1 Introduction

The second half of the first century BC was a dynamic period and rich in important transformations for the Marche region. Around 49 BC, numerous municipia arose in the agrus Picenus and agrus Gallicus (i.e. that part of territory that passed under direct Roman control in the third century BC). Their creation involved the foundation of real towns with suitable urban structures and important buildings that reflected the new political-administrative situation [1]. The site of the Roman municipium of Forum Sempronii—cited by Strabo, Pliny, and Ptolemy in the context of the VI regio of Augustus—was located 2 kilometers to the east of the present-day urban centre of Fossombrone (Province of Pesaro-Urbino; Fig. 1), in the district of “S. Martino del Piano”, where numerous ancient items have been revealed during rural land works since the fifteenth century [1–3]. The built-up area developed during the middle of the second century BC on a wide plain (ca. 35 hectares made of Pliocenic marls and gravels) in the final segment of the Mataurus (Metauro) valley. It was closely connected with the “via publica Flaminia” (220 BC), an important road that favoured its expansion and determined its principal urban road order [1]. The Forum was most probably planned and built by Gaius Sempronius Graccus during the period of activity of the triumviral commission constituted following the lex Sempronia (133 BC). The presence of the Roman magistrate in the lower Metauro valley is part of the intensive reorganization of the ager publicus in the fertile coastal plain [1].

Fig. 1
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Geographic location of the Fossombrone village corresponding to the ancient town of Forum Sempronii

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Excavation and research activities carried out in all the area since 1974 by the University of Urbino have furnished significant information concerning the planning of the town (regular typology, with straight, parallel roads, and right angled crossroads) and yielded many archaeological finds—architectural elements, some Imperial portraits and statues, friezes, ornamentals stones, and volcanic flagstones imported from the Euganean Hills to pave the local stretch of the ‘Via Consolare Flaminia’ [4]—that date the abandonment of the settlement to the fifth century AD [5].

The identification of the coloured stones and marbles found at the archaeological site of Forum Sempronii was the subject of a recent paper [6]. Carystium, Taenarium, Scyreticum (from Greece), Lucullaeum, Phrygium, Troadense (from Asia Minor), and Greco Scritto (from Asia Minor or Algeria) [7–10] marbles were the coloured lithotypes identified.

The present study focuses on the determination of the provenance of the white marbles used for the sculptures from the same site and currently recovered at the Archaeological Museum “A. Vernarecci” of Fossombrone. The provenance of the white marbles was determined on the basis of both their mineralogical-petrographic features and isotopic signatures. A description of the analysed sculptures (all found at the ancient Forum Sempronii—S. Martino del Piano area) is reported below.

1.1 The studied sculptures

# FS 1—Frieze with procession of riders (Fig. 2a)

Rectangular slab with bas-reliefs of three young riders, moving right, with lance, shield, and helmet, and three figures on foot: a soldier and a camillus on the left and a character checking the mouth of a horse on the right. The scene has been recognized [11] as depicting a moment of a ludus iuvenalis, the exhibition of the members of the collegia iuvenum, which was strongly encouraged by Augustus both in the municipia and provinces. The stylistic characteristics and the hair styles enable the frieze to be dated to the early decades of the first century AD, a period in which the iuventus associations flourished all over Italy.

Fig. 2
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(a) Frieze with a procession of riders (# FS1), Archaeological Museum of Fossombrone; (b) Fragment of frieze depicting a dancing girl (# FS2), Archaeological Museum of Fossombrone; (c) Portrait head of Caligula (# FS3), Archaeological Museum of Fossombrone; (d) Portrait head of Hadrian (# FS5), Archaeological Museum of Fossombrone

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# FS 2—Frieze depicting a dancing girl (Fig. 2b)

Within the frame, formed by a fillet slightly in relief, a bas-relief depicts a dancing female figure moving energetically toward right. Due to the movement, the figure discloses the himation, while the lower part of the chiton widens in ample folds. It follows another similar figure, which is only partially preserved. The frieze is closed to the right by a candelabra, which is repeated after the edge. Its dimensions as well as formal and stylistic characteristics suggest that the frieze is related to the previous item (#FS1), with which it also shares the chronology (Gori, personal communication).

