1. Introduction
The Chiaravalle Cross, a mediaeval jewelry masterpiece, is a processional cross made of silver and gold laminas, combined to filigrees, gemstones, cameos and red jasper plates. This piece from the Museo del Duomo’s collection was restored during “Restituzioni 2016” [
1]. Ancient documents on the Chiaravalle Cross are rare,
1 inaccurate and contradictory. In the late 19th century other scholars added different conflicting interpretations.
2 Finally, in the twentieth century, having cleared the field of erroneous attributions to earlier times, the Cross was recognized as a work of Venetian art that could be inserted into a large group of masterpieces of goldsmithery, dated to the end of the 13th century. In particular, the Chiaravalle Cross is acknowledged as a work of art executed in Venice at the end of the 13th century [
6], likely commissioned by the archbishop of Milan Ottone Visconti (who died in 1295). However, some decorations (like one angel statue and some gems) seem to have been produced after this period.
The chronological attribution was one of the important research aspects, but the rich decoration and complex iconography of this piece of art deserved further insights. The production and manufacturing techniques of a complex piece of art like the Chiaravalle Cross involve much expertise, and in order to derive a comprehensive knowledge many different multidisciplinary skills are required. Metalworking and decoration techniques used in the production of jewelry can, for example, give indications on the period of fabrication, while the choice for different materials suggests information on the provenance of an ancient object of unknown origin. Several scientific investigations were applied during the restoration of the Cross, including in situ and laboratory measurements, addressing different materials. The analyses, part of a multidisciplinary protocol, completely characterized the gemstones adorning the Cross, the cameos, the gold, silver, jasper plates and glass parts, to derive indications on materials provenance, conservation status and authenticity issues [
7].
The Cross was under restoration at Franco Blumer laboratory (Bergamo, Italy): this was an invaluable circumstance to undertake selected and focused scientific analyses. A multidisciplinary study was carried out, including neutron and nuclear analysis, gemological analysis, Raman characterization, X-ray fluorescence (XRF) and scanning electron microscope (SEM) measurements, whose main results will be discussed below.
This project gathered many different experts and all the contributors shared their findings in a very fruitful collaboration, resulting in an in-depth knowledge of the masterpiece in order to clarify several open questions as stated above.
2. Materials and Methods
The Cross is a precious jewelry masterpiece of rather complex construction, as can be seen from the pictures (see
Figure 1). The core of the Cross is a walnut wooden structure covered on the front with 12 plates of red jasper, on which the main figures of the crucified Christ, of the Virgin and of Saint John the Evangelist were applied, all made of lost wax-cast silver, finely chiselled, gilded with mercury amalgam. The filigree in gilded silver entirely surrounds the outline and it is adorned with jeweled settings (cameos, transparent and opaque colored gemstones). The perimeter thickness of the Cross is entirely covered by a smooth and shiny silver sheet made in a single segment, fixed by nails. On the back there are embossed, chiselled and gilded silver laminae, covered with natural quartz lenses, ialine variety (commonly known as “rock crystal”). Many other details, visible before and after the restoration, are available in [
1] as well as the restoration report by F. Blumer (see contributions by F. Blumer in [
1], and [
7], pp. 237–271).
The restorer had access to the inner part of the artefact, since a few parts were dismounted to perform a better cleaning. In some cases, few fragments were available for the analysis. The use of non-destructive and non-invasive analysis was very important for the whole study. However, for dating analyses some very small fragments were used. We will focus on three different types of analysis: (a) radiocarbon dating, (b) gemological study and (c) SEM measurements. Further analyses were carried out, mainly neutron-based techniques, whose details can be found elsewhere [
8,
9]. Concerning the golden filigree, this deserves a particular attention, since the materials used and the techniques of fabrication were very peculiar. Therefore, an in-depth study performed both by SEM and particle-induced X- and gamma-ray emission (PIXE and PIGE) is still ongoing and it will be the subject of a presentation at the forthcoming IBA2019 congress (Antibes, October 2019 [
10]).
