Attenuated total reflectance microspectroscopy mapping for the characterisation of bronze corrosion products

Abstract

Samples collected from an archaeological object and an outdoor bronze monument as well as a bronze coupon naturally aged in an outdoor marine environment have been analysed in order to determine the composition of the corrosion and deposition compounds and map their stratigraphic distribution. Cross-sectioned samples and the bronze coupon have been placed onto the x-y-z motorized stage under the objective of an FTIR microscope and both punctual analysis and mapping of small areas was performed in ATR mode. The combination of visible light microscopy, which allows the microphotograph documentation of chemically heterogeneous samples, with FTIR-ATR mapping, is of the outmost importance in the characterisation of the corrosion features and behaviour of a wide variety of corroded bronze samples streamlining the selection of the most appropriate conservation materials and cleaning procedures to be adopted by conservators. Limitations, such as the spatial resolution of about 20 μm and silicon crystal opacity, are compared with the great advantage of obtaining distribution false colour plots of the different compounds. As non-destructive of the sample analytical technique ATR mapping can further be combined with other analytical techniques such as metallography, X-ray EDS and Raman spectroscopy. The limitation of the ATR technique in identifying important corrosion products, which do not absorb in the mid infrared, and the need to be combined with other type of analysis is also highlighted.