# Characterization of Waves in the Roman Wall Paintings of the Herculaneum Site (Italy)
###### Abstract
Waxes are solid at room temperature, highly hydrophobic, have a plastic character, become soft and workable when warm, have a melting point in the 60-80 \({}^{\circ}\)C range and are stable to chemical and enzymatic agents [10]. Beeswax, produced by worker bees of the _Apis mellifera_ genus, represents the most used natural wax in history. In ancient times, it was thought that it came from the flowers; only at the end of the XVIII century, it was understood that it was produced by the bees themselves [13]. The use of beeswax as a sealing agent in pottery and for lighting has been known since the Neolithic [14; 15]. The Egyptians employed beeswax for balms to be used for the mummification process, in shipbuilding, to polish the surface of paintings and to make statues and writing tablets [16; 17]. Beeswax was particularly appreciated by Greeks and Romans for its shine and waterproofing properties and was largely applied on stone surfaces for both aesthetic and protective purposes [18]. According to the ancient sources, beeswax was used as a binder in a painting technique named encaustic [2; 19]: by Romans and Egyptians in mummy portraits, as in Fayum mummy portraits [20; 21; 22], and by Greek, Roman and Byzantine artists in ceramic, wall paintings, in polychrome sculptures and other artworks made by other materials [23]. For a long time, due to the brilliant colors of the paintings and their extraordinary preservation and also to the complex interpretation of [PERSON] and [PERSON]'s texts, it was thought that Herculaneum and Pompeii wall paintings had been made with the encaustic technique. Studies in recent years have actually found the presence of beeswax in Pompeii's paintings, but from the observation of the cross sections of the collected samples, it has been possible to localize the wax only on the pictorial surface: this suggests the application of wax during later conservation treatments [24; 25]. In particular, research on Insula del Centenario in Pompeii was carried out to verify the presence of protein and lipid materials as binders for the pigments and to identify organic compounds on the paintings from previous restoration works [24]. Samples collected from wall paintings from different rooms were studied by Fourier transform infrared spectroscopy (FTIR) and gas chromatography coupled with mass spectrometry GC/MS. Analytical results show that these Roman wall paintings were made without the use of lipid and proteinaceous binders, most probably with a fresco technique. In addition, wax, egg and animal glue have been identified on the surface of the paint: their presence could be associated with previous restoration works. The use of lipid and protein compounds for protective purposes and/or to restore wall paintings to their original brilliance has been documented in the restorations carried out in Pompeii in the nineteenth century [5; 24; 25]. An investigation carried out by [PERSON] et al. [26] has employed several analytical techniques to study Roman wall paintings from House of the Villa of Papyri in Herculaneum and the Golden Bracelet in Pompeii. Organic binding media have been studied by means of infrared spectroscopy and Py-GC/MS. The results of FTIR spectroscopy indicate the presence of some glue- or gum-like compounds. The results of this study showed vegetable wax, beeswax and some types of protein and gum compounds, thus underlining the presence of a dry paint [26]  
Another study has been carried out in the Villa of the Papyri in Herculaneum, in order to identify the organic binders [27]. The authors, by means of GC/MS, detected egg, in some samples, wherein the pigments required _a secco_ technique. They also investigated the presence of waxes employed as superficial treatments of the wall paintings. The fact that they did not find beeswax in all the considered samples gives additional evidence that the encaustic technique was not used in the Villa of the Papyri in Herculaneum [27].