Calibration of Raman Spectroscopy at 1064 nm for Beeswax Quantification

Abstract

In the early sixties, coating with molten beeswax was considered a valuable method for preventing the erosive action of weather and/or salinity on the surface of granite sculptures and monuments. This technique had been traditionally employed by the Galician stoneworkers for partial repair of historical monuments. For this purpose, beeswax was applied to the Renaissance Frieze in the Cloister of the Cathedral of Santiago de Compostela in Galicia (Northwest Spain). The beeswax treatment was counterproductive. An intense grain disaggregation of the granite can be observed in the Frieze, owing to the crystallization of salts. As a consequence, the restoration of the Cloister presents many problems. This fact imposes the need for an exhaustive study of the wax–stone system and the demand for a nondestructive method to measure the beeswax thickness at the stone surface. The aim of this contribution is the evaluation of a laser-based method, namely Fourier transform Raman spectroscopy, for analyzing the wax presence in specific rocky material of the Frieze to be restored. To obtain a reliable quantitative calibration, we prepared beeswax films of five different thicknesses on aluminum plates (26.6–97.2 μm). Nylon was selected as external reference to obtain the Raman emission independently from the laser beam power. The ratios of the relative intensities of the Raman bands corresponding to beeswax and nylon were used for the construction of a calibration curve used for the quantitative analysis. The intensities at 2879 cm⁻¹, I_c2879, and 2880 cm⁻¹, I_n2880, for beeswax and nylon, respectively, in the Raman spectra of each material were used. A linear dependence was found for the ratio I_c2879 / I_n2880 with the beeswax thickness. The validation of this calibration curve was tested with a second validation set of samples that spans beeswax film thicknesses both inside and outside the calibration range (12.1 to 180 μm), in order to evaluate in addition the accuracy of the model at extrapolation. Without complex sample preparation, near-infrared Raman spectroscopy resulted in an effective technique for localizing the wax with lateral resolution of tens of micrometers, and for determining wax layer thickness in the stone with an uncertainty of a few micrometers.