Micro-Raman Analysis of the Pigments on a Crucifix in Calabria

Abstract

This study was carried out on a crucifix located inside the church of St. Mary Major in Acri. The story of this crucifix is not very clear, and its dating is still uncertain, as well; however, it ranges between the 15th and the 14th century. The wooden sculpture of the crucified body of Jesus was painted and the conservation state of the color appeared good. A detailed analysis of the pigments was considered very useful in verifying the restoration history of this crucifix, and to discriminate between the original painting and later restoration activities. A micro-Raman analysis reveals the presence of the classical pigments expected for the estimated age of the crucifix, such as gypsum, terra di Siena, vermilion, carbon black, and others. In addition, other pigments of more recent use were found, such as Prussian blue, titanium white, lithopone, and chromium yellow, mostly in the thong around body of Jesus, which appears green. This is a clear indication of a relevant modification of the artifact in modern times, when the thong was totally painted again; smaller modifications were surely revealed by the presence of “modern” pigments on other points of the body.

1. Introduction

Among the artefacts belonging to cultural heritage, wooden sculptures are among those most widespread in many churches of Europe. Most of them date from the Middle Ages, as well as during later times labeled as Renaissance and Baroque. Wooden crucifixes are a particular kind of sculpture, and are omnipresent in churches and in public places of Christian settlements.

Raman spectroscopy is an non-destructive technique which is widely used to study the materials of cultural heritage [1,2,3,4].

In this work, we present a micro-Raman spectroscopic investigation of the pigments used on a wooden crucifix present in the church of St. Mary Major in Acri, near Cosenza, in the region of Calabria.

The story of this cross is not very clear, and its dating is still uncertain, as well. According to some studies, it can be affirmed that the cross originates from the 15th century, although some recent studies reveal that the origin of the cross might be older than the previously mentioned period. During the following centuries, the cross has undergone several undocumented restorations, and in one of these, the decision was made to cover it with several papier mache layers, completely changing the aesthetics of the artwork.

In the last restoration in 2012 the papier-mache layers covering the crucifix were eliminated. After cleaning and removing the not-original parts from the cross, it became possible to perform a micro-spectroscopic analysis of the pigments used on the original artwork by taking several fragments extracted from various zones of the body, to identify the pigments using a para-destructive technique. This procedure was utilized to conduct spectroscopic tests without damaging the artwork.

2. Materials and Methods

Very small, pigmented fragments were obtained during the last restoration work by using para-destructive methods, i.e., extracting them from zones that are not visible, or else from the most damaged part of the crucifix; six specimens were collected in this way from the different parts of the body, as shown in Figure 1.

On each specimen, several Raman spectra were collected. The micro-Raman apparatus is a LabRAM spectrometer (Horiba Jobin Yvon, Piscataway, New Jersey, USA) consisting of a single grating spectrograph, interfaced with an Olympus microscope, an edge filter and a Peltier cooled CCD detector (256 × 1024 pixels). Different microscope objectives were used for visual analysis of the samples; for the collection of the spectra, carried out using backscattering geometry, the 50× long-working-distance objective was used. A He-Ne laser (633 nm) internal to the spectrometer was used as an excitation source. The grating used was with 1800 grooves/mm. The spectral resolution was about 1–2 cm⁻¹. The Raman spectra shown in this work were baseline-corrected.

3. Results and Discussion

Figure 2 shows photos of the six specimens investigated using Raman spectroscopy.

For each specimen several points were analyzed, in order to provide detailed information on the pigments used. For instance, both samples S_4 and S_5, from the thong, appear green; however, the pigments found are not green, but yellow and blue.

In general, a strong luminescent background is present in all the spectra. In some cases, it totally covers the Raman signal. By using the He-Ne laser for excitation, we obtain a better ratio of Raman signal over luminescence. However, we performed many measurements at different points on the samples in order to obtain a reasonable Raman-signal-to-noise ratio.

Representative Raman spectra collected from the studies on different specimens are displayed in Figure 3.

Because of the fine mixture of pigments present, in many cases, the spectral features originate from different materials.

A summary of the various colors found in the Crucifix and studied via micro-Raman analysis is given in

Table 1. Labels, and origin of the specimen on the sculpture and its dominant color.

