Unveiling the Invisible in Uffizi Gallery's Drawing 8P by Leonardo
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applied sciences Article Unveiling the Invisible in Uffizi Gallery’s Drawing 8P by Leonardo with Non-Invasive Optical Techniques Alice Dal Fovo , Jana Striova , Enrico Pampaloni and Raffaella Fontana * National Research Council—National Institute of Optics (CNR-INO), L.go E. Fermi 6, 50125 Firenze, Italy; [email protected] (A.D.F.); [email protected] (J.S.); [email protected] (E.P.) * Correspondence: [email protected] Abstract: Until recently, the study of drawings by old masters has been confined to the art history conservation field. More specifically, scientific investigations of Leonardo’s drawings are still very few, possibly due to the latter’s extreme fragility and artistic value. However, analytical data are crucial to develop a solid knowledge base of the drawing materials and techniques used by artists in the past. In this work, we report on the application of non-invasive optical techniques on a double- sided drawing by Leonardo belonging to the Uffizi Gallery (8P). We used multispectral reflectography in the visible (Vis) and near-infrared (NIR) regions to obtain a spectral mapping of the drawing materials, to be subsequently integrated with technical information provided by art historians and conservators. Morphological analysis by microprofilometry allowed for the identification of the typical wave-like texture impressed in the paper during the sheet’s manufacture, as well as of further paper-impressed traits attributable to the drawing transfer method used by Leonardo. Optical coherence tomography revealed a set of micrometric engraved details in the blank background, which lack any trace of colored material, nor display any apparent relation to the drawn landscape. Citation: Dal Fovo, A.; Striova, J.; The disclosure of hidden technical features allowed us to offer new insights into Leonardo’s still Pampaloni, E.; Fontana, R. Unveiling under-investigated graphic practices. the Invisible in Uffizi Gallery’s Drawing 8P by Leonardo with Keywords: Leonardo da Vinci; drawing; multispectral reflectography; microprofilometry; optical Non-Invasive Optical Techniques. coherence tomography Appl. Sci. 2021, 11, 7995. https://doi.org/10.3390/ app11177995 Academic Editor: Asterios Bakolas 1. Introduction Scientific analysis of artworks is often performed within the limits of non-invasiveness Received: 27 July 2021 requirements, which automatically exclude any material sampling or contact measure- Accepted: 23 August 2021 ment. Meeting such requirements, however, may prove especially challenging when Published: 29 August 2021 studying paper-based drawings and paintings, whose extremely light-sensitive nature often demands fixed lighting conditions. Minimizing light exposure during measurements Publisher’s Note: MDPI stays neutral without compromising data significance means striking an effective balance between spa- with regard to jurisdictional claims in tial sampling (pixel size) and spectral resolution— an extremely delicate task at best [1–3]. published maps and institutional affil- Furthermore, the limited variety of artistic materials typically found in ancient drawings iations. 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 art- works 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 Copyright: © 2021 by the authors. imaging processing has been successfully combined with site-specific chemical methods, Licensee MDPI, Basel, Switzerland. e.g., Raman spectroscopy [6,7], X-ray fluorescence (XRF) [8,9], fiber optics reflectance spec- This article is an open access article troscopy (FORS) [10], and particle-induced X-ray emission (PIXE) [11], mainly for the distributed under the terms and identification and mapping of pigments in medieval illuminated manuscripts and painted conditions of the Creative Commons books [2,4,8,12,13]. Stratigraphic analysis of miniatures and ancient books has also been Attribution (CC BY) license (https:// performed with infrared thermography (IRT) [14,15] and XRF [15] to highlight the presence creativecommons.org/licenses/by/ of structural defects, such as detachments of the gildings. However, fewer scientific data 4.0/). Appl. Sci. 2021, 11, 7995. https://doi.org/10.3390/app11177995 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 7995 2 of 12 Appl. Sci. 2021, 11, 7995 2 of 12 performed with infrared thermography (IRT) [14,15] and XRF [15] to highlight the pres- ence 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 fra- are available when it comes to drawings on paper, possibly due to their extreme fragility. gility. As