Screening and quantification of proteinaceous binders in medieval paints based on μ-Fourier transform infrared spectroscopy and multivariate curve resolution alternating least squares

This work aims at screening the presence and predicting the quantity of the binding media used to produce red and blue medieval paints based on the combined use of μ-Fourier-transform infrared (μ-FTIR) spectroscopy and multivariate curve resolution alternating least squares (MCR-ALS). Using different modes of application, MCR-ALS is proposed with a dual objective: 1) classical MCR-ALS is used for a first qualitative analysis of the presence/absence of certain binders in historical samples; 2) correlation constrained MCR-ALS is applied for quantitative analysis of binding media used to produce red and blue medieval paints and to provide structural spectral information about all compounds in the sample. Correlation constrained MCR-ALS introduces a constraint to establish internal calibration models for quantitative analysis in the resolution algorithm. This approach has been first applied to synthetic binder mixtures in order to check for the possibility of designing an appropriate calibration model to validate the quantitative analysis of the three binders that were potentially used in the medieval paints under study: parchment glue, egg white and egg yolk. Finally, classical and correlation constrained MCR-ALS were applied in a set of historical medieval manuscripts (illuminations) of three Portuguese Monasteries to provide qualitative and quantitative information about their binding media. Parchment glue and egg white have been often found separately or in mixtures of different proportions, whereas the egg yolk binder seems to be generally absent in all samples (or in very minor amounts in a few of them). Correlation constrained MCR-ALS resolution has provided reliable results with less rotational ambiguity than classical MCR-ALS with softer constraints. Quantitative values in historical samples should be interpreted only as current values, since there is no available knowledge about the effects of aging processes on the binders of these paints.