The effects of selected cleaning agents on indigo paint films

Abstract

Indigo is a blue organic colourant produced from species of the plant genus Indigofera. Its primary use is as a textile dye, but it is also used as a pigment. Although extensively used as an artist's pigment throughout the ages, indigo's poor colourfastness was always known. Fading of indigo is primarily a result of photo oxidation, but it is also susceptible to alter due to chemical reagents. This study focuses on the effects of selected reagents used by conservators on indigo and linseed oil paint films. It aims at identifying some of the physical, optical and chemical change of indigo paint films due to cleaning. Artificially-aged replica paint films are used as to simulate original paint films whilst also allowing for more invasive test methods and analyses. Based on the apparatus available, the analyses applied to this study were Scanning electron microscopy (SEM), Fourier-Transform Infrared spectroscopy (FTIR) and Spectrophotometry. Complementary to the results observed, Nuclear Magnetic Resonance (NMR) was also used. Reagents from different chemical classes were applied by cotton swab rolling to the surface of indigo oil paint films and indigo and lead-white paint films. Surface alterations due to mechanical action were observed in SEM photomicrographs. Using spectrophotometry, colour change was noted as an effect of water. FTIR results, although limited, showed that organic solvents swell the paint film but do not affect it chemically; whereas aqueous reagents can possibly interact with the paint through hydrogen bonding. Overall, it was observed that lead white increases the resistance of the dried paint film.