Recipe analysis, reconstruction, and re-enactment: historical technological sources on the cleaning of easel paintings

Analysis of over 180 historical documentary sources from 1600 to 1900 covering the cleaning of oil paintings established that the use of alkalis was mentioned in over 50% of references to aqueous cleaning. Alkalis could be employed as a paste with wood ashes and water or in the form of lye. Historical alkali cleaning recipes based on wood ashes were reconstructed and characterised using titration and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Results indicate that pH depends on the wood-ash source, the age of both the ashes and the lye, as well as on the proportion of ash to water. Reconstructions of lye solutions and wood-ash pastes were applied to naturally aged model oil paints and oil paintings. Their visual impact was investigated using high resolution imaging, and chemical changes were studied with infrared spectroscopy in attenuated total reflection (ATR-FTIR) and focal-plane-array Fourier transform infrared spectroscopy (FTIR-FPA) as well as scanning electron microscopy equipped with energy-dispersive X-ray analysis (SEM-EDX). These latter investigations determined the presence of potassium within the upper layers of paint samples after exposure to wood ash lye. The potassium remained in the oil network after extensive water rinsing. This research adds information to our existing knowledge about historical cleaning methodologies and contributes to a better understanding of the current condition of paintings previously cleaned with alkalis.