A smartphone-based innovative approach to discriminate red pigments in roman frescoes mock-ups

The characterization of red pigments in frescoes wall paintings has been of great interest for researchers to better understand raw material procurement dynamics, pigment receipts, stylistic evolution and to assess their conservation state. In this study a non-destructive colorimetric approach implementing a smartphone-based method was developed in order to be able to distinguish between three pigments made from minium, haematite and cinnabar minerals, and also mixed pigments, preparing frescoes mock-ups following the roman receipt described by Vitruvius. Portable FT-IR, Raman spectroscopy, portable XRF and visible reflectance spectra analyses were carried out as reference methods for smartphone colorimetry results validation. Employing a reference colour sheet to control changing lighting conditions, different chemometric approaches have been developed and tested, cross-referencing standard analytical results with the data obtained by smartphone. Overall, using only colour parameters from the smartphone, a Linear Discriminant Analysis and a Support Vector Classifier were tested to efficiently classify each sample based on the red pigment used, with low prediction errors. This work shows the potential of smartphones as cheap, fast and user-friendly analytical devices for the screening of frescoes, and as a prior selective step before carrying out further more expensive and specialized analyses.