1.2.4. X-ray Fluorescence Spectroscopy (XRF)

A *home built* portable X-ray system (SANDRA) was used for recording the elemental composition of the studied object, this system and its applications are described in detail elsewhere [17]. In brief, the system employed a Mo X-ray source collimated to a 1 mm area, and an Amptek SDD detector with detection angle fixed at 45◦ . Calibration of the system is achieved by using a NIST multicomponent glass standard reference material (SRM 1412). Irradiation parameters were 45 kV, 0.200 mA with a recording time of 90 seconds, and no filters were used during the spectra acquisition. Spectra analysis and deconvolution was performed by means of ESRF-BLISS PyMca software.

Due to the existence of an underlying painting, interpretation of XRF data could be complex, since the range of photons in matter allow them to penetrate hundreds of micrometers, encompassing not only the surface, but most of the layers of the painting. However, the physics ruling the interactions between X-rays and solids, such as the attenuation of photons in matter, provide a testing ground for determining the depth from which the recorded photons arise. We considered the Kα/K<sup>β</sup> (or Lα/Lβ, depending on

the studied element) relative intensity ratio of the representative elements in each pigment, since the attenuation strongly depends on the energy (especially for the energy range involved). When the ratio corresponded (within uncertainties) with predictions from databases, XRF data was then related with the surface layer, otherwise data were associated with deeper layers pretation of the results provided by the spectroscopic techniques. Imaging results are presented as mosaics, where the different acquired images of one region are contrasted: visible; UV fluorescence; IR pseudo-color (IRPC); spectral angle mapping (SAM); a pseudo-

*Heritage* **2021**, *4* FOR PEER REVIEW 5

of photons in matter, provide a testing ground for determining the depth from which the

recorded photons arise. We considered the Kα/Kβ (or Lα/Lβ, depending on the studied

element) relative intensity ratio of the representative elements in each pigment, since the

attenuation strongly depends on the energy (especially for the energy range involved).

When the ratio corresponded (within uncertainties) with predictions from databases, XRF

data was then related with the surface layer, otherwise data were associated with deeper

Results of the spectroscopic techniques (FORS, XRF) are grouped by the binding me-

dia and the materials associated by the color palette of this painting: golden; blue; red;

brown; green; flesh tones. Results from the imaging techniques are grouped by following

the main areas of the painting composition: the Virgin's face and hands; the crown; and

the right-side angel. These images provided the most relevant information of the upper

layer version of Mary. The analysis always considered the underlayer versions registered

by the radiographic images, which are described in detail in [1], to achieve a correct inter-

## **2. Results and Discussion** color image composed by the three main PC (PC-1,2,3); principal component 1(PC-1); and

layers

**2. Results and Discussion** 

Results of the spectroscopic techniques (FORS, XRF) are grouped by the binding media and the materials associated by the color palette of this painting: golden; blue; red; brown; green; flesh tones. Results from the imaging techniques are grouped by following the main areas of the painting composition: the Virgin's face and hands; the crown; and the right-side angel. These images provided the most relevant information of the upper layer version of Mary. The analysis always considered the underlayer versions registered by the radiographic images, which are described in detail in [1], to achieve a correct interpretation of the results provided by the spectroscopic techniques. Imaging results are presented as mosaics, where the different acquired images of one region are contrasted: visible; UV fluorescence; IR pseudo-color (IRPC); spectral angle mapping (SAM); a pseudo-color image composed by the three main PC (PC-1,2,3); principal component 1(PC-1); and three selected different principal components (PC-n). three selected different principal components (PC-n). *2.1. Purísima Concepción: Materials and Painting Technique*  Results from the spectroscopic techniques are divided in a description of the binding media and color palette. In a first approximation, the color palette appears to be simple, with predominance of golden, blue, and red hues. However, the interpretation of the studies applied show a great diversity of materials employed to achieve effects such as bright-
