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Sciences at Ca’ Foscari Spectroscopic analysis of wall paintings from Rocca Manerba Gian Antonio Mazzocchin, Alessandra De Lorenzi Pezzolo, Alvise Vianello

Overview of the Rocca Manerba stronghold and its location on the Garda lakeshore.

Spectroscopic analysis of wall paintings from Rocca Manerba Gian Antonio Mazzocchina, Alessandra De Lorenzi Pezzoloa*, Alvise Vianellob

a

* b

Department of Molecular Sciences and Nanosystems, Ca’ Foscari, University of Venice, Dorsoduro 2137, 30123 Venezia, Italy. Institute for the Dynamics of Environmental Processes, CNR, Venice, Italy. e-mail: [email protected]

Received: 2013-11-14 Accepted: 2013-12-07 Abstract: In this paper the characterization of pigments and materials from wall fragments found at Rocca Manerba (on the Garda lakeshore, Italy) is presented in order to contribute to setting this archaeological site within a historical context. Although a stylistic analysis of the paintings was almost impossible owing to the high fragmentation of the samples, analytical techniques such as Fourier Transform Infrared (FTIRS), R aman, Scanning Electron Microscopy-Energy Dispersion (SEM-EDS) Spectroscopy, and X-R ay Diffraction (XRD) showed the presence of common pigments (yellow and red ochres, green earths, calcite white, and carbon black) as well as of the precious cinnabar, mixed with hematite. Blue pigments were not found, whereas a yellow-green one containing lead and tin used since the XVth Century was found in some over-paintings. Keywords: Analysis, Pigments, Rocca Manerba, Wall Paintings, Spectroscopic Techniques, Cultural Heritage, Xth-Regio. DOI: 10.7361/SciCF-504

1. Introduction

The wall painting fragments studied in the present work were found in Rocca Manerba (see Fig. 1a), a rocky landmark located in Northern Italy, on the South-West of the Garda lakeshore, during the archaeological campaigns of 1997-1999 and 2009 directed by Prof. G.P. Brogiolo of the University of Padua. Set on top of the Monte Sommo, the Rocca (stronghold) is in ruins since 1574 when the Venetian local administrator, Giacomo Soranzo, decreed its destruction due to its use as a refuge by the Zamari bandits. Of great strategic value because of its domineering position, the place bear evidence of having been frequented starting from the Late Bronze Era (XIth-Xth Century B.C.). During Roman times (late Republican Age - early Imperial Age, Ist Century B.C.-Ist Century A.D.) it seems that a small temple devoted to Minerva was built. In the Early Middle Age (IXth-Xth Century) the fortifying of the place begun and the building activity lasted till the destruction of the stronghold ([1] and references quoted therein). As it 34

can be seen in Figure 1b where the map of the site is reported, the ruins present three defensive curtain walls, the inner one (dating to the XIIIth-XVth Century A.D.) encircling the main tower, a cistern and other structures of unknown use. Hundreds of wall fragments were found throughout the site, resulting from the collapse of the buildings or from the fillings of holes in the ground. The samples studied in the present work come from different places: US 7046 and US 7080 were found in the area between the tower, the cistern and the first wall encircling the fortress, US 7044 under the floor of the D area, and US 9012 near the tower. Generally, the wall paintings are monochrome or painted with bands or panels of different colours; only very few samples of US 7044 show vegetal decorations with branches and hastate leaves [2]. The aim of this paper, that follows a thesis work [3] and three short reports [2, 4, 5], is the characterization of the pigments and materials in order to set the pieces of this archaeological site into the historical context of the study of wall paintings of the Xth Regio (Venetia et Histria). sciences at ca’ foscari

2. Experimental

As reported also in our previous papers on Roman age building materials [3, 4, 6-17], the sampling was performed by the archaeological team, encompassing the fragment varieties and differences of colour. The dimensions of the specimens analysed, all covered by mould and rubble, ranged from two to twenty square centimetres. 2.1. Fourier Transform Infrared Spectroscopy (FTIRS)