2.1. Radiocarbon Dating
14C-AMS (radiocarbon accelerator mass spectrometry) dating was performed at the CIRCE laboratory (Caserta, Italy). Three samples were made available for the analysis: two pieces of wood (few grams, from two different locations on the Cross) and some stucco coming from the rear of a gem setting. In particular, we coded samples from the wood as RC442 (coming from the node of the Cross, at its bottom part, FZ region-see
Figure 1a,b) and RC443 (coming from the central part of the Cross), while RC439 specimen was the stucco one (taken from the rear of jasper in R1, see
Figure 1b). The samples were prepared in the CUDAM laboratory (University of Milano-Bicocca).
The measurements have derived the ratios C14/C12 and C13/ C12 for carbon isotopes, extracted by chemical and physical treatments from the specimens. From these data, the percentage of modern carbon (pMC) has been derived, and the conventional age (tRC) calculated. This age has been calibrated from OxCal v.4, calibration curve INTCAL04 [
11] to obtain the calendar age.
2.2. Gemological Study
To identify and characterize all the gemstones set in the Chiaravalle Cross, we used the standard gemological tools [
12]. The gems dimensions, when possible, have been measured using a digital dimensiometer, Presidium model, with a tolerance of ±0.01 mm. The setting prevented the direct detection of gemstones weights by the scale method. The estimated carat weights were obtained on the basis of the dimensions and known average density. On the unremovable stones, the observations have been performed by the achromatic aplanatic 10× triplet gemological loop; while the partially unset stones allowed the dark field microscope analysis. We used a Leica S6E optical microscope with 16× ocular and 0.63–4.0× magnification.
The species and the variety identification were obtained crossing the refractometer analysis, where applicable, with the optical observation. The standard gemological tests were completed by fluorescence analysis exciting the gemstones with long wave (365 nm) and short wave (354 nm) light sources.
For the main 17 gemstones set in the front of Chiaravalle Cross, the limitations imposed by the mounting in the characterization performed by the standard methods were undertaken by applying a portable spectrometer of new generation: XRAMAN, made by XGlab company [
13]. This instrument is able to perform in situ, fast and non-contact combined elemental and molecular analyses, by the complementary EDXRF (energy-dispersive X-ray fluorescence) and Raman techniques. The XRF system is fully integrated into a compact detection head and is equipped with a 20 mm
2 fast silicon drift detector (SDD) and digital readout electronics. The exciting source is a high-efficiency and compact 50 kV X-ray tube, 200 µA (max 10 W) with Rh anode. The X-ray tube is coupled with three automatically software-selectable different collimators between 0.5 mm and 2 mm and a set of X-ray filters to improve low detection limit capability in special applications. Usual measuring conditions for XRF were the following: X-ray tube 50 kV, 80 µA current, and acquisition time 30 s. Regarding the Raman system, the instrument is equipped with a cooled CCD detector with 6–7 cm
−1 of resolution and 100–4000 cm
−1 analytical range. The excitation source is a compact 785 nm stabilized laser source with output power regulable from 0 to 500 mW. Common measuring conditions were power 400 mW and acquisition time 180 s. However, the length of the measurements (for both XRF and Raman spectra) could be different in order to acquire a good statistic.
Thanks to the XRAMAN alignment system, made by a couple of lasers (axial and focal) and a micro-camera, able to observe a 2 × 2 cm2 area at 10× magnification, the tests were performed at 1 cm distance, without any direct contact between the gemstones and the instrument. The analyzed area corresponds to about 1 mm diameter with perfect coincidence for both techniques.
2.3. Scanning Electron Microscope (SEM) Measurements
A scanning electron microscope (SEM) equipped with an energy-dispersion (EDS) detector was used in a non-invasive approach, i.e., without any treatment of the artifact, to study some elements of the Cross (like glass, filigrees, collets, embossed sheet and figures in relief). Their sizes, though large, could be easily housed in the sample chamber.
