The combined use of SEM-EDX, Raman, ATR-FTIR and visible reflectance techniques for the characterisation of Roman wall painting pigments from Monte d'Oro area (Rome): an insight into red, yellow and pink shades

Abstract

The aim of this work has been the identification of the painter's materials employed in the wall decoration of some destroyed buildings dating approximately between the first century B.C. and the first century A.D. This research originates from a previously started joined archaeological and analytical investigation concerning a varied group of findings that resulted from a rescue excavation performed by Soprintendenza Archeologica in the area of Monte d'Oro in Rome. The focus of this study progression has been directed to a numerous selection of monochrome red, pink and yellow-pigmented fragments. The analyses were performed by means of scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDX) combined with Raman and Fourier transform infrared (FTIR) spectroscopies; visible reflectance measurements have also been carried out and the relevance of this technique in such a kind of archaeological studies has been highlighted. Most attention has been given to the assessment of the performances of non-destructive techniques achieved by portable Raman, and visible reflectance instrumentation to test their diagnostic capabilities. In addition to the expected and well-known pigments such as cinnabar, red ochre, hematite for the reds and yellow ochre for the yellows, the study highlighted a diffuse use of mixed colours and in some cases the possible presence of overlapped painted layers and confirmed the presence of gildings. Among the mixtures of pigments, the most singular outcome concerns the pink fragments revealing the possible application of bone white, which seems to be rather uncommon as a pigment in Roman wall decorations.

Keywords: ATR-FTIR; Ash; Bone white; Gold; Raman; Roman wall paintings; SEM-EDX; Visible reflectance.

Fig. 1

Some of Monte d’Oro’s analysed samples: 1. Bright red with gilding (n.inv. 607354); 4. Pink (n.inv. 608044), 5. Light red (n. inv.), 6. Yellow (n.inv. 608054), 7. Red (n.inv. 608047), 8. Yellow (n.inv. 608056), 11. Violet (n.inv. 608042)

Fig. 2

Raman spectra acquired by a portable Raman spectrometer with 1064 nm excitation (solid line) and by a micro-Raman spectrometer with 785 nm excitation (dotted line) on samples: (a) red 7, (b) red 7, (c) light red 5, (d) light red 5, (e) light red 5, (f) violet 11 and (g) violet 11. Triangles indicate ochre/hematite peaks, asterisks indicate calcite and circles quartz peaks

Fig. 3

ATR-FTIR spectra of red 2, 7 and light red 5 samples; the peaks of calcite are marked with an asterisk, some quartz bands are indicated by a curly bracket

Fig. 4

Raman spectra acquired by a portable Raman spectrometer with 1064 nm excitation (solid line) and by a micro-Raman spectrometer with 785 nm excitation (dotted line) on samples: (a) yellow 6, (b) yellow 8, (c) and (d) single different yellow grains on sample 8. Asterisks indicate peaks of calcite

Fig. 5

ATR-FTIR spectra of yellow samples 6 and 8. Peaks due to calcite are indicated with an asterisk. Some quartz bands are indicated by a curly bracket

Fig. 6

SEM-EDX analysis on sample 4: on top the whole EDX spectrum obtained from the area of about 12 mm² corresponding to BSE image in a; in b and c the distribution of respectively phosphorous (K series) and vanadium (K series) on the same area

Fig. 7

Raman and micro-Raman spectra of cinnabar obtained with 1064 nm excitation (solid line) and 785 nm excitation (dotted line) on bright red and pink samples

Fig. 8

Micro-Raman spectra (λ_exc=785 nm) acquired on different grains on the surface of pink sample 4. From bottom to top: (a) cinnabar; (b), (c), (d) cinnabar, calcite (peaks marked with an asterisk) and litharge in different proportions; (e) litharge and calcite

Fig. 9

ATR-FTIR spectrum obtained for pink sample. Bands of calcium phosphate are highlighted by the ellipsis; asterisks indicate calcite

Fig. 10

SEM-EDX analysis on an area of about 650 × 500 μm² of the gilded surface of sample 1: (a) S.E. image; (b) BSE image; (c) and (d) distribution of respectively gold (M series) and calcium (K series)

Fig. 11

ATR-FTIR spectra of (a) micro-sample collected from the red surface of fragment 1 and (b) micro-sample collected from the gilded surface of the same fragment. The bands of the organic binder are indicated by arrows; calcite (*) and silicates (×) bands are also pointed out

Fig. 12

On the left, visible reflectance spectra of bright red (1 and 3), pink (4), violet (11), red (7), light red (5) and yellow (6) samples; on the right, their first derivative. The vertical dotted grey lines indicate the position of maxima of first derivatives of pink (left) and bright red (right)