Contents lists available at ScienceDirect Journal of Archaeological Science: Reports journal homepage: www.elsevier.com/locate/jasrep A multivariate analysis for enhancing the interpretation of infrared spectra of plant residues on lithic artefacts Alessandro Nucara a , Stella Nunziante-Cesaro b , Flavia Venditti c,d , Cristina Lemorini d, ⁎ a Department of Physics, “Sapienza” University of Rome, Italy b Scientific Methodologies Applied to Cultural Heritage (SMATCH), Italy c Institute of Archaeology, Tel Aviv University, Tel Aviv 69978, Israel d LTFAPA Lab., Department of Classics, “Sapienza” University of Rome, Italy ARTICLE INFO Keywords: PCA analysis FTIR analysis Residues analysis Lithic flint replicas Experiments ABSTRACT The reliability in using chemical techniques for the characterization of organic and inorganic ancient residues has been proved by several scientific works in the archeological literature. Among the different proposed approaches, Fourier Transform InfraRed Spectroscopy (FTIR) demonstrated to be a valuable method to detect and interpret the use-related residues entrapped in lithic chipped stone tools after their use. However, FTIR is affected by some restrictions that could be partially overcome thorough a statistical treatment of its results. With this contribution, we propose a methodological study based on Principal Component Analysis (PCA) as a statistical method to analyze infrared spectra collected on experimental residues on stone tools used to process vegetal materials. Here we show the ability of the PCA multivariate analysis to cluster samples with similar characteristics and to mine subtle spectral differences. According to our results, we proved the technique to be very suitable to discriminate vegetal materials, especially if belonging to herbaceous plants and underground storage organs (USOs) species where the likelihood to be recognized account for about 86% of possibilities. Our results demonstrate how the application of statistical methods such as PCA may amplify the interpretative capacity of the InfraRed Spectroscopy, especially when applied to archeological materials. Moreover, we contribute to build a representative spectral database of vegetal residues useful for scholars dealing with infrared measurements focused on the interpretation of the function of the archaeological artefacts. 1. Introduction The first systematic attempt to estimate the potential of FTIR technique to detect micro-residues trapped in the lithic surface of pre- historic artefacts was carried out by S. Nunziante-Cesaro and C. Lemorini within a project financed by the Wenner Gren Foundation, ICGR Grants 2009, 2011. The core of the project was the production of a reference collection of FTIR spectra of replicas of chipped stone tools used to process animal, plants and mineral materials useful for the functional interpretation of similar archaeological items. As a result, it was demonstrated that, both on experimental and archaeological lithic tools, this non-destructive technique allows to detect: 1) mineral pig- ments such ochre and cinnabar, 2) mineral components of organic materials such bone (hydroxyapatite) and shells (calcium carbonate), 3) presence of fat acid salt resulting from the degradation of the lipidic components of organic tissues, 4) lignin and cellulose (Nunziante- Cesaro and Lemorini, 2012; Lemorini et al., 2014). Furthermore, these articles highlighted the importance to integrate FTIR analysis with the use-wear observations. The combined application of these two in- dependent approaches proved to be crucial for cross-checking the re- sults and for the validation of the inferences obtained through each of the two methods of analysis. The project of S. Nunziante-Cesaro and C. Lemorini stimulated a fruitful debate regarding strengths and weaknesses of the application of FTIR technique in use-related residues investigation on chipped stone tools. Prinsloo and colleagues (Prinsloo et al., 2014) put emphasis on the possible technical problems of reflection and transmission/absor- bance using a microscope without an ATR (Attenuated Total Reflection) device and, as a result, in the difficulties of interpreting the resulting spectra. Since FTIR is a technique which allows the characterization of residues in situ (without the need of removing them from the artefacts), the authors suggested also producing spectra of the underlying stones (or other materials to which residues are attached) in order to verify the possibility that the spectrum from the substratum could eventually hide https://doi.org/10.1016/j.jasrep.2020.102526 Received 13 March 2020; Received in revised form 15 July 2020; Accepted 11 August 2020 ⁎ Corresponding author. E-mail address: cristina.lemorini@uniroma1.it (C. Lemorini). Journal of Archaeological Science: Reports 33 (2020) 102526 Available online 01 September 2020 2352-409X/ © 2020 Elsevier Ltd. All rights reserved. T