Non-invasive, non-destructive and portable identification of Indian yellow: Moving from highly purified standards to real watercolour, oil and historical samples

Abstract

The unambiguous identification of Indian yellow using spectroscopic techniques is challenging. Not long ago (2017), a reference spectrum of the pigment was produced using FT-Raman (an invasive and non-portable technique). Following this, the spectra of Indian yellow was also able to be acquired using confocal Raman microscopy (also non-portable) using VIS-NIR light sources. Finding a method to identify Indian yellow using portable instrumentation has many advantages when objects cannot be taken to a laboratory or is unfeasible to take microsamples. Here, results are described for the Identification of the pigment Indian yellow in unpurified historical samples and real samples in different media (watercolour, oil and tempera). The following techniques were used: fluorescent imaging, portable Raman (1064 nm) and dispersive Visible - Near Infrared (VIS-NIR) as well as Fourier transform infrared (FTIR) diffuse reflectance. The advantages and problems associated with the use of such instrumentation were discussed. It is possible to identify Indian yellow using portable instrumentation (Raman at 1064 nm excitation and chemometric analysis using NIR diffuse reflectance).