Raman spectroscopic study of the degradation of a middle age mural painting: the role of agricultural activities

Abstract

The present work was focused on the investigation of the original and decayed materials as well as the degradation phenomena affecting the middle age mural painting preserved in the Assumption's church of Alaiza (Basque Country, Spain). Portable Raman spectroscopy assisted with energy dispersive X‐ray fluorescence was used to identify the nature of the original compounds and to distinguish them from the following: (1) those used in recent restorations and (2) those considered as decaying compounds. The information obtained by this technique was used to select the sampling areas to perform laboratory Raman measurements and soluble salt test, supported by the use of ion chromatography in an attempt to clarify the degradation processes of the mural paintings. These analyses suggested the connection of the deterioration problems with the agricultural activity of the environment surrounding the church. For example, the use of industrial common salt (unpurified NaCl) against the snow and ice, as well as common fertilizers such as NH4NO3 and (NH4)2SO4, generates Na+ and NH4+ cations together with Cl−, NO3−, and SO42−anions that ascent the basal part of the walls causing the decarbonation of calcite and the subsequent disintegration of the mortar. Sulfate compounds were detected in the lower parts of the walls (below half a meter), while nitrate compounds were detected in the higher parts, at heights greater than 1 m. This work is a clear example of the conservation problems in cultural heritage related to the characteristics of the surrounding environment. Copyright © 2014 John Wiley & Sons, Ltd.