# Characterization of Waves in the Roman Wall Paintings of the Herculaneum Site (Italy)
## 3 Results and Discussion
### Analyses of Archeological Samples  
#### 3.2.1 FTIR and Raman Spectroscopies  
Spectroscopic FTIR and Raman analyses were carried out on the samples from Herculaneum to identify organic materials and the composition of pigments. The mineral compound identification was performed by comparison with reference spectra [42; 57; 58; 59; 60]. In the FTIR spectra of red samples AC 21, AC 23 and AC 26 (Figure 5), the characteristic peaks of iron oxide/hydroxide, probably hematite, at 568 and 478 cm\({}^{-1}\), were identified. In addition, in all samples, calcium carbonate both in the calcite (1435 and 876 cm\({}^{-1}\)) and argonite (1490 and 856 cm\({}^{-1}\)) phases and gypsum (3529, 3401, 1682, 1620, 1104 and 668 cm\({}^{-1}\)) were clearly visible. The characteristic bands at 1322 and 775 cm\({}^{-1}\) (marked in figure by asterisk) of oxalates, due to degradation phenomena, were also present [60; 61]. Features at 2922, 2852, 1734, 1469, 1419, 1379, 1174 and 722 cm\({}^{-1}\) are typical of waxes [49; 50; 62]. The FTIR spectra, collected on the AC 18 and AC 22 samples, blue in colour, exhibited the typical absorption bands of gypsum and probably of Egyptian blue: antisymmetrical Si-O-Si stretching bands (1160, 1056 and 1008 cm\({}^{-1}\)) and symmetrical Si-O-Si stretching bands (800, 755, 664 and 595 cm\({}^{-1}\)) [63]. The main bands of Egyptian blue (1160, 1060, 1008 cm\({}^{-1}\)) overlap those of gypsum. In addition to the typical bands of waxes, in AC 22, the features of proteinaceous material (animal glue, gelatin, egg) at 3350 cm\({}^{-1}\) and 1680 cm\({}^{-1}\) were also identified [64]. To discriminate between the kinds of wax, we propose an empirical method (mainly used in beeswax adulteration studies [56; 65]) based on the ratio between the intensities of the FTIR absorption main bands corresponding to the stretching vibrations of the C=O and CH groups. The integrated areas of \(\
u\)(CH\({}_{n}\)) and \(\
u\)(C=O) bands have been determined, in the 3050-2750 and 1800-1650 cm\({}^{-1}\) ranges, respectively, by band deconvolution using pseudo-Voigt functions (i.e., linear combinations of Gaussian and Lorentzian curves) using LASPEC software [41]. The ratio values are 0.23, 0.22, 0.26, 0.16 and 0.19 for beeswax, spermaceti, lanolin, candellilla and carnauba, respectively. The results for AC 18 and AC 23 (a ratio value of about 0.23) may suggest the presence of animal wax. For AC 21, AC 22 and AC 26 (0.09, 0.07 and 0.1 ratios, respectively), it is not possible to identify the kind of wax by this method. In all Raman spectra (Figure 6) acquired on the archaeological samples AC 18, AC 21, AC 22, AC 23 and AC 26, the typical features of waxes (indicated in figure by **w**) are recognizable, but it was not possible to establish the kind of wax (animal, vegetable, mineral or artificial). In the Raman spectra of the red samples (AC 21, AC 23 and AC 26), hematite (\(\alpha\)-Fe\({}_{2}\)O\({}_{3}\)), which is mainly responsible for the red color, at 224, 245, 291, 411, 611, 660 cm\({}^{-1}\); calcite (CaCO\({}_{3}\) trigonal crystal system) at 155, 281, 711, 1085 cm\({}^{-1}\); and aragonite (CaCO\({}_{3}\) orthorhombic system) at 703, 1085 cm\({}^{-1}\) were identified, indicating the use by Roman artists of red ochre [66]. Furthermore, gypsum (CaSO\({}_{4}\cdot\)2H\({}_{2}\)O) at 1008 cm\({}^{-1}\) and quartz (SiO\({}_{2}\)) at 465 cm\({}^{-1}\) were detected. In the Raman spectra of AC 18 and AC 22 samples (Figure 6), the features of Egyptian blue (calcium copper silicate with formula (CaCuSi\({}_{4}\)O\({}_{10}\))) at 432, 478, 572, 783, 1085 cm\({}^{-1}\)[63] are clearly evident, in addition to those of calcite, quartz and gypsum. Egyptian blue is one of the oldest synthetic pigments: it is obtained by heating a mixture of siliceous sand, lime, copper compounds and a flux (soda, natron or plant ash) [67]. Its use was very common in the Roman period.