The instrument used is a TESCAN Mira XMU series, coupled with an EDAX system in EDS, and the operating conditions for the collection of images and microanalysis were as follows: beam acceleration voltage: 20 KV; beam current: 40 mA; working distance: 15.8 mm; counting for microanalysis: 100 s; correction factor for microanalysis: ZAF; analysis area 100 × 100 µm2.
The study has returned images in high resolution, both in secondary electrons (SE) and in back-scattered electrons (BSE) of the surfaces, of the texture of the material, of the relations (microstructures) between the different metal alloys and crystalline and/or amorphous phases.
EDS measurements, when conducted on morphological surfaces, should be considered purely indicative of the chemical composition of the sample, since it is possible that, during the measurement, a direct proportionality between the signal output from the sample and the counts to the detector is not guaranteed, due to the lack of flatness of the surface.
4. Discussion
The Chiaravalle Cross is a processional cross that came from the abbey of Chiaravalle (close to Milan, Italy). Several non-destructive (or micro-invasive) techniques were used, tailored to this case study. Wood, stucco, gems, cameos, and metals were deeply investigated and many interesting considerations can be derived.
Lacking any indication on the date of the production of this artefact, since no documentation has been found until now, we obtained by radiocarbon dating a date in agreement with the current attribution of this masterpiece to the late 13th century, since for the wooden part of the Cross we derived an age not earlier than 1165–1265 AD. Concerning the modern date of the stucco behind a jasper gem, this could be related to a different chronological placement but also to a later restoration improving the adhesion of the gem setting. Yet, two previous restorations were recorded: the first in 1539 when the Cross was found again after having been stolen in 1521, and the second in the 17th century [
7] (p. 240). In addition, F. Blumer reported a recent restoration too, finding a small paper with the inscription “Agostino Figini orafo—Milano, restaurò anno 1950” (literally meaning “Agostino Figini goldsmith—Milan, did restore in 1950 AD”, see the restoration report in [
1] and Blumer, F. “Il restauro” in [
7], pp. 239, 262–263).
Regarding the characterization of the gems on the Chiaravalle Cross, it is possible to focus the attention on several discussion points. First of all, on the basis of the statistical data reported in
Table 1,
Table 2 and
Table 3 it has been possible to observe differences in the number, quality, dimensions and color of the gemstones between the front and rear of the Cross. The dominant red color is the first sensation observing the front of the Cross, hand in hand with the preciousness of the gem materials.
Considering all the differences found between the front and the rear, it is possible to deduce a deep planning ability beyond this magnificent opera. The gemstones come from several parts of the world, as it was known during the 13th century. Due to that the gemstones set could be considered as a symbol of the commercial links existing from Italy, Venice particularly, to the Asian regions. A proof for these facts is the presence of the four trapiche sapphires, probably from Burma or Ceylon.
The four trapiche sapphires open an interesting window regarding where are they from and how old they are. In the modern market the trapiche corundum, rubies and especially sapphires, are very rare gemstones. Regarding the geographic origin, the first trapiche rubies arrived on the market from MongHsu mines, Myanmar (Burma), around 1995 and subsequently from the Vietnam mines, as described by Schmetzer, et al. [
23,
24] and Garnier, et al. [
25]. About sapphires, the discovery of the trapiche variety is a recent fact, too and the rare ones found are typically from modern and magmatic geographic origin, such as Madagascar, Australia, Montana mines [
20]. Due to that and to the limitations imposed by the mounting, using only the gemological applicable methods it is not possible to give a unique answer to these questions, but crossing all the other knowledge obtained on the Chiaravalle Cross during this work, several hypotheses could be formulated. On the basis of the data obtained analyzing the metal and particularly the settings, it is possible to define them as original and untouched; however, since we could not date the fabrication age of all the decorative parts, we cannot be sure about the chronological arrangement of the four trapiche sapphires. However, the cut and the inclusion pattern observed on them suggest that the trapiche sapphire tablets could have been cut from the same, big, hexagonal “barrel”-shaped rough stone. Therefore, considering the edge and all the gemological characteristics, the four trapiche sapphires could come from only two geographic areas: Burma or Ceylon, actually known as Myanmar and Sri Lanka, respectively.
