Unmixing-based cleaning methods evaluation for re-polychromated plasterwork

Abstract

The cleaning of re-polychromated plaster is a rather challenging task for Conservators. The optimal choice of solvents and its application instruments may depend on the original polychromy (binder and pigment) that aims to be revealed and the overlay polychromy (binder and pigment) as well. We present a new image-processing based method that uses hyperspectral imaging to obtain a non-invasive quantitative evaluation of cleaning procedures. It is based on the application of unmixing techniques for this problem. The method computes three quality indices: efficiency, destructivity, and alteration, that are based on the residual amounts of overlay pigment, the detection of excessive abrasion in the sample and the change in color that the cleaning produces. It produces as well pixel-by-pixel concentration maps and presence maps for plaster and overlay pigment, and it can self-evaluate by computing the error map comparing the modelled spectral reflectance with the measured spectral reflectance from the samples.