Absorption spectra in the mid-IR region (400 - 4000 cm-1) were collected using a FT-IR Perkin Elmer Spectrum One spectrometer. Pellets of a diameter of about 13 millimeters were obtained from a few milligrams of sample diluted in IR-grade potassium bromide (Merk). Spectra were acquired with a resolution of about 2 cm-1 and thirty-two scans average. 2.2. Raman Spectroscopy

The R aman spectra were obtained using a confocal Jobin Yvon Labram R aman microscope, employing the 632.8 nm laser line and a power of about 1 mW. The detector was a Peltier-cooled 1100-pixel CCD (330 x 1100). The spectral range investigated was varied depending on the sample characteristics, but in all cases the resolution was about 4 cm-1 and the measurements were extended down to 100 cm-1. The spectra were recorded without any preliminary preparation of the samples, simply placed on a metal holder. A fter focusing on a crystal through a microscope and a CCD camera, the spectra were recorded with a variable number of scans or accumulating time according to their intrinsic intensity. 2.3. Scanning Electron Microscopy – Energy Dispersive Spectroscopy (SEM-EDS)

Figure 1. The Rocca Manerba, Northern Italy, on the Garda lakeshore: a. the ruins; b. plan of the fortress.

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SEM images were obtained through a Jeol JSM 5600 LV equipped with an Oxford Instruments 6587 EDS microanalysis detector. The images were taken under low vacuum conditions where samples did not show charging effects so that it was possible to avoid coating with a high conductance thin film. EDS microanalysis was performed to gain information on the elemental composition of the samples.

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2.4. Optical Microscopy

The samples, illuminated using a movable optical fibre system, were observed by means of a WildLeitz M8 optical stereo microscope with a 19.2× to 256× zoom. 2.5. X-Ray Diffraction (XRD)

Powder X-Ray diffraction was used to identify the different crystalline phases present in pigments and sand recovered from mortars. A Philips X’Pert vertical goniometer with Bragg-Brentano geometry, connected to a highly stabilized generator, was used for XRD analysis. Cu-Kα Ni-filtered radiation, a graphite monochromator on the diffracted beam and a proportional counter with pulse height discriminator were used. Measurements in a 5 to 60° range were taken with a step size of 0.05° and 2 seconds per point. 3. Results and discussion 3.1. Pigment analysis

In Figure 2 the images of eight wall fragments, representative of the colour varieties of most of the samples recovered at Rocca Manerba, are reported. As it can be seen, the investigated samples show a fairly restricted palette of colours, mainly red, black, yellow, green, and white [2, 5]. The sample 2a (from US 7046) is green with traces of underlying black and the 2b one (from US 7080) is red. The five specimens from US 7044 present a comparatively larger colour variety: the first (sample 2c) and the second one (sample 2d) combine red and green-yellow bands; the third (sample 2e) and fourth (sample 2f) ones are mainly yellow, the latter showing a broad white band; the last sample from US 7044 is dark gray-black in colour (sample 2g); finally, the specimen from US 9012 (sample 2h) is red and black with greenish hues. The FTIR spectrum of sample 2a is reported in Figure 3. In this spectrum the narrow bands of the terminal OH group are evident near 3550 cm-1, typical of green earths, whose presence in the sample is confirmed by the couple of bands at 1100-1072 and 984-954 cm-1 related to green earths of glauconite or celadonite [17, 18]. Absorption bands due to calcite (at 1429, 872, and 710 cm-1) can be seen in the spec36

Figure 2. Representative samples of Rocca Manerba wall paintings.

