Pigment identification of a XIV/XV c. wooden crucifix by means of the Raman spectroscopic technique

Abstract

The historical polychrome pigments used originally and those applied for later restoration were identified on a wooden crucifix (about 1400 A.D.) from the collection of the National Museum in Gdansk by means of the Raman spectroscopic technique. Characteristic bands associated with the pigments on different areas of the object were observed in the spectra of the paint layers and also on their cross sections. Sets of bands corresponding to the original white, red and green pigments allowed the identification of chalk (283, 1087 cm −1 ), vermilion (253, 284, 343 cm −1 ) and red lead (313, 390, 548 cm −1 ), and also numerous bands ascribed to malachite and azurite. Bands corresponding to the pigments Prussian blue, (Fe[Fe 3+ Fe 2+ (CN) 6 ] 3 ) (282, 538 cm −1 ), and chrome yellow, (PbCrO 4 ) (338, 360, 403 cm −1 ), were also observed. As these pigments have been known since 1704 and 1803, respectively, it is concluded that partial retouching of some areas of the statue had taken place. For confirmation, elemental analysis was performed using LIBS (laser-induced breakdown spectroscopy) and recorded for the green paint retouching after excitation at 248 nm by a KrF laser. In the plasma emission, the elements Cu, Pb, Cr, Fe, CN, C 2 , Ca were identified, supporting the pigment assignment based on the Raman spectra. Results of this work enabled the mapping of the polychrome pigments and also the layer stratigraphy, contributing in this way to the restoration of the statue.

Complementary use of the Raman and XRF techniques for non-destructive analysis of historical paint layers

“…During the last years the systematic research, e.g. on the pigment collections, and also case studies show, that the combination of analytical techniques such as the elemental, compositional, structural ones, etc., strongly enhance the reliability of the results [10][11][12].…”

Analytical study of polychromy on exterior sculpted stone

“…Raman spectroscopic studies (Fig. 3) identified the red minerals such as cinnabar (Raman bands at 252, 284 and 341 cm −1 [12] ), red lead (151, 224, 313, 388 and 547 cm −1 ) [12] and hematite (221, 241, 288, 408, 496, 608 and 1315 cm −1 [18,19] ). All these compounds appeared mixed in the layer in similar quantities as revealed by SEM-EDX.…”

“…It has been reported that this compound can give evidence of the gradual alteration of red lead. [12,23] The pigment alteration leading to lead sulfate is assumed to occur through sulfating processes controlled by environmental parameters, namely humidity, temperature and sulfate concentration, which depend on the oxidative dissolution of atmospheric SO 2 pollutant. [24] In addition, although only traces of As were found by SEM-EDX, Raman spectra of orpiment (As 2 S 3 ) with characteristic bands at 153, 177, 201, 291, 309, 352 and 381 cm −1 (Fig.…”

“…[24] In addition, although only traces of As were found by SEM-EDX, Raman spectra of orpiment (As 2 S 3 ) with characteristic bands at 153, 177, 201, 291, 309, 352 and 381 cm −1 (Fig. 4) [12] and excellent signal-to-noise ratio were recorded in this layer at different locations. This clear identification of orpiment suggested that it was used here as a pigment and that probably the thinness www.interscience.wiley.com/journal/jrs of the layer and the overlapping of bands due to Pb and As led to an underestimation of the As amount by SEM-EDX.…”

“…They were characterized by the use of clear, welldefined and bright colors. [9] Exterior polychromies have suffered much more deterioration [10,11] in comparison with the often wellpreserved and studied indoor pieces of art, wooden objects, [12,13] polychromed statues [14] and wall paintings. [15] Thus, the study of paint layers on external sculpted stone is particularly complicated.…”