An application of near-infrared reflectance imaging spectroscopy on historical studies: differentiation of painting ground layers from the 15th and 16th centuries

During 15th and 16th centuries, while painters in Flanders or Italy used chalk (calcium carbonate) in ground layers of panel paintings, artists in Spain employed gypsum (calcium sulphate dihydrate). Thereby, the identification of these minerals in such artworks is relevant for historical studies, especially for that period owing to the active artistic exchange. Visible and near-infrared reflectance imaging spectroscopy is already applied to characterize pigments and binders. Besides, there are investigations suggesting that, owing to the transparency of some pigments mixed with binder in the near-infrared range, the signal coming from the subjacent substrate may be detected. This work proposes a methodology involving near-infrared reflectance imaging spectroscopy, for distinguishing between chalk- and gypsum-based ground layers. The methodology has been validated using reference samples of oil pigments applied on both preparation layers. Spectra of them reveal the distortions on the reflectance curves of pigments due to the ground layers. The hyperspectral images have been acquired with the VARIM 2.0 system, endowed with an InGaAs detector camera, coupled to an image spectrograph, covering the range 900-1700 nm. Several real cases have been examined. Study areas takes into account zones affected by retouching, rectifications and conservative interventions, previously identified by imaging techniques. The results prove the capability to discriminate gypsum and chalk on ground layers of panel paintings using a noninvasive methodology that involves near-infrared reflectance imaging spectroscopy and without sampling. Consequently, this technique can be a new tool to study the provenance of panel paintings from the 15th and 16th centuries.