Raman scattering or fluorescence emission? Raman spectroscopy study on lime-based building and conservation materials

• A considerable set of building materials Raman spectra is given.
• Lime binder spectra excited with the 1064 nm are a fluorescence effect.
• Fluorescence bands are observed in the 1064 nm-spectra of carbonated lime.
• The origin of the fluorescence of lime-based materials are discussed.

This work presents an in-depth study on Raman spectra excited with 1064 and 532 nm lasers of lime binders employed in the past as building materials and revealed today as valuable conservation materials. We focus our interest on the bands of strong intensity, which are present in the spectra of all binders acquired with laser excitation at 1064 nm, but absent in the corresponding spectra acquired with laser excitation at 532 nm. We suggest, that the first group of spectra represents fluorescence phenomena of unknown origin and the second true Raman scattering. In our studies, we also include two other phases of lime cycle, i.e. calcium carbonate (a few samples of calcite of various origins) and calcium oxide (quicklime) to assess how structural and chemical transformations of lime phases affect the NIR-Raman spectral profile. Furthermore, we analyse a set of carbonated limewashes and lime binders derived from old plasters to give an insight into their spectral characteristics after excitation with the 1064 nm laser line. NIR-Raman micro-mapping results are also presented to reveal the spatial distribution of building materials and fluorescent species in the cross-section of plaster samples taken from a 15th century chapel. Our study shows that the Raman analysis can help identify lime-based building and conservation materials, however, a caution is advised in the interpretation of the spectra acquired using 1064 nm excitation.

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