Optimisation of Raman analysis of walnut oil used as protective coating of Galician granite monuments

In the middle of last century, many valuable monuments of the Galician cultural heritage showed disintegration of the granite in the outdoor walls due to chemical erosion. To repair and conserve the stone from further deterioration, restorers decided to apply protective coatings that might act as consolidants and as hydrofugants. It is important to study these coatings in order to understand the processes that affect their conservation and deterioration and to design protocols for any future restoration action that might be necessary. Raman spectroscopy presents several advantages for this kind of application since it is a versatile technique, does not need sample preparation and provides a non-destructive method. In this work, different types of walnut oil are analysed. A comparative study was performed using five different laser excitation wavelengths, and 532 and 488 nm turned out to be the most appropriate wavelengths for oil detection on aluminium substrates. Additionally, Raman spectroscopy allows the study of the evolution of the treatment once applied, as well as the determination of the curing times and chemical transformation. Raman analysis of walnut oil over Roan granite using excitation at both 532 and 488 nm are viable for in situ testing, but 532 nm offers higher sensitivity. Copyright (C) 2010 John Wiley & Sons, Ltd.