Figure. 3 FTIR spectrum of sample 2a (US 7046).

trum of Fig. 3 as well [19]. The identification of these pigments is confirmed by the corresponding EDS spectrum (not shown) where the peaks of the main constituents of green earths (iron, magnesium, aluminium, silicon, and potassium) together with those of calcium from calcium carbonate were observed. In Figure 4 the R aman spectrum of this sample is shown; the bands typical of green earths are desciences at ca’ foscari

tected at 704, 553, 466, 396, and 383 cm-1, while that of calcite can be seen at 1088 cm-1 [17, 18]. In addition, spectral features ascribable to carbon black as broad bands occurring at about 1600 and 1355 cm-1 are found [20, 21]. In Figure 5 the FTIR spectrum of sample 2b is reported. The spectrum is characterized by the presence of the bands characteristic of an ochre containing kaolinite at 3692 and 3615 cm-1; absorption bands attributable to calcite at 1429, 872, and 710 cm-1, to silicate groups near 1031 cm-1, and to hematite at about 540 cm-1 are also found [19]. These data indicate that the red pigment of this sample can be identified as a red ochre, as confirmed also by the corresponding EDS spectrum (not included), that shows the characteristic peaks of iron, aluminium, silicon, and calcium. Two samples coming from US 7044 (2c and 2d) present characteristics different from those of US 7080 (2b); although the FTIR spectrum of the red portion of samples 2c and 2d (not included) is similar to that of sample 2b, the corresponding EDS spectrum, reported in Figure 6a for sample 2d, shows the presence of mercury, together with sulphur and iron, suggesting that this red colour is mainly a cinnabar pigment. It is rather surprising that such a precious and expensive pigment [22] was used in this context, although mixed with hematite (corresponding peak at about 540 cm-1 in Figure 5). The R aman spectrum of this pigment, shown in Figure 6b, corroborates the identification of cinnabar showing its typical peaks at 345 and 257 cm-1; additional bands characteristic of hematite at 297 and 613 cm-1 and magnetite at 666 cm-1 (weak) confirm that this pigment, known to be progressively scarce starting from the Low Roman Empire [23], was here diluted with iron III oxides. The band of calcite at 1088 cm-1 can be observed in Fig. 6b as well. The FTIR spectra (not included) of the yellowgreen bands of samples 2c and 2d, overpainted on a red film, suggest the presence of a green earth; in the corresponding EDS patterns, however, besides the expected peaks corresponding to iron, magnesium, and potassium, those characteristic of lead and tin are observed. In Figure 7a a SEM image of a detail of sample 2c where these metals appear as bright crystals is shown, whereas the EDS spectrum of the yellow-green pigment can be seen in Figure 7b.

Figure. 4 Raman spectrum of sample 2a (US 7046).

Figure 5. FTIR spectrum of sample 2b (US 7080).

Figure 6. Sample 2d (US 7044): a. EDS spectrum of the red portion of the sample; b. Raman spectrum of the red portion of the sample.

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Figure 8. X-Ray diffraction spectrum of the yellow-green pigment of sample 2c (US 7044).

Figure. 7 Sample 2c (US 7044): a. SEM image of a detail of the sample; b. EDS spectrum of the yellow-green pigment.

US 7044) and 2h (from US 9012), found near the tower, show the presence of carbon black as confirmed by its disappearance when the samples are heated in an oven at about 400 °C (in the case of sample 2h in agreement with [5]). The red pigments of sample 2h are made of red ochres (in agreement with [5]), while the graygreen ones are green earths. No blue pigments, that would have conveyed important information about the execution period of the decoration [33], were found in the wall fragments [5]. Finally, no intentional use of the white employed by Romans to enhance the adhesion of the overpaintings to the substrate obtained mixing calcite with aragonite or dolomite [10] was found in the wall fragments analysed.

