Cultural heritage study: Microdestructive techniques for detection of clay minerals on the surface of historic buildings

Clay minerals often occur in the finishing layers covering the surface of historic and architectural artefacts. They may come from ochres intentionally applied or from atmospheric dust naturally deposited. In the first case the finishes could testify to treatments applied for the stone conservation and/or aesthetic purposes, whose study is of great interest in conservation activities.In this paper we report on the application of microdestructive analytical techniques in order to identify the presence and the origin of clay minerals found on the surfaces of several historic buildings located in the Apulia region (Southern Italy). Optical and SEM observations, along with EDS microanalyses, and FTIR analyses were performed. The microdestructive character of these techniques made it possible to carry out the analyses on the same specimen, thus reducing the sampling. XRD analyses were also performed to confirm the presence of clay minerals. The complementary analytical findings were helpful in most cases in establishing the real origin of the clay minerals; the detection of organic materials arising from man-made treatments allowed to identify clays as pigments. However, in some cases the uncertainty remained between ochre, intentionally employed, and soil–dust deposition.

Research Highlights
► Clay minerals contribute to composition of superficial patinas of historic buildings.
► Integrated microdestructive analyses on the same specimen allow reduced sampling.
► Clay minerals in patinas are usually pigments where organic binders are also present.
► Clay minerals in patinas may frequently come from soil–dust deposition.