The Polychromatic Inscriptions on the Relief Sculpture Deposition from the Cross by Benedetto Antelami in Parma Cathedral, Italy

Abstract

This paper reports on the studies carried out on engraved inscriptions of the Deposition from the Cross by Benedetto Antelami (1150–1230), a relief sculpture conserved in the Cathedral of Parma (Italy). The inscriptions between the characters show residues of colored material in alternating red- and dark-colored stripes. The purpose of this specific investigation was to identify the materials (pigments and organic binders) used for the polychromy that are still present on the relief sculpture. Seven microsamples were taken to carry out laboratory analyses. In red-colored letters, mercury, and sulfur, constituents of the cinnabar (or vermilion) pigment were easily identified by SEM-EDS. This result is confirmed by Raman spectroscopy and XRD measurements. In the dark letters, carbon, iron, and lead are observed. The presence of materials containing metals is compatible with a mastic encrustation technique. FT-IR, Raman, and XRD techniques clearly detect beeswax, which was probably used as a polishing material. Amino acids and lipids that are typical of eggs have been identified by means of GC-MS investigations, suggesting their use as organic components of the mastic encrustation.

1. Introduction

The Deposition from the Cross (Figure 1) in Parma Cathedral, Italy, is a relief sculpture by Benedetto Antelami made by red Verona marble. It is a sedimentary rock composed mainly of the mineral calcite, whose hue, given by iron oxide and fossils, ranges from brick-brown red to a pinkish color, with veins. The marble slab is preserved in the right transept of the Cathedral. Its dimensions are 2.30 m long and 1.10 m wide. The artwork, dated 1178, was originally part of a pulpit in the presbytery area. It was commissioned by the bishop of Parma Bernardo II but was destroyed by Girolamo Mazzola Bedoli in 1564–1566 due to the redesign of the staircase. Other slabs of the pulpit were mainly lost, and some are now preserved in the Diocesan Museum of Parma [1]. Antelami was an architect and sculptor who has been long studied. In his works, it is possible to identify the first signs of renewed aesthetics with respect to Romanesque solutions, with openings towards new image models of the Early Gothic in France. In the inscription “ANNO MILLENO CENTENO SEPTVAGENO/OCTAVO SCVLTOR PATVIT MENSE DICTV SECVNDO/ANTELAMI SCVLPTOR FVIT HIC BENEDICTVS”, the date of execution and the signature of the author seem evident, despite some doubts about them [2]. As far as we know, the only scientific investigation on the materials found in Antelami’s Deposition from the Cross consists of a study by fiber optic mid-FTIR reflectance spectroscopy of the patinas on the marble surface of the artwork. The mid-FTIR portable set up allowed us to identify a wide contamination of oxalates, silicates, and sulfates, and a past protective layer made of proteinaceous and lipidic components [3]. It should be mentioned, only for the sake of curiosity, that a recent study investigated a relief placed in a church of Saint-Germain-en-Laye, France, resembling very closely the Deposition from the Cross in the transept of Parma Cathedral, and it reported the same date, 1178, and signature of Benedictus Antelami. According to the results of archival and historical research and scientific investigations on the materials, this copy of the original relief sculpture conserved in Parma was created in the early 1900s [4].

Several scientific studies have been carried out on medieval sculptures showing polychromies in engraved marble [5,6,7,8,9,10,11,12,13]. Generally, these marble incisions were obtained by digging the rock, and the shaped cavities were filled with different mixtures. This technique is often reported as mastic encrustation [14]. It is similar, as often reported [1,15], to the niello technique, in which black metal alloys or mixtures of sulfur, copper, silver, and lead are used as an inlay on engraved metals. For the stones, mixtures of iron oxides and chalk with resinous, bituminous, or waxy material containing pigments were used. For the mastic encrustation, fine-grained black crushed coal or carbon black were employed [4,14]. A final layer of polishing was applied using waxy substances to allow greater durability of the surface [15,16]. In Italy, the use of mastic encrustations dates to Roman times and it was subsequently diffused in the Mediterranean area between the 10th and 13th centuries [4]. The relief sculpture by Benedetto Antelami in the Cathedral of Parma represents an example in the Tuscan–Emilian area [14]. Although significant parts have been lost, the remains present in the colored inscription have been investigated, and the results are reported here.

