A multi-analytical approach to investigate the state of conservation of the wall paintings of Insula 104 in Hierapolis (Turkey)

• A protocol for on site and in laboratory analyses of salts is proposed for a very famous archaeological site.
• Nondestructive and microinvasive techniques were applied.
• The presence of soluble salts was related to water circulation and environmental parameters.
• The role of environmental conditions on state of conservation of wall paintings was investigated.

Insula 104 is a residential area dating to the proto-Byzantine period (second century AD), inside the wide archaeological site of Hierapolis of Phrygia (Turkey). Today, the domus, which probably belonged to one of the aristocratic families of the city, has been damaged due to salt crystallization on the wall paintings of some of the rooms. In order to select the best type of conservation interventions, various diagnostic techniques were used to determine the nature and the origin of the soluble salts. In particular, the wall paintings from room A 1207 in the House of the Doric Courtyard and room A 1267 in the House of the Painted Inscription were investigated by non-invasive techniques, followed by sampling for laboratory analyses. Rising damp combined with the presence of salts is a critical issue for the aforementioned wall paintings. On site screening by portable techniques (IR thermography, Quantofix test strips) was performed to address the microsampling for the analyses. The data on both the chemical and mineralogical composition of the efflorescences revealed the presence of sulphates (mainly thenardite, aphthitalite and syngenite), sodium and potassium nitrates, and chlorides (halite and sylvite). The original and restoration mortars were also analysed to examine their impact on the conservation state of the wall paintings. The distribution of the salts and their typologies inside each room was correlated to the different uses of the rooms over the centuries and to previous restoration works.