**1. Introduction**

The study of painting in México is an important challenge due to the number of preserved artworks over a long period (16th–21st centuries) of time, with different pictorial traditions. In this research area, each technical study provides a piece of the puzzle, and contributes to the understanding of the heritage legacy developed through generations.

*Purísima Concepción* is an artifact of time and technique, and an artistic document in which different plastic and formal solutions can be studied. Its research strategy, defined by the study of its technology, materiality, and state of conservation through imaging and spectroscopic techniques, can complement the understanding of its temporality and material meaning of its production related to the 16th, 17th, and 18th centuries. By using imaging techniques, underlayer versions representing the Virgin Mary were recently discovered, as shown in [1], and the study of those layers will help understand the changes and formulations of its meaning. Meanwhile, the upper layer composition of Virgin Mary coronated by the angels, related to the 18th century, and the relevance of the *Museo Exconvento San Agustín Acolman-INAH* are discussed in this paper, and should provide a starting point for further and deeper studies of the currently hidden layers. The painting technique from the 18th century Novo-Hispanic period is scarcely studied, and it is unknown if there was a continuity of materials, anonymous painters, and geography. This research also seeks to provide an insight on such subjects.

The study of cultural heritage requires the use of non-invasive techniques and a multidisciplinary approach, in order to generate significant knowledge while preserving

**Citation:** Pérez, M.; Cano, N.; Ruvalcaba-Sil, J.L.; Mitrani, A.; de Lucio, O.G. Technical Non-Invasive Study of an 18th Century Novo-Hispanic Panel Painting. *Heritage* **2021**, *4*, 3676–3696. https://doi.org/10.3390/ heritage4040202

Academic Editors: Daniel Vazquez-Molini and Antonio Alvarez Fernandez-Balbuena

Received: 31 August 2021 Accepted: 13 October 2021 Published: 19 October 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the studied objects. It has been necessary to develop methodologies where complementary techniques provide pieces of information that, when assembled, deliver the *whole picture* on the material properties and historical importance of an object. It is in this fashion that XRF spectroscopy data provides unequivocally elemental information in order to discriminate the material composition of a pigment, and FORS information provides a chemical signature of pigments and binders. Both are known to be powerful techniques, but are limited to analyze a single point, and usually require complex interpretations if a detailed description of the manufacture of an object is required. In contrast, some imaging techniques, such as digital radiography, ultraviolet photography, and even visible photography, can provide global information on a given object, and usually enhance details on the production of the studied object.

Hyperspectral imaging (HIS) combines properties of both spectroscopic and imaging techniques. Therefore, it is not limited to a local analysis (as with spectroscopic techniques), but instead, it can deliver a global description of the studied object. The generated images contain physical or chemical information, describing the distribution maps of certain materials. Therefore, results are not limited to a mere material study, but they can also provide—through an adequate analysis and interpretation—insights on the manufacturing process of the object, information regarding later interventions or modifications, and even its preservation state. HSI analysis has recently proven to be useful for heritage sciences [2–6]. In fact, it has become a well appreciated analysis technique due to its intrinsic characteristics: it is a non-invasive and portable technique which provides results over large areas in a relatively short time.

As recent examples of the use of HSI in heritage science studies, we have: the work by Pérez et al. [7], where different manufacturing stages were identified in the painting *The Pentecost* by Baltasar de Echave Orio (1558–1619); the work by Padoan et al. [8], in which HSI was used to monitor preservation and aging conditions of historical documents; the study by Sun et al. [9], where different degrees of flaking deterioration were evaluated in mural paintings at the *Mogao Grottoes* by using NIR-his; and the work of Hou et al. [10], where a combination of HSI information and multivariable analysis algorithms were employed to create a virtual restoration of ancient Chinese Qing dynasty paintings.
