More than XRF Mapping: STEAM (Statistically Tailored Elemental Angle Mapper) a Pioneering Analysis Protocol for Pigment Studies

Abstract

Among the possible variants of X-Ray Fluorescence (XRF), applications exploiting scanning Macro-XRF (MA-XRF) are lately widespread as they allow the visualization of the element distribution maintaining a non-destructive approach. The surface is scanned with a focused or collimated X-ray beam of millimeters or less: analyzing the emitted fluorescence radiation, also elements present below the surface contribute to the elemental distribution image obtained, due to the penetrative nature of X-rays. The importance of this method in the investigation of historical paintings is so obvious—as the elemental distribution obtained can reveal hidden sub-surface layers, including changes made by the artist, or restorations, without any damage to the object—that recently specific international conferences have been held. The present paper summarizes the advantages and limitations of using MA-XRF considering it as an imaging technique, in synergy with other hyperspectral methods, or combining it with spot investigations. The most recent applications in the cultural Heritage field are taken into account, demonstrating how obtained 2D-XRF maps can be of great help in the diagnostic applied on Cultural Heritage materials. Moreover, a pioneering analysis protocol based on the Spectral Angle Mapper (SAM) algorithm is presented, unifying the MA-XRF standard approach with punctual XRF, exploiting information from the mapped area as a database to extend the comprehension to data outside the scanned region, and working independently from the acquisition set-up. Experimental application on some reference pigment layers and a painting by Giotto are presented as validation of the proposed method.

Atomic spectrometry update: review of advances in X-ray fluorescence spectrometry and its special applications

“…Even if only some of the spectra could be attributed to recognisable pigments, by combining the elemental distribution data and SAM maps a mixture of materials could be identified that matched the description given by the artist in his “ Small Treatise on Pictorial Technique ” (De Chirico in Abscondita, 2019). The importance of the macroXRF spectrometry scanning technique in the investigation of historical paintings was highlighted by Orsilli et al , 93 who discussed its advantages and limitations and emphasised the challenges of handling the large data set and the inhomogeneity of the data itself. An impressive pioneering protocol called STEAM (Statistically Tailored Elemental Angle Mapper) was based on the SAM algorithm and combined macroXRF standard data with point XRF spectrometry data.…”

Contribution of X-ray Fluorescence Techniques in Cultural Heritage Materials Characterization

“…Again on MA-XRF, the work by Orsilli and co-authors [7] summarizes the advantages and limitations of MA-XRF, here considered as an imaging technique. This allows a better synergy with other hyperspectral methods, or the combination with spot investigations.…”

Atomic spectrometry update: review of advances in the analysis of metals, chemicals and materials

“…A third paper to report the use of macro-XRF for the analysis of pigments was presented by Orsilli et al 369 These authors combined macro-XRF with Statistically Tailored Elemental Angle Mapper for the analysis of a painting by Giotto. The paper is partially an overview (with 54 references) that discussed the relative merits and limitations of using macro-XRF.…”