cases, the microscopic effect of the elongation/compression motion is the stretching of the intermolecular bonds, within the limits of elastic deformation, after which the atomic bonds rearrange, causing irreversible deformation. The archaeological evidence indicates that the existence of springs dates back to the Bronze and Copper Ages. Examples of metal artefacts functioning as springs or containing spring actions are: fibulae widely used in the Mediterranean area, certain fastenings of necklaces, bracelets, earrings, and diverse typologies of hair binders, the subject of the present work.The origin of the spring is beyond the scope of this article, but we can start analyzing some peculiar aspects of these items, useful for the identification of the intentional creation of an elastic system. A metal presents a crystalline structure constituted by grains of different shape and dimension, dependant on a series of factors, including mechanical and thermal stresses. Particular working processes are necessary in order to exploit the cold plastic properties of any metallic material. In fact, only by employing cold-working processes, such as hammering or drawing, is it possible to directionally elongate the grains. Not all metals are suitable for making springs, and specific compositions of alloys, as well as particular mechanical procedures, are necessary to enhance characteristics of elasticity. The impossibility of any metallographic sampling of the precious object only allowed an analytical study based on the morphology of the surface and on the microstructure and chemical composition of small areas. Anticipating more detailed studies, the information derived from the understanding of the working process suggests that spirals are not to be considered solely as simple hair decorations, but mainly as a technological achievement.
6Laurentina Necropolis tomb 133 – The analyzed object is a silver strip spiral (ø 0.750 mm) covered by an electrum sheaf, with a high Ag content, circa 40 wt%, as revealed by EDS analysis. An electrum wire, produced by torsion of a small lamina with a composition similar to the lamina of the spiral body, makes up the filigree decoration on the terminal parts (Fig. 1). Beyond the possible economic reasons for the choice of a two body system consisting of an inner strip of silver mechanically enclosed in a lamina of electrum, it is suggested here that the artefact’s system was also developed for its elastic properties and general wear resistance to further unintentional cold plastic deformation. Longitudinal lines all along the strip’s length are observable on the internal side of the lamina, suggesting the use of a burnishing tool to further cold harden the electrum lamina and ‘iron out’ eventual buckles and kinks that tend to form on the inside surface and lateral inner edges while forming the spiral.
7The formation of Ag corrosion compounds, such as AgCl and Ag2S, has been detected by SEM-EDS analysis.The natural formation of Ag corrosion products on the internal wire is the likely cause of electrum lamina detachments. Furthermore, the preferential corrosion of the silver alloyed in the electrum produces a black patina on the gilded coating (Carraro et al., 2005). The irreversibility of these physical-chemical processes involving the metal microstructure allows their consideration as diagnostics in authenticity tests.
8Concerning the soldering process employed to fix the filigree elements, EDS analysis revealed the presence of silver soldering, where the Ag content was 100%. The use of silver soldering is typical for filigree work from the Classic and Imperial Roman periods (Ferro et al., 2008), owing to its high degree of wettability, leaving intact the decoration’s tridimensionality. In this case, in which the filigree was made with an electrum alloy with a low melting point, the excessive heating caused the loss of relief definition.
9Laurentina Necropolis tomb 74 – A simple gold wire forms a regular spiral with the terminals decorated with filigree. The filigree wire (ø 0.2 mm) was produced by torsion of a gold strip (Fig. 2). The precious metal composition of the coil and of its decorations shows a high gold content (Au 94 wt%), with silver and copper as the other major elements, a composition typical for precious objects from the Roman period.
Figure 2: Spiral in gold from Laurentina Necropolis tomb 74.
Figure 2 : Spirale en or provenant de la tombe 77 de la Nécropole Laurentina.
10The use of high gold is not optimal for the production of an ‘elastic’ metal, despite cold work hardening procedures. Longitudinal and parallel traces all along the wire’s length are well visible also in this coil, suggesting the use of a burnishing tool employed to further surface harden the wire while creating a polished finish.
11From the verified invariance of the spring wire’s diameter (0.4 mm), it is possible to assume the use of a procedure employing a calibration system. The filigree soldering was carried out with silver, found in the form of snippets, as observed in the false colour EDS map, with silver in red, copperin green and gold in blue (Fig. 3). The fact that the choice of silver as a solder is verified once more suggests an improvement of the scientific knowledge derived from empirical experience: in fact, the wettability of silver is higher than that of most brazing alloys, due to its superficial tension properties.
Figure 3: EDS false colour X-ray map of the filigree soldering area in the spirals from tomb 74: Ag – red, Au – blue, Cu – green.
Figure 3 : Cartographie de rayons X en fausses couleurs de la soudure du filigrane sur la spirale de la tombe 74: Ag-rouge, Au-bleu, Cu-vert.
12With respect to the spring from tomb 133, the filigree appears more detailed, because the heat induced to melt silver in this case had no influence on the nearly pure gold, with a much higher melting point.
13The investigation of the two hair coils is part of a more complex study of the technology applied in their production. The results obtained on the hair coils from Via Laurentina, compared with those obtained on other similar items originating from Etruscan areas (Carraro and Ferro, 2008), allows the identification of different fabrication procedures; however, all of them were carried out according to common rules and design. Further analyses on such samples, reproduced by experimental archaeology, will be necessary in order to identify distinctive elements for this type of common, yet precious, objects.
The authors are grateful to David Loepp for the editing and to Vania Virgili for her collaboration.