**1. Introduction**

Scientific analysis of artworks is often performed within the limits of non-invasiveness requirements, which automatically exclude any material sampling or contact measurement. Meeting such requirements, however, may prove especially challenging when studying paper-based drawings and paintings, whose extremely light-sensitive nature often demands fixed lighting conditions. Minimizing light exposure during measurements without compromising data significance means striking an effective balance between spatial sampling (pixel size) and spectral resolution— an extremely delicate task at best [1–3]. Furthermore, the limited variety of artistic materials typically found in ancient drawings makes it difficult to assess their provenance and authenticity in the absence of proper historical documentation [4]. Numerous studies indicate that analysis of paper-based artworks is best performed when applying a synergic approach that combines non-invasive analytical tools and complementary optical techniques [5]. In the past few decades, spectral imaging processing has been successfully combined with site-specific chemical methods, e.g., Raman spectroscopy [6,7], X-ray fluorescence (XRF) [8,9], fiber optics reflectance spectroscopy (FORS) [10], and particle-induced X-ray emission (PIXE) [11], mainly for the identification and mapping of pigments in medieval illuminated manuscripts and painted books [2,4,8,12,13]. Stratigraphic analysis of miniatures and ancient books has also been performed with infrared thermography (IRT) [14,15] and XRF [15] to highlight the presence of structural defects, such as detachments of the gildings. However, fewer scientific data are available when it comes to drawings on paper, possibly due to their extreme fragility.

**Citation:** Dal Fovo, A.; Striova, J.; Pampaloni, E.; Fontana, R. Unveiling the Invisible in Uffizi Gallery's Drawing 8P by Leonardo with Non-Invasive Optical Techniques. *Appl. Sci.* **2021**, *11*, 7995. https://doi.org/10.3390/ app11177995

Academic Editor: Asterios Bakolas

Received: 27 July 2021 Accepted: 23 August 2021 Published: 29 August 2021

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As a result, analysis of paper-based drawings has thus far been primarily conducted within the historical-conservative field, with only a handful of published studies on drawings by old masters over the 15th–17th centuries contributing to the relevant literature [16–19]. Despite such limitations, the supporting value of scientific data in this regard has proved increasingly crucial in order to gain further insights into drawing materials and techniques used by artists in the past. For instance, art history investigations on Leonardo da Vinci's drawings carried out in the past two decades have successfully integrated analytical measurements to identify constituting materials, delineate the artist's modus operandi, and characterize material deterioration over time [20–22]. Suffice to mention Leonardo's drawings belonging to the Biblioteca Reale in Turin, which were recently analyzed with macro X-ray Fluorescence (MA-XRF), μ-Raman spectroscopy, and atomic force microscopy (AFM) to identify their constituting materials and assess their state of conservation [23,24]. This line of research clearly points to the need for further integration of scientific methods to gain a thorough understanding of Leonardo's creative process.

In this work, we report on the application of non-invasive multi-modal analysis on a double-side drawn sheet by Leonardo, *Drawing 8P* (Figure 1), from the Uffizi Gallery of Florence, Italy. Our study is part of a research campaign carried out by Opificio delle Pietre Dure in collaboration with the National Research Council (CNR) and the National Institute of Nuclear Physics (INFN) within the framework of the exhibition held in Leonardo's birth town (Vinci) to commemorate the five hundredth anniversary of his death (2 May 1519) [25]. The commemoration year also saw other research campaigns, such as the set of investigations on this sheet performed at the University of Bologna (for further reference see [26]). We used multispectral reflectography in the visible (Vis) and nearinfrared (NIR) to perform the spectral mapping of the drawing materials, whose elemental composition had been previously characterized by XRF [27]. Morphological micro-features, i.e., not visible to the naked eye, were revealed thanks to the combined application of laser scanning microprofilometry and spectral-domain optical coherence tomography (Sd-OCT). The resulting visible set of hidden details related to the drawing technique allowed us to provide new insights into Leonardo's still under-investigated graphic production.

**Figure 1.** Leonardo da Vinci, Landscape, 1473, Firenze, Gallerie degli Uffizi, Gabinetto dei Disegni e delle Stampe, inv. 8 P, size 194 × 285 mm, drawing on paper. RGB images of the recto (**a**) and the verso (**b**) acquired via multispectral scanner. The black dashed rectangles highlight the mirrored left-handed (*i*) and the right-handed (*ii*) writings magnified below. Region (*i*) is shown as it is and flipped horizontally (central image) to facilitate the reading of the inscription.