Regarding the cameos, on the basis of the gemological analyses, it is possible to identify three type of materials: sardonyx, chalcedony and artificial glass, divided into two groups (see results). Both groups are older than the creation of the Cross, and reused on it because felt as “ancient” and then “precious”—a phenomenon well known since the Early Middle Ages
3—as demonstrated by the fact that a cameo from group (a), lacking its upper half, has been nonetheless maintained on the Cross, even if integrated with colophony, as stated after autoptical examination and experimental remake by the conservator F. Blumer (see his report in [
1]) and moved from the front to the rear side of the precious artifact.
The natural untreated lapis-lazuli gemstone is a complex and variable mixture of minerals, commonly constituted by lazurite and sodalite with minor amount of calcite, nosean, hauyna and pyrite [
28]. No bands corresponding to these typically associated mineral phases were observed in the Raman analysis of FSQ1. Moreover, the chemical spectrum of FSQ1 presents barium and zirconium traces not reported for natural lapis-lazuli chemicals [
17]; a chemical composition as reported in
Figure 4b is compatible with a mixture used to create a gemstone imitation [
29]. The presence of the imitation of lapis on the Cross is not a surprise, because the use of lapis-lazuli simulants or dyed are common and well known in the gemstones on the market, also from ancient ages [
30]. Due to that, the presence of lapis imitation is easily explicable thanks to the observations made on the metal setting during the restauration process; indeed, Blumer [
7] indicates a settings alteration corresponding to FSQ1 and FDQ1 gemstones. The presence of this modern lapis could be compatible with the restoration in 1950 documented by Blumer (see the restoration report in [
1] and Blumer, F. “Il restauro” in [
7], pp. 239, 262–263).
The red jasper on which the crucifix has been fixed was macroscopically identified as likely coming from Giuliana, province of Palermo, Sicily (see [
7], pp. 207–209). This source of beautiful green, yellow and red jaspers was known since the Hellenistic period and much exploited in Roman and Baroque times when it was exported throughout the Italian peninsula [
31]. This information, together with the connection to the atelier working at the Norman court of the Hautevilles in Palermo (for some cameos provenance) opens new scenarios for the Cross production, linking the acknowledged golden filigree fabrication in Venice to some peculiar materials provenance from Sicily.
The analytical data collected on the transparent glass-gems, although only indicative of their chemical composition, indicate the main components of the glass recipe: SiO
2 as a vitrified, mixed-alkali as a flux, CaO as a stabilizer. The yellow color of two of the three gems is due to the presence of FeO; the colorless glass-gem has been obtained by adding manganese to the vitrifiable mixture [
32].
We cannot forget that each ancient artefact is the testimony of a series of gestures and actions (
chaine operatoire), whose sequence often comes to us incomplete and devoid of operational details sometimes fundamental to the success of the object. The ancient artefact also comes to us with the overprint of its entire “life path”, including the use, ordinary maintenance and extraordinary maintenance [
33]. The presence of lead (
Table 4) found in the glass could be also attributed to the polishing phases of the fake gemstone, as reported in technical cookbooks [
34,
35] certainly known in the early Middle Ages and, therefore, may not be a component of the glass recipe.
Finally, concerning metal objects we observed, they are made of an alloy of silver, containing copper and gold as minor elements. The proportions between these elements seem to show a correlation with the technique of realization of the single decorative elements (
Figure 8). A striking result followed the observations and measurements on the two central angels’ statues, confirming the hypothesis that the realization of the two artefacts could have taken place in different times and probably in different workshops.
Other metallic processional crosses were characterized in the literature [
36,
37], but this is the first really multidisciplinary study, addressing different materials and fabrication techniques, published as a journal paper. The study of a complex masterpiece like the Chiaravalle Cross has taken advantage from the exchange of knowledge and skills in the field of gemology, gliptic, restoration, art history, physics, and archaeometallurgy.