a result, analysis of paper-based drawings has thus far been primarily conducted As a result, analysis of paper-based drawings has thus far been primarily conducted within within the historical-conservative field, with only a handful of published studies on draw- the historical-conservative field, with only a handful of published studies on drawings ings by old masters over the 15th–17th centuries contributing to the relevant literature by old masters over the 15th–17th centuries contributing to the relevant literature [16–19]. [16–19]. Despite such limitations, the supporting value of scientific data in this regard has Despite such limitations, the supporting value of scientific data in this regard has proved provedincreasingly increasingly crucial incrucial order in to order gain further to gain insights further into insights drawing into materialsdrawing andmaterials techniques and techniquesused by artists used in by the artists past. in For the instance, past. Forart inst historyance, art investigations history investigations on Leonardo on Leonardo da Vinci’s dadrawings Vinci’s drawings carried out carried in the out past in two the decadespast two have decades successfully have successfully integrated integrated analytical ana- mea- lyticalsurements measurements to identify to constituting identify constituting materials, materials, delineate thedelineate artist’s the modus artist’s operandi, modus andop- erandi,characterize and characterize material deterioration material deterioration over time [20 over–22 ].time Suffice [20–22]. to mention Suffice Leonardo’sto mention draw-Leo- nardo’sings belonging drawings to thebelonging Biblioteca to the Reale Biblioteca in Turin, Reale which in were Turin, recently which analyzed were recently with macro ana- lyzedX-ray with Fluorescence macro X-ray (MA-XRF), Fluorescenceµ-Raman (MA-XRF), spectroscopy, μ-Raman and spectroscopy, atomic force microscopy and atomic (AFM) force microscopyto identify their(AFM) constituting to identify materialstheir constituting and assess materials their state and of assess conservation their state [23 of,24 conser-]. This vationline of [23,24]. research This clearly line pointsof research to the clearly need for points further to the integration need for of further scientific integration methods of to scientificgain a thorough methods understanding to gain a thorough of Leonardo’s understanding creative of process.Leonardo’s creative process. InIn this this work, work, we we report report on on the the application application of of non-invasive non-invasive multi-modal multi-modal analysis analysis on on a a double-sidedouble-side drawn drawn sheet sheet by by Leonardo, Leonardo, DrawingDrawing 8P 8P (Figure(Figure 1),1), from the UffiziUffizi Gallery of Florence,Florence, Italy. Italy. Our Our study is part of a research campaigncampaign carriedcarried out out by by Opificio Opificio delle delle Pietre Pie- treDure Dure in collaborationin collaboration with with the the National National Research Research Council Council (CNR) (CNR) and theand National the National Institute In- stituteof Nuclear of Nuclear Physics Physics (INFN) (INFN) within within the framework the framework of the of exhibition the exhibition held inheld Leonardo’s in Leo- nardo’sbirth town birth (Vinci) town to(Vinci) commemorate to commemorate the five th hundredthe five hundredth anniversary anniversary of his death of his (2death May (21519) May [ 251519)]. The [25]. commemoration The commemoration year alsoyear sawalso saw other other research research campaigns, campaigns, such such as theas theset set of of investigations investigations on on this this sheet sheet performed performed at at the the UniversityUniversity ofof Bologna (for (for further further referencereference see see [26]). [26]). We We used used multispectral multispectral reflectography reflectography in in the the visible visible (Vis) (Vis) and and near- near- infraredinfrared (NIR) (NIR) to to perform perform the the spectral spectral mapping mapping of of the the drawing drawing materials, materials, whose whose elemental elemental compositioncomposition had had been been previously previously characteriz characterizeded by by XRF XRF [27]. [27]. Morphological Morphological micro-features, micro-features, i.e.,i.e., not not visible visible to to the the naked naked eye, eye, were were reveal revealeded thanks thanks to to the the combined combined application application of of laser laser scanningscanning microprofilometry microprofilometry and and spectral-domain spectral-domain optical optical cohe coherencerence tomography tomography (Sd-OCT). (Sd-OCT). TheThe