All these data suggest that the yellow-green pigment of samples 2c and 2d from US 7044 is a lead-tin In short, the analyses performed on the eight wall yellow known as “giallorino” used starting from the fragments showed that the decoration was based on a XVth Century [24-29]. This identification is supported restricted number of pigments as summarized in Table 1. by the corresponding X-R ay diffraction pattern, reported in Figure 8 for sample 2c, where the peaks Table 1. Pigments identified in the eight samples of Rocca of the lead tin oxide type II (PbSnO3) [30] are found Manerba wall paintings. sample yellow red ochre green calcite carbon cinnabar “giallorino” along with those of calcite and quartz, the latter ochre hearths white black possibly a residual from the preparation procedure 2a (US7046) × × × of the “giallorino” itself [30-32]. The same analyses performed on the remaining samples showed that, contrary to what found in the samples 2c and 2d from US 7044, the yellow pigments of samples 2e and 2f, also from US 7044, are not lead and tin yellows but common hydrated iron oxides. The white pigments of the specimen 2f consist only of calcite containing just natural traces of dolomite or aragonite. The black or gray areas of samples 2g (from 38

2b (US7080)

×

2c (US7044)

×

×

×

2d (US7044) 2e (US7044)

×

2f (US7044)

×

×

×

×

×

× ×

2g (US7044) 2h (US9012)

×

×

×

×


3.2. Plaster analysis

back to Roman execution times or belong to buildings designed to particular functions or uses [2].

A cross section of a typical sample of the plaster mainly used at Rocca Manerba is shown in Figure 9. In this figure the absence of a true “intonachino” rich 4. Conclusions in calcite under the painted layer is evident, whereas a superficial layer smoothed by slaked lime and sand The site of Rocca Manerba was developed on a on which the pictorial layer was spread is observed. very restricted rocky area and subjected to continuous building activities until the final destruction of the fortress in 1537 A.D. In the present work eight representative wall fragments found inside the inner wall of the fortress were studied and characterized to obtain data useful to set this archaeological site into the appropriate historical context. In all the samples analysed the painted layers are spread directly on the smoothed mortar surface. Both the absence of the calcite-rich preparatory layer called “intonachino” and the quite low values (about 1.0) of the aggregate/ binder ratio found in the plasters are indicative of a building technology different from that typical of the Roman age. The pigments used in the paintings are essentially common: yellow and red ochres, green earths, calcite white, and carbon black. The precious cinnabar is also found but mixed with hematite, suggesting that the decorations took place when the supply of the former from Spain got difficult, that is during the Low Roman Empire or the High Middle Ages. Also the absence of Egyptian blue, typical of Roman Figure 9. Cross section of a plaster sample. painting, suggests a later execution date. The use of the calcite alone for the white and of The aggregate is composed by small rounded lith- the limewash to transfer colours to a painted surface ic fragments and fine gray sand. The FTIR analysis instead of the appropriate whites containing also of the lime plaster (not reported) showed the pres- dolomite and/ or aragonite known by the Romans ence of calcite and traces of dolomite and quartz. To for overpainting is indicative of a period after the enhance silicate features, FTIR spectra were carried Roman Empire. out also on plaster powders treated with 1M HCl Characteristic of the Medieval period is the preswhere most of the calcite derived from the slacked ence of a yellow-green pigment (“giallorino”) synlime was eliminated. Thus, the presence of quartz, thetized starting from lead and tin oxides found in illite, and feldspars was highlighted. A powder XRD some samples, whose use is not reported prior to spectrum (not shown) of the aggregate indicates cal- the XVth Century A.D. cite, albite, illite, orthoclase, quartz, and dolomite as the main components. The mean aggregate/ binder ratio, measured in 5. Acknowledgements different plaster samples, showed rather low values, near to 1.0; this is a value smaller than those usually The authors are very grateful to Prof. Pietro Baralfound in plasters from Roman villas, that typically di of the University of Modena and Reggio Emilia fall in the 2-3 range [34]. for the R aman measurements, to Tiziano Finotto of Only very few samples of the 1977 campaign [2], the University of Venice for the X-R ay diffraction showed different features with an evident “intonachi- pattern, and to Danilo Rudello of the University of no” layer of pure calcite; such samples could date Venice, Dr. Isabella Colpo of the University of Padua, 39


and Dr. Silvia Minghelli of the University of Modena and Reggio Emilia for their appreciated cooperation.

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