In the present study, the interest is focused on the characterization of pigments and organic binders used in polychromy observed in some of the engraved inscriptions between the characters. The Deposition from the Cross represents an episode described in the Gospel, the detachment of Christ’s body from the cross, but resumes the Byzantine iconography, inserting other characters and symbols (such as the personification of the sun and the moon within Clypeus garlands) [15]. From left to right (Figure 1), between the heads of the characters represented in the relief sculpture, their names are written on several lines colored with red and dark colors: Salome (SALOME), Mary, mother of Jacob (MARIA: IA/COBI), Mary Magdalene (MARIA: MAG/DALENE), Saint John (SIO/HS), the Virgin Mary (S:MA/RIA), the personification of the Catholic Church with the banner (ECCLA/EXALTA/TVR), Joseph of Arimathea (JOSEPH/AB ARI/MATHIA), Jesus (IHVS: NAZARENVS REX IVDEMX), Nicodemus (NICODEMVS), the personification of the synagogue with the banner (SINA/GOGA/DE/PO/NI/TVR), the Roman centurion (VERE: ISTE/FILIVS/DEI ERAT), the seated soldiers who contend for the tunic, and the procession. In the upper part, always from left to right, there is the sun (SOL), the Archangel Gabriel (GABRI/EL), the Archangel Raphael (RAPHA/EL), and the moon (LVNA) [16].

Microsamples taken from the artwork have been subjected to laboratory analyses to provide significant data on the presence of pigments and binders in the engraved letters. The morphology and elemental analysis were assessed by scanning electron microscopy with energy-dispersive spectroscopy; the compositional crystalline phases were determined by means of X-ray powder diffraction. Fourier-transform infrared and micro-Raman spectroscopies contributed to the identification of organic and inorganic compounds; gas chromatography coupled with mass spectrometry was important for differentiating the organic binders.

2. Materials and Methods

2.1. Sample Collection

Seven samples of a few micrograms (20–40 μg) were collected from both the red and dark areas of the engraved inscriptions using a thin-bladed scalpel.

Table 1. Description of samples and analytical techniques.

Sample	Sampling Image	Sampling Point	Techniques
A1		Red pigment from the letter C of NICODEMVS; inscription under the left arm of Christ.	FT-IR spectroscopy Raman spectroscopy SEM-EDS
A2		Dark material from the E of IOSEPH under NICODEMVS.	FT-IR spectroscopy Raman spectroscopy GC-MS
A3		Dark material from letter R in S:MA/RIA above Christ’s right arm.	FT-IR spectroscopy GC-MS
A4		Red pigment from A of SINA/GOGA above the banner of the synagogue.	FT-IR spectroscopy SEM-EDS XRD
A5		Dark material from the E of MAG/DALENE under Archangel Gabriel.	FT-IR spectroscopy SEM-EDS
A6		Red pigment from the letter D of DE/PO/NI/TVR taken on the dark material in the synagoguebanner.	FT-IR spectroscopy Raman spectroscopy
A7		Dark material from the letter H in ARI/MATHIA to the right of Christ.	FT-IR spectroscopy Raman spectroscopy SEM-EDS XRD

shows the sampling points and the investigations carried out.

After the Raman measurements, the samples were divided into two portions: one was metalized and investigated by means of SEM, and the other was subjected to FTIR spectroscopy and XRD analyses. As highlighted in Table 1, the samples subjected to GC-MS analysis (performed after FTIR measurements) are different from those on which the SEM investigation was carried out.

2.2. Instruments and Methods

2.2.1. Scanning Electron Microscopy Coupled with Energy-Dispersive X-ray Spectroscopy (SEM-EDS)

The morphology and elemental composition of samples A1, A4, A5, and A7 were investigated by means of a Jeol 6400 SEM-EDS scanning electron microscope (Jeol Ltd., Tokyo, Japan) equipped with an Oxford Instruments (Abingdon-on-Thames, UK) Link Analytical Si (Li) Energy-Dispersive System (EDS) detector. Experimental parameters: excitation voltage 20 kV, beam current 0.28 nA, ~1 mm beam diameter, and 60 s counting time. The samples were covered with a thin film of graphite to avoid charging effects. SEM images were obtained using a backscattered electron detector and were processed using Oxford INCA V7.2 software.

