Chemical studies of Roman wall paintings
Since the discovery of Pompeii an d Herculaneum in the 18th century, to the present days, determination of the painting technique used in Roman wall paintings has been subject to numerous controversies and conflicting results1 . The most recent scientific studies of Roman wall painting techniques yield extremely divergent and inconsistent results, and therefore the present state of knowlegde on Roman wall painting techniques is extremely confused. As a consequence, the suitability of current conservation treatments of these paintings is also unreliable and controversial.
Some analysis of Roman wall paintings indicate that they were made with an organic paint medium whose composition has not been determined2 . These studies rule out the possible use of fresco painting. On the other hand, some studies suggest that Roman wall paintings were executed on fresco3 , while others consider, depending on the authors, with mezzofresco (lime painting)4 , or “a secco” with an organic paint media of undetermined composition5 .
There are compelling reasons to consider that Roman wall paintings could not have been painted with fresco. Regarding possible secco techniques, the studies that identified a paint medium of emulsified beeswax stand out, as they propose an artist`s quality painting technique which is in agreement with the ancient written sources. These studies indicate that this water soluble encaustic of beeswax and soap was a common painting technique in Roman murals.
By means of microchemical techniques, Selim Augusti studied a great number of wall painting samples from Pompeii, belonging to the four Pompeian styles (2nd century BC – 1st century AD), as well as samples from Herculaneum coming from excavation that had not been subjected to any restoration. Augusti identified in all the samples studied the presence of beeswax and soap both in the painted backgrounds and in the upper paint layers6 .
Two recent studies show results close to those of Augusti:
– Durán et al. who analized the binding medium in unrestored wall painting samples from the House of the Golden Bracelet in Pompeii and the Villa of Papyri in Herculaneum. The study by pyrolisis-chromatography-mass spectrometry, identifyied beeswax and soap7 .
– IR spectra of a Roman wall painting conserved in the National Archaeological Museum of Naples show bands attributed to beeswax and saponified compounds8 .
Other chemical studies also detect beeswax or components of the soap in Pompeiian wall paintings: G. Malquori identified these paintings the presence of fatty substances which would have produced a calcium soap9 . The study carried out by H. Kuhn via infrared spectroscopy indicated presence of beeswax in a Pompeiian mural10 .
The use of wax-and soap encaustic was not limited to the Vesubian area. Jorge Cuní carried out a chemical study of samples of Roman wall painting from Mérida (Spain), Complutum (Spain), and Marsala (Sicily) by means of gas chromatography – mass spectrometry and infrared spectroscopy. Samples had not been subjected to any restoration. All paint layers of the samples studied showed presence of beeswax and fatty acids. The study of fatty acids indicated that they were partially saponified, sugesting that the paintings were executed with a water-soluble encaustic composed of beeswax and soap11 .
The analysis by FT-IR and GC-MS of unrestored Roman wall painting samples from Ampurias, Cartagena and Baelo Claudia in Spain also showed presence of beeswax and soap both in the painted background colours and in the upper paint layers12 .
The study of infrared spectra of Roman wall painting samples from different periods and geographical areas, suggests a widespread use of water-soluble encaustic in Roman murals. At right, infrared spectra of a reference sample of beeswax and soap and of differant samples of Roman wall painting. It can be observed the correspondence between the composition of the reference sample and that of Roman binding media. Below, absorption bands corresponding to the components of beeswax and soap.