Abstract
Spectral preprocessing data and chemometric tools are analytical methods widely applied in several scientific contexts i.e., in archaeometric applications. A systematic classification of natural powdered pigments of organic and inorganic nature through Principal Component Analysis with a multi-instruments spectroscopic study is presented here. The methodology allows the access to elementary and molecular unique benchmarks to guide and speed up the identification of an unknown pigment and its recipe. This study is conducted on a set of 48 powdered pigments and tested on a real-case sample from the wall painting in S. Maria Delle Palate di Tusa (Messina, Italy). Four spectroscopic techniques (X-ray Fluorescence, Raman, Attenuated Total Reflectance and Total Reflectance Infrared Spectroscopies) and six different spectrometers are tested to evaluate the impact of different setups. The novelty of the work is to use a systematic approach on this initial dataset using the entire spectroscopic energy range without any windows selection to solve problems linked with the manipulation of large analytes/materials to find an indistinct property of one or more spectral bands opening new frontiers in the dataset spectroscopic analyses.
Highlights
Classification of different types of pigments extracting the characteristic variables with chemometrics and non-destructive spectroscopic techniques are the new trend scenario [1,2].X-ray fluorescence (XRF) and Fourier transform infrared spectroscopy (FTIR) stand out [3].Significant shifts of the characteristics bands from pigments made it a challenge to interpret the spectra [3].In the case of painted surfaces [4], micro-sampling is required to confirm the type of the pigment by additional destructive techniques or model samples to compare the data [5]
Along the PC1 that is associated to the elemental distribution, the samples are distributed according to their colour scale: in the Cu group, for example, the Verdigris (GCU), Malachite (GM), Azurite (BLA), and Chrysocolla (GC) are distributed from the green to the blue/green colours; a similar trend is observed for the red/yellow ochre that spans from the dark reddish-brown (RM) to the yellow (YG, IV quadrant)
The association with the colour and its shadows is linked with the predominant elemental marker/chromophore that is the element with the higher peak area
Summary
In the case of painted surfaces [4], micro-sampling is required to confirm the type of the pigment by additional destructive techniques or model samples to compare the data [5]. In this framework, a lot of parameters need to be considered such as the raw materials [6,7,8,9,10,11], the specific technique and the specific setup [12,13]. Their composition provides complex spectrum in which the interpretation of spectral features for extraction of information would be ambiguous [14]
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