2.2.2. Fourier-Transform Infrared Spectroscopy (FT-IR)

Infrared spectroscopy investigations were performed on all samples, in ATR mode, using a Perkin Elmer FT-IR Spectrum Two spectrophotometer (Waltham, MA, USA) equipped with a diamond (n = 2.41) crystal ATR accessory. Typical ATR parameters: penetration depth 2 μm, effective pathlength of the IR beam ≈ 4.36 μm at 1000 cm⁻¹ for θ = 45° (angle of incidence), and a substrate of ns = 1.5. Infrared spectra were recorded in the 4000–400 cm⁻¹ range with 4 cm⁻¹ resolution and 32 scans.

2.2.3. Micro-Raman Spectroscopy

Raman spectra were acquired on 4 samples (A1, A2, A6, and A7) with a Jobin Yvon LabRam micro-spectrometer (HORIBA, Jobin Yvon, Kyoto, Japan) equipped with an integrated Olympus BX40 microscope (Olympus Corporation, Tokyo, Japan). A 15 mW He-Ne laser (at 632 nm) was used as the excitation source. The spectral resolution was 2 cm⁻¹. To avoid damage to the samples or uncontrollable thermal effects, the average power on the sample was kept, by density filters, less than 1 mW. Spectra were collected using both 100× or long-distance 50× microscope objectives. The measurements were recorded with 30–60 s exposures and 3–5 times accumulation in the range of 50–3500 cm⁻¹. The silicon Raman feature at 520.6 cm⁻¹ was used to calibrate the instrument. The data were processed by HORIBA LabSpec v. 6 software.

2.2.4. X-ray Diffraction (XRD)

X-ray diffraction data were collected on powdered samples A4 and A7 with a Rigaku Smartlab Multipurpose diffractometer equipped with a HyPix3000 two-dimensional detector (Rigaku, Tokyo, Japan). The diffraction patterns were collected under ambient conditions in Bragg–Brentano geometry using CuKα (1.5406 Å) radiation (40 kV and 30 mA) in the 2θ angular range of 5°–80° at 10°/min scan rate with a step size of 0.02°. The identification of crystalline phases was performed for comparison with the literature data and JCPDS (Joint Committee on Powder Diffraction Standard) cards. Origin 8 v. 9.6.5 software (OriginLab Corporation, Northampton, MA, USA) was used to elaborate the data.

2.2.5. Gas Chromatography–Mass Spectrometry (GC-MS)

The GC-MS analysis was performed on samples A2 and A3 by a 6890 N GC system gas chromatograph equipped with a split/splitless injection port and coupled with a 5973A mass selective detector mass spectrometer (Agilent Technologies, Santa Clara, CA, USA). A Varian FactorFour (Varian Inc., Palo Alto, CA, USA) VF-5 fused-silica capillary column (30 m × 0.25 mm × 1 μm) coated with a 0.25 μm film of methyl silicone (5% Phenyl) was used for the separation. The mass spectrometer operated in electron impact positive mode (70 eV), and the mass spectra were acquired in the scan range of 40–500 m/z. The MS transfer line temperature was set to 280 °C.

Following the methodology reported in a previous work [17], fatty acids were extracted from 20 µg of selected samples treated with HCl (4N) in methanol (1 mL) and n-hexane (1 mL) for 2 h at 50 °C. Heptadecanoic acid (50 µL of a 0.1 mg/mL w/v solution) was added as the internal standard. After centrifugation, the hexane phase containing fatty acid methyl esters was taken, and 1 µL of extract was analyzed.

For the amino acid extraction, after evaporation to dryness of the methanol phase, the residue was dissolved in 6N hydrochloric acid (2 mL), and Norleucine (50 µL of a 0.1 mg/mL solution w/v) and Norvaline (50 µL of a 0.01 mg/mL solution w/v) were added as internal standards. The hydrolysis was performed for five hours at 100 °C. After evaporation to dryness, the hydrolysate was esterified using 3 mL of HCl (2N) in propan-2-ol at 90 °C for one hour. Then, the solvent was vacuum evaporated until dry, and the obtained residue was dissolved in 0.2 mL of dichloromethane and derivatized with 0.2 mL of trifluoroacetic anhydride at 60 °C for one hour and again brought to dryness. The final residue was then redissolved in 0.2 mL of dichloromethane, and the solution, containing N-trifluoroacetyl-O-2-propyl esters amino acids derivatives, was used for GC-MS analysis (1 µL).

