Direct microextraction for red lakes detection in painting layers by Raman spectroscopy

Identification of red lakes--obtained by precipitation or adsorption of dyes onto an inorganic substrate--is of fundamental importance for the study of their manufacturing technology and for their conservation, but this poses an analytical challenge. We show how the detection of red lakes in painting layers by Raman spectroscopy can be enhanced. We propose to exploit the solubility of red lakes and their subsequent recrystallization in a hydrogel as a way to isolate the lake from the painted matrix. This analytical method has been found to be an effective tool for the identification of cochineal and madder lakes present in painting layers. The obtained results are compared to available experimental data and DFT calculations, and it is possible to define the organometallic complex present in the hydrogel after the micro-extraction process. We test and demonstrate the efficiency of this methodology on naturally aged mock-up panel paintings and on a historical painting. Reflectance spectra and colorimetric measurements by Fiber Optics Reflectance Spectroscopy are employed to support the Raman data and to evaluate the colorimetric parameters following the micro-extraction.