Aware of such controversies, Radke has proceeded with careful deliberation. In October 2008, he presented his thesis about the silver figures to colleagues at the Provo/Athens Renaissance Sculpture Conference, a quadrennial meeting of experts. Some were convinced, some not. "My main hesitation is to attach big names to works of art about which we know very little in terms of how the workshops in which they were produced functioned," says Sally Cornelison, associate professor of Italian Renaissance art at the University of Kansas. "I'm not going to say that it's not Leonardo, but I think we need to be cautious. We don't know that much about people who worked as goldsmiths and silversmiths during the Renaissance. It could just as easily be by an extremely capable but unknown artisan."
Martin Kemp, who did not attend the conference, is inclined to accept Radke's attribution on the basis of photographic evidence and the way the two "Leonardo" figures reflect light. Leonardo's handling of light was always more "painterly" and sensitive to the nuances of surface, he says, while Verrocchio tended toward the blunt and the sculptural. "What is absolutely right is that there are different hands and eyes at work in that panel," Kemp adds, but he speculates they might be Verrocchio's in the "Leonardo" figures and a lesser assistant elsewhere. Or was there another apprentice as talented as the young Leonardo?
As Radke himself notes, no contemporary attributions to a Leonardo sculpture have won unqualified acceptance. "I believe that until we discover some new written documents or other evidence, neither will the two figures in the silver altar," he says. "But what can one expect in a situation where no documented work has survived? That said, I do believe that there is more visual evidence for my attribution than any previously proposed."
Ann Landi is a contributing editor of ARTnews and the author of the Schirmer Encyclopedia of Art. She is based in New York.