**4. Discussion and Conclusions**

At present there are very few scientific studies supporting the historical-artistic interpretation of paper-based artefacts, particularly those by Leonardo da Vinci. Our results offer a useful contribution to the literature on the drawing technique used by Leonardo in his early production. *Drawing 8P* was analyzed with complementary optical techniques that fully preserve its material integrity. Scientific data were accurately integrated with technical information provided by art historians and conservators involved in the measurement campaign to obtain an exhaustive and reliable characterization of the examined artwork.

False-color and PCA processing of the multispectral images revealed traces of a preliminary sketch of the landscape on the recto, which was drawn with a lead stylus, consistently with other drawings by the same author. Reflectance imaging and spectroscopy highlighted the presence of two distinct superimposed drawings, both made with irongal ink, and yet displaying different absorption properties. This seems to indicate that Leonardo used a diluted ink solution for the main composition, and a more concentrated one for the definition of specific details. Furthermore, our analysis revealed a similar superimposition of materials in the main sketch on the verso, also attributed to Leonardo, who outlined the scene with a lampblack pastel and then redefined it with ink and pen. The identification of the pastel technique in the early work of Leonardo is particularly significant, as it is believed that the artist himself was responsible for the introduction of said technique to Italy. The other sketches, caricatures, and writings observed on the backside of the sheet are made with ink, red chalk (sanguine), lead point, lampblack, and metal stylus. The concurrent presence of such diverse tools and materials suggests that they are the result of occasional drawing exercises not only by Leonardo but also by other artists attending the workshop of Verrocchio in the same period, or even later. The poor definition of the least visible sketches, in particular, does not allow for their precise attribution. The only exception are the sanguine sketches, which can be attributed to a later author, since the material used was extremely rare in the years comprised between the time the landscape was drawn and 1480 [42].

Morphological analysis of the sheet by microprofilometry showed the typical wavy texture and chain line/laid line impressions related to the papermaking method available at that time. The 3D model also highlighted the presence of deep incisions, possibly resulting from the use of transparent paper for transferring the preparatory drawing onto the final sheet—a method described by Leonardo himself, yet still poorly documented in terms of scientific investigations.

OCT analysis allowed for the visualization of micrometric details of traits engraved in the white background with no trace of coloured material and no apparent relation to the drawn landscape.

Our work provides corroborating evidence of the suitability of multimodal application of non-invasive techniques to gain new insights into Leonardo's under-investigated modes of creation. The systematic application of complementary analytical methods proved essential for a deeper understanding of such complex and extremely valuable artworks.

**Author Contributions:** Conceptualization, R.F., A.D.F. and J.S.; methodology, R.F. and A.D.F.; software, E.P.; validation, R.F., J.S. and E.P.; formal analysis, R.F. and A.D.F.; investigation, R.F. and A.D.F.; resources, R.F.; writing—original draft preparation, A.D.F.; writing—review and editing, A.D.F.; visualization, R.F., J.S. and E.P.; supervision, R.F.; project administration, R.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Regione Toscana (POR FSE 2014–2020, "Giovanisì", Intervention Program "CNR4C", CUP B15J19001040004).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** Marco Ciatti, Cecilia Frosinini, Conservator L. Montalbano from Opificio delle Pietre Dure are gratefully acknowledged.

**Conflicts of Interest:** The authors declare no conflict of interest.