# FS 3—Portrait head of Caligula (Fig. 2c)

Emperor’s portrait with crown of oak. The face is slightly rotated toward the left and it features deep-set eyes, small lips, and a relatively pointed chin. The hairstyle has a series of small, regular curved locks on the forehead, which are covered by the mass of the crown “ob cives servatos”. Some traces of red colour are preserved locally. Most probably, this type of portrait dates from around AD 38, when the senate designated Caligula as the pater patriae and saviour of the citizens [5, 12–14].

# FS4—Imperial torso with lorica

Statue of standing figure. The right leg, forward and flexed, is at rest, while the left leg bears the weight of the body and rests on a tree trunk. The figure is wearing a short tunic under armour, with three rows of pendants decorated with animal protoma, heads of divinity and phytomorphic themes. The breastplate shows a typical sacrificial scene with nikai burning incense on a thymiaterion.

The decorative themes and the stylistic features of the statue indicate an urban production dating back to the early decades of the second century AD (Trajan or Hadrian), which was surely influenced by models elaborated during the Flavian age [12, 15, and references therein].

# FS 5—Portrait head of Hadrian (Fig. 2d)

The face, under a ruffled crown of locks, is weighed down by an incipient fatness; the slight wrinkling of the forehead and two folds at the base of the nose create a thoughtful and intense expression. The portrait depicts the emperor Hadrian in his late maturity. Following the classical classification of Hadrian’s portraits proposed by [16], it belongs to the well-known typology called “Emperors 32”. Portraits of this type are usually associated with the identification of the emperor as pater patriae (AD 128) and with the almost contemporaneous celebration of the decennalia [17–19].

# FS 6—Female statue rich in archaisms

The statue depicts a young girl dressed in chiton and short himation fastened on the right shoulder and crossing the breast diagonally. The weight of the statue is borne on the right leg while the left, which is visible between the folds of the chiton, is slightly forward. The left arm was lowered to lift the hem of the dress, the right, which is stretched forward, held something. The meaning and function of this statue are difficult to establish because of the limited amount of identifiable attributes; nevertheless, images like this one were frequently used on coins to represent Spes. The statue replicates the Greek model elaborated at the end of the archaic period in relation to the so-called korai and it is consistent with the style, rich in archaisms, which characterized the Roman world from Augustan to Antonine times; in this case stylistic and executive features suggest the second half of the second century AD (Gori, personal communication).

# FS 7—Portrait of women

Female head just a little smaller than life-size. It is characterized by a “turban” hair style that appears for the first time on a coin of Galeria Valeria, the daughter of Diocletian, and which became relatively common in the Constantinian age. The portrait presents a persistent symmetry and a notable formal abstractionism, and it is evidently the work of an artist open to oriental influence [19–22].

#FS 8—Winged lion

A cymatium fragment of a funeral altar with a depiction of a winged lion sitting on its back legs with front legs erect; the outspread wings support a volute with a central floral theme. The representation belongs to the usual scheme of this subject, which has both symbolic and decorative values, and it dates from the first–second century AD (Gori, personal communication).

2 Sampling and analytical techniques

Small flakes (≅1×1×0.5 cm) were detached with a sharp chisel and hammer in hidden areas of these eight white marble sculptures coming from different excavation areas investigated at different periods (Table 1). Each sample yielded a thin section for the petrographic investigation; another part of each sample was finely ground and subjected to C and O isotopic measurements and X-ray powder diffraction (XRPD) analyses.

