It is not to be excluded that Leonardo may have in his youth, while living in Florence, taken the opportunity to visit Siena during a period when these two city-states were not at war. It is certain that he made a visit there during the reign of Cesare Borgia, around 1502. Leonardo notes its location prominently on map RL 12278, indicates it on map RL 12277 and mentions it on RL 12682.
In addition, there is a reference on the second cover folio of Ms. L, where Leonardo mentions “Pagolo Vannoccio in Siena”, the father of Vannoccio Biringuccio. In his treatise on metallurgy, De la pirotechnia, the latter described Leonardo as a “scultore eccellente” and spoke of the fusion technique that the artist had intended to use for his monumental equestrian statue dedicated to Francisco Sforza. Leonardo also mentions on folio 33v the clapper of the bell in the city’s Torre del Mangia: “… the bell of Siena, that is, the manner of its motion and the place of attachment of its clapper”, with a note on the volume of the bell (ca. 600 x 240 cm).
Finally, there are two bibliographic references worth noting. Leonardo cites around 1507 in the Codex Madrid II (f. 2v) a “work by San Bernardino da Siena” which he thought of including in a panel that he was composing for the altarpiece of a church in Brescia. In the Codex Atlanticus, on f. 559r the artist mentions The Preservation of Health, perhaps a treatise by Ugo Benzi of Siena printed in Milan in 1481.
Leonardo’s frequentation of intellectual and artistic circles in Siena in the early Renaissance period was crucial to his formation, contributing to his technical knowledge and exposing him to the classics of antiquity (including Spiritali, the Italian edition of the authoritative work on pneumatics by Hero of Alexandria).
The importance of the work of Sienese engineers – from Taccola (the author of De ingeniis) to Francesco di Giorgio (who was one of the colleagues who in 1490 worked together with Leonardo on the Fabbrica del Duomo in Pavia) – on the scientific formation of Leonardo is demonstrated by Il Trattato di Archittetura in the Biblioteca Mediceo Laurenziana in Florence, which contains twelve annotations in Leonardo’s hand (datable to the opening years of the 16th century) and by at least twenty-seven pages of notes in the Codex Madrid II that Leonardo had copied from a manuscript by Francesco di Giorgio.
In turn Leonardo’s art would later influence painters working in and around Siena, from Giovanni Antonio Bazzi (known as Il Sodoma) to Andrea del Brescianino and Domenico Beccafumi.