Simulating the composition and structuration of coloring layers in historical painting from non-invasive spectral reflectance measurements

Abstract

Reflectance spectroscopy is a powerful non-invasive technique for determining the material composition of historical polychromies, since the measurement is fast, simple, and contactless. However, reflectance spectra of complex color mixtures can sometimes be hard to interpret from a compositional point of view. In these cases, theoretical optical simulations can provide useful additional data. The main issue is the choice of the optical model that must be adapted to the measurement protocol and the material structure of the coloring layer, this latter being generally unknown. Simple models based on analytical formulas are preferred, as they can be easily inversed to deduce the optical and structural properties of the materials from the measured spectral reflectance of the object. In this paper, we address this issue to investigate the material composition of the colors of the Codex Borbonicus, a 16th-century Mesoamerican manuscript. Two models dedicated to two different types of material structures are presented: the Kubelka–Munk model with Saunderson correction, suitable for one homogenous layer, and the Clapper–Yule model used for continuous colorant layer, suitable when a weakly scattering paint is on top of a diffusing support. The results of the simulation provide new insights into the way the coloring materials were combined in the document, either as mechanical mixture before application or as superimposition.

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