Table 1 Chronological context, mineralogical and petrographic features, and isotopic signatures of the marble artefacts and their probable quarry sources. Qtz: quartz; Ms: muscovite; Gr: graphite; Ore min.: ore minerals; Pl: plagioclases; Ep: epidotes; Dol∗: dolomite, detected by X-Ray diffraction analysis; MGS: maximum grain size; He: heteroblastic; Ho: homeoblastic; +++: very abundant; ++: abundant; +: present; ±: traces
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The thin sections were observed under a polarizing microscope to describe and estimate the most significant petrographic features useful for differentiating the ancient white marbles and widely used in archaeometric studies (see, for example [23–27]): fabric, maximum grain-size (MGS), calcite boundary shapes, occurrence and distribution of accessory minerals. The petrographic data obtained were compared with those reported in the literature and with reference samples taken from ancient quarries (the LAMA Collection, Venice).

The XRPD analysis was conducted with a PANalytical EMPYREAN diffractometer (Cu-K α –Ni filtered, obtained at 40 kV and 40 mA) to evaluate the presence of dolomite into marbles (detection limit: 3 %).

In regard to the isotopic analyses, following the procedure suggested by [28], CO2 was extracted from calcite by reaction with H3PO4 100 % at 25 °C using a high-vacuum line; the relative abundances of the C and O isotopic ratios were determined by a Finnigan Mat Delta E mass spectrometer at the Bureau de Récherche Géologique et Minières of Orléans (France). The values of isotopic composition are expressed in terms of δ 13C and δ 18O, in ‰, relative to the international reference standard PDB [29], and then plotted in the updated diagrams proposed by [25] with supplementary data from [26, 27].

3 Results and discussion

Mineralogical and petrographic features as well as isotopic signatures of all samples are shown in Table 1 and Figs. 3 and 4. The results are summarized as follows.

Fig. 3
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The isotopic signatures of the white marbles sculptures of Forum Sempronii (isotopic fields from [25] with supplementary data after [26] and [27]). (a) Fine grained marbles; (b) Medium- to coarse-grained marbles. C: Carrara; Ca: Campiglia (Ca-1, white variety); D: Dokimion, Afyon; Pe: Penteli; Ts: Tinos; Pa: Paros (Pa-1, Stefani; Pa-2, Lakkoi; Pa-3, Karavos); Aph: Aphrodisias; Pr1-2: Proconnesos, Marmara (Pr-1, main marble; Pr-2, marble from Camlik area); N: Naxos; T: Thasos (T1-2: Aliki-Fanari; T-3, Cape Vathy)

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Fig. 4
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Microphotogaphs of thin sections of some of the studied sculptures (crossed Nicols; long side length is 1.6 mm). (a): Carrara marble (# FS7) showing a homeoblastic fabric with triple points; (b): Pentelic marble (# FS3) showing a detail of a heteroblastic fabric formed by calcite crystals with curved boundaries and potassic mica; (c): Thasian dolomitic marble (# FS5). It shows a heteroblastic fabric formed by calcite crystals, also including epidotes, with typical sutured-to-embayed boundaries

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Samples FS1-FS2-FS4-FS7-FS8

All these sculptures are made of a fine-grained (<2 mm) pure white marble characterised by a homeoblastic (FS1-FS2-FS7) or slightly heteroblastic (FS4-FS8) fabric, formed by a polygonal mosaic of small to very small calcite crystals somewhere forming triple points (FS7-FS8; Fig. 4a). The calcite crystals usually show straight-to-curved boundary shapes and their maximum grain sizes range from 0.56 (FS4) to 0.85 mm (FS7). The most frequent accessory phases are graphite, in powdery particles mostly dispersed inside the calcite crystals, white mica in small needles (FS1-FS4-FS8), epidote (FS2-FS7), opaque minerals, and rare plagioclase (FS4). These mineralogical and petrographic features correspond to those known for the marble from Luna (Carrara) ([25] and references therein). The isotopic values are conclusive, confirming this provenance (Table 1 and Fig. 3a).