The mass spectrometer was operated in the selected ion monitoring mode (SIM). The following target ions for the fatty acid methyl ester were considered: m/z 153 for azelaic acid, m/z 270 for palmitic acid, and m/z 298 for stearic acid, and for amino acid derivatives, m/z 140 for alanine (Ala), m/z 168 for norvaline (Nval), m/z 182 for leucine (Leu) and norleucine (Nleu), m/z 126 for glycine (Gly), m/z 166 for proline (Pro), m/z 164 for hydroxyproline (Hyp), m/z 184 for aspartic acid (Asp), m/z 198 for glutamic acid (Glu), and m/z 91 for phenylalanine (Phe) were selected.

3. Results and Discussion

Samples A1, A4, A5, and A7 were studied by SEM-EDS. Figure 2 shows the EDS spectra acquired on areas and points as shown.

Table 2. Elements present in samples A1, A4, A5, and A7 detected by SEM-EDS.

Sample	Hg	S	Ca	Fe	Pb	Si
A1, A4	X	X	X			X
A5		X	X	X		X
A7			X		X	X

summarizes the main elements identified.

In the red-colored A1 and A4 samples, mercury and sulfur suggest the use of the red pigment cinnabar or vermilion and mineral or synthetic HgS, respectively. We did not attempt to discriminate between these two forms.

In the dark samples, iron (sample A5) and lead (sample A7) were found. The presence of calcium in all the spectra may be due to the underlying marble substrate. Elements such as Si, Mg, Al, K, Cl, Fe, and Pb, clearly evident in the dark samples, can be attributed to the use of the mastic encrustation technique [18,19], in which a melted mix was used as a filler of the engraved stone [14].

The infrared spectroscopy that was carried out on all samples (Figure 3 and Figure 4) aimed to identify organic substances and salts. Figure 3 reports the IR spectra acquired on the A2, A3, A5, and A6 samples. Typical bands of calcium carbonate (1428, 875, and 712 cm⁻¹), silicates (1020 cm⁻¹), calcium oxalate (1624, 1322 and 780 cm⁻¹) [20], and hydrated calcium sulfate (3537, 3407, 1620, 1110, 671, and 598 cm⁻¹) are observed. Furthermore, the vibrational bands related to the presence of organic substances are evident: 3300–3100 cm⁻¹ (overlapping amide A, amide B, and OH stretching vibrations), 2952–2848 cm⁻¹ (stretching of CH and CH₂), 1640 cm⁻¹ (amide I overlapping vibrational bands of calcium oxalate and sulfate), and 1540 cm⁻¹ (amide II) due to a proteinaceous material, presumably due to a protein binder.

In the FT-IR spectra collected on the A1, A4, and A7 samples that are reported in Figure 4A, the features of calcium sulfate (indicated by dotted lines) and wax are found. Wax identification is possible through a comparison (Figure 4B) of the FT-IR spectrum of sample A1 and beeswax, spermaceti, lanolin, carnauba, and candelilla standards [21]. The wax may be unequivocally identified as beeswax. The Supplementary Materials (Figure S1) also report the comparison with the wax standards in the range of 1000–1600 cm⁻¹. Beeswax is characterized by long aliphatic chains identified in the spectra by asymmetric and symmetric CH₂ stretching vibrations at 2916 and 2848 cm⁻¹, respectively, and by the typical doublets attributed to the bending and rocking vibrations at 1472, 1462 cm⁻¹ and 730, 718 cm⁻¹, respectively [22].