Specimen Number	Origin of the Specimen on the Sculpture	Color
S_1	Head of Christ (upper side, hairs)	Brown
S_2	Face of Christ (lower right side)	Carnation
S_3	Ribs (blood on the wound)	Red
S_4	Thong (upper zone, left side)	Green
S_5	Thong (lower zone, right side)	Green
S_6	Foot (blood on the wound)	Red

below.

Some of the collected spectra are shown in the following section. Various pigments were identified using the databases provided [5,6,7,8,9].

The first set of representative Raman spectra of the pigments found on the crucifix under examination is reported in Figure 3. Shown are pigments of ancient use, which are expected in a sculpture made in the XV century, after the most reasonable estimate. The Raman micro-analysis, however, also reveals the presence of other pigments, introduced in use more recently: besides the titanium white, discussed above, the spectra of some modern pigments are shown separately in Figure 3.

First at all, we report, in Figure 3a, the spectrum of gypsum (calcium sulphate dihydrate), which was derived from the preparatory layer and was found in different samples, alone or in combination with other pigments. In addition, in the spectrum of gypsum, reported in Figure 3a, the presence of titanium white (anatase) is also revealed: in fact, the strong, low frequency peak at 143 cm⁻¹ is derived from it, as are the shoulders at 395 cm⁻¹ and 636 cm⁻¹, which overlap with the typical internal mode of the sulphates.

In Figure 3b, the spectrum of hematite can be seen—called red ochre by painters—which was found in the red regions of our samples. This pigment, largely used, has been well known since prehistoric times for painting red and red-like zones.

Another material used as a red color is cinnabar (mercury sulfide)—called vermillion in the artistic milieu—herein reported in Figure 3c, found in some regions of our samples. Finally, in the darker zones of this painted crucifix, carbon black was found, and its typical spectrum is reported in Figure 3d, corresponding to amorphous carbon. Some of the spectra associated with the modern pigments are reported in Figure 4.

For instance, in the samples coming from the green zones of the sculpture, the occurrence of lithopone is revealed (see Figure 4a). In other measurements on the dark green regions, the spectrum of Prussian blue was collected. This organometallic complex was discovered at the end of the 18th century, and its use in painting starts from the 19th century. Even in the presence of a strong, luminescent background, the characteristic Raman modes are clearly observed, particularly the one at 2152 cm⁻¹, which is quite a specific marker.

The complete list of the pigments found in this study is summarized in

Table 2. List of all pigments found using micro-Raman spectroscopy analysis with the historic records of use and the principal Raman peaks.

Pigment	Composition	Main Raman Peaks (cm−1)	History: First Use
Gypsum	CaSO4 • 2H2O	1138 w; 1007 s; 670 vw; 411 w	All ages
Titanium white	TiO2	636 w; 394 vw; 141 s	After 1920
Lithopone	ZnS + BaSO4	984 s; 617 vw; 457 w; 450 w	End of 19th century
Siena earth	Fe2O3 + silicates + MnO2	638 vw; 386 w; 271 m; 210 m	All ages
Red ochre	Fe2O3	603 w; 400 m; 285 m;217 m	All ages
Vermilion	HgS	340 m; 281 w; 249 vs	All ages
Minium	Pb3O4	543 s; 385 w; 114 s	All ages
Chrome yellow	PbCrO4	838 s; 356 w	After 1818
Lead yellow	PbO	138 s	All ages
Prussian blue	Fe4 [Fe(CN)6]3	2152 m; 1083 w; 544 m; 270 w	18th century
Carbon black	Amorphous carbon	1355 s (br); 1595 s (br)	All ages

.

4. Conclusions

In this work, the identification of different kinds of pigments on a Crucifix from the Church of St. Mary Major in Acri (Calabria), obtained via para-destructive micro-Raman analysis, was discussed. Gypsum, titanium white, lithopone, Siena earth, red ochre, vermilion, minium, chrome yellow, lead yellow, Prussian blue, and carbon black were found.

Considering their widespread applications in the past, from the middle of the 18th century to the 19th century, and even more recently, the presence of such pigments can support the hypothesis of several undocumented restoration works on this crucifix—probably during the 19th century and maybe one in the 20th century—before the application of “papier mache”, finally removed in the last restoration of 2012.