Sample FS3

The fine-grained, brilliant white marble of the “Portrait of Caligula” shows a heteroblastic fabric characterised by a mosaic of small calcite crystals with a slight preferred orientation (lineation). The calcite crystals show curved, sometimes embayed, boundary shapes (Fig. 4b) and their MGS is 1.05 mm. The accessory minerals are graphite in very small particles, white mica, occasionally forming little trains, and quartz in tiny crystals. Petrographic characteristics compare well with available data of Pentelic marble [25, 30]. An origin from Mount Pentelikon (Athens, Greece) is also clearly in agreement with the isotopic data (δ 13C: +3 ‰; δ 18O: −4 ‰) due to the unequivocal position of the sample on the δ 13C vs. δ 18O diagram of Fig. 3a.

Samples FS5–FS6

The white medium-grained marbles used for the “portrait of Hadrian” (FS5) and the “female statue” (FS6) show very similar petrographic features. In fact, they contain an abundant amount of dolomite, clearly prevailing on the calcite content in the XRPD spectra, and show (i) a heteroblastic fabric (Fig. 4c) with an MGS of 2.05 mm and 2.15 mm in the samples FS5 and FS6, respectively; (ii) crystal boundaries of a sutured-to-embayed shape; (iii) a typical deformation in the trace of polysynthetic twinning. Epidote (FS5), graphite and rare opaques (FS6) are the accessory minerals. All these features indicate a Thasian origin from the district of Vathy, located in the northeastern sector of the island [25, 31–33]. The isotopic signatures of the samples are apparently not completely diagnostic (Fig. 3b). In fact, the sample FS5 falls close to but outside of the Thasian (T-3) field, within the Naxian domain; the sample FS6 falls within the field of Thasos-3 (T-3), with an almost partial overlapping of the Proconnesian field (Pr-1). In this uncertain frame, a conclusive remark is represented by the much coarser grain size of the Naxian marbles [24, 25] with respect to those of our samples and by the classical mortar-like fabric of the Proconnesian marble [25], which does not characterise the sample FS6. Considering these textural differences, the origin for samples FS5 and FS6 is unequivocally Thasos-3, corresponding to the district of Vathy. Finally, the position of the sample FS5 suggests a small enlargement of the Thasian T-3 isotopic field.

4 Final remarks

The mineralogical-petrographic and isotopic analyses indicate that for the statuary and sculptures so far discovered in the Roman municipium of Forum Sempronii, Pentelic (Greece), Thasian (Greece), and Lunense (Carrara, Italy) white marbles were used.

It should be noted that the samples (FS1–FS2) from the two friezes dated from the early decades of the first century AD are made up of marble from Carrara district. In particular, their isotopic signatures are very similar and imply that they come from the same locus in the quarrying area (or even from the same block of marble), supporting a likely single commission and workshop as hypothesised by archaeologists (Gori, personal communication).

All the analyzed items (with the single exception of FS7—“statue of woman”) refer to the most flourishing period of the town (mainly first-second centuries AD), which saw considerable urban transformation and decoration [1] and also included a sculpture programme for public places and limited importation of coloured marbles [6]. As pointed out by some inscriptions related to other stone objects and statues of that period (e.g. statua decreta…iam comparata advehitur, C.I.L. XI 6123; [15]), now housed in the local archaeological museum, almost certainly the marble sculptures discovered in Forum Sempronii were commissioned directly in Rome and imported as finished artefacts. Effectively, the local workshops were qualified to work local limestones and sandstones (especially belonging to the “Calcare Massiccio” and “Marnosa Arenacea” geological formations, respectively), but they did not have the ability to complete the work in marble. From the results so far obtained, it seems that the earliest imported sculptures (early decades of the first century AD) were made of marble from Carrara district and Mt. Pentelikon (a stone rich in symbolic value and prestige). In the second century AD, the presence of the Lunense marble persists (early decades), but statues made of Greek Thasian marble were also commissioned (second half of the century). Finally, in the fourth century AD—when the activity and the economic condition of the city were declining and the production of the so-called “cultured” statuary is appreciably replaced with small-to-average size items addressed essentially to the late-ancient potentiores—the “autochthonous” marble from Carrara district remains the only one.