Raman spectra were collected on the A1, A2, A6 (Figure 5A), and A7 samples (Figure 5B). In the red-colored samples (A1 and A6), the presence of cinnabar or vermilion is confirmed by the peaks at 250 and 345 cm⁻¹ [23], in agreement with the SEM-EDS results. In addition, in the A1 sample, the bands in the range of 1300–1600 cm⁻¹ indicate the presence of amorphous carbon on dark material under the red pigment. In sample A2 taken from the dark letter (E from NICODEMVS), amorphous carbon was clearly found. Figure 5B reports the Raman spectrum of sample A7 (dark letter, H from ARI/MATHIA). The characteristic peaks of beeswax are found [21], i.e., the peaks assigned to the symmetric stretching vibrations of the methyl and methylene groups at 2880 cm⁻¹ and 2842 cm⁻¹, respectively, and the peak at 1461 cm⁻¹ is attributed to the bending vibrations of CH₂ groups of the aliphatic chain. The strong peaks at 1435 cm⁻¹ and 1417 cm⁻¹ are due to the asymmetric and symmetric bending vibrations of CH₃, respectively. The comparison between the Raman spectra recorded on the A7 sample and the beeswax standard is demonstrated in Figure 5B. The arrows in the A7 sample spectrum at 1349 and 1590 cm⁻¹ indicate the features characteristic of amorphous carbon.

The presence of amorphous carbon (e.g., carbon black) and beeswax both in the red and black samples confirms their use in mastic encrustations, as reported by van der Werf et al. [14].

The XRD patterns of powdered samples A4 and A7 are displayed in Figure 6. The diffractograms show a high-intensity reflection for both analyzed samples at 2θ = 21.5° and a medium-intensity reflection at 2θ = 23.7°, which correspond to the reflection planes typical of beeswax fatty acid chains [24,25,26]. Calcite features were also identified in both samples. In addition, for sample A4, characterized by the red color, the reflection planes typical of the HgS profile [27] used in these engravings can clearly be distinguished.

Through the spectroscopic techniques and XRD analysis, beeswax was unequivocally identified. On the contrary, it was not possible to discriminate the proteinaceous material with FT-IR investigation on samples A2, A3, A5, and A6.

GC-MS analysis was carried out on samples A2 and A3. Both samples show fatty acids due to lipidic constituents and amino acid content. The chromatogram in Figure 7 suggests the presence of fatty acids (sample A3). Azelaic (nonanedioic acid, saturated dicarboxylic acid), palmitic (C16:0), and stearic (C18:0) acids were detected. In this sample, the value of the azelaic acid/palmitic acid ratio (A/P) is lower than 1. According to the literature [28], this value would lead to excluding the presence of drying oils. The mass spectra of derivatized azelaic, palmitic, and stearic acid are shown in Figures S2–S4.

Figure 8 shows the amino acid chromatographic profile of sample A2, taken from the dark-colored material. The amino acid mass spectra are shown in Figures S5–S16.

To characterize the proteinaceous binding media, the percentage content of amino acids was compared to those from a dataset of 61 reference samples of egg (whole, egg white, egg yolk), casein, animal glue, and a mixture of them, considered the most widespread proteinaceous binders in the Western tradition, belonging to the reference collection of the Opificio delle Pietre Dure in Florence, Italy [29]. Using these data as variables, multivariate statistical analysis, namely, principal component analysis, PCA [30], was carried out on the correlation matrix of the relative percentage contents of eight amino acids (aspartic acid, glutamic acid, proline, hydroxyproline, phenylalanine, alanine, glycine, and leucine) [31]. The PCA score plot is described in Figure 9.

The first two components, PC1 and PC2, account for 61.67% and 23.70% of the total variance, respectively. The A2 and A3 samples that are collected are placed on the Cartesian diagram of the PCA, not far from the egg references, suggesting that there are eggs in the A2 and A3 samples. We also believe that the lipid fraction, due to azelaic, palmitic, and stearic acids detected in the examined samples, is due to the lipid content of the egg yolk.

4. Conclusions

This paper contributes to the study of the polychromy used by Antelami in Parma, Italy, in the colored inscriptions between the characters in the relief sculpture Deposition from the Cross. SEM-EDS, Raman spectroscopy, and XRD suggest the use of cinnabar or vermilion in the red letters. Amorphous carbon is identified by means of Raman spectroscopy, indicating the use of carbon black in dark inscriptions. Small quantities of calcite are probably due to the marble substrate. Other elements such as silicon, iron, and lead in dark materials were used by Antelami in the marble decoration by a mastic encrustation technique. GC/MS analysis of amino acids and fatty acids confirms the presence of eggs as a component of the mastic preparation. Beeswax was also found by means of FT-IR and Raman spectroscopies and XRD investigations. As well as being used as a component of the mastic encrustation, beeswax was used as a final treatment to give more intensity to the color and greater durability over time.