Characterisation of organic colourants in ukiyo-e prints by Fourier transform near infrared fibre optics reflectance spectroscopy

Author Summary: The materials used in Japanese woodblock prints called ukiyo-e, particularly colouring matters, function as indicators on the resources available at a given time and allow us to deepen our knowledge of the cultural purposes and the socio-economic (trading activity, import of new techniques…) impacts of these artworks. The aim of this work is to apply non-invasive near infrared (NIR) spectroscopy which may be helpful for the identification of organic colourings widely employed in ukiyo-e, using fibre optics such as the Fibre Optics Reflectance Spectroscopy (FT-NIR FORS), that has been applied to the cultural heritage field for two decades because of its fast response time in any geometrical configuration. The present work reports results obtained on five organic pigments (indigo, safflower, gamboge, dragon’s blood and cochineal), considered as the first step for the building of a database dedicated to specific Japanese colouring matters in order to interpret future hyperspectral data recorded in the Short Wave InfraRed (SWIR) range on a collection of Japanese ukiyo-e prints. The data were obtained with a FT-NIR spectrometer using a probe to collect the specular reflection within the 11,000–4000 cm–1 range, transformed in pseudo-absorbance with log(1/R) conversion. Because large bands were observed, originating in overtone and combination modes, a pre-processing smoothing and derivative procedure of the data, together with Principal Component Analysis (PCA) were applied to discriminate the pigments, notably dragon’s blood.