It’s amazing what you find when you’re not looking for it. As part of the research for the exhibition Andrea del Sarto: The Renaissance Workshop in Action, I was examining an infrared image that showed the underdrawing beneath the paint layers of Andrea del Sarto’s large painting Sacrifice of Isaac from about 1528, part of the collection of the Old Masters Picture Gallery in Dresden.

Here’s a tracing—in yellow lines—of what I saw.

It seemed unrelated to the Andrea del Sarto painting, and yet clearly included some sort of architectural studies. When we isolated it and turned it upside down, it became (slightly) clearer. Maybe it could be found in the background of one of Andrea’s paintings? I couldn’t think of one, but immediately flipped the pages of various del Sarto books…and no such building was anywhere to be seen.

And yet it seemed such a familiar image. What first came to mind was The Marriage of the Virgin by Raphael from around 1504, in the collection of Milan’s Pinacoteca di Brera, but that wasn’t quite the same.

The origins of Raphael’s composition came from a painting of the same subject, now in Caen, northern France, by his teacher Pietro Perugino. When I looked at that painting, I felt a growing sense of excitement: all the tiny details of architecture matched, and even the few lines that could be made out in the figures matched too. The link was clear.

What an odd discovery! It was one of those moments I’ll never forget. It’s humbling to realize how little we really know about major artists who worked so long ago, and a little glimpse such as this makes that all the more apparent.

So what on earth is the compositional underdrawing of a Pietro Perugino painting from just after 1500 doing underneath Andrea del Sarto’s Sacrifice of Isaac from a quarter century later? We don’t know, but in due course I hope that three of us—myself along with Marcia Steele, the paintings conservator from the Cleveland Museum of Art who took the infrared images, and Andreas Henning from the Dresden Gallery—can figure this out.

In the meantime, my working hypothesis is that Andrea del Sarto reused an abandoned panel by Perugino. The underdrawing bears little in common with that found under Andrea’s paintings, and it would have been odd for him to want to copy a (by then) extremely outdated composition. Perugino had studios in both Perugia and Florence and died in 1523. We could imagine that Perugino started work on a copy of his Caen painting, but it was left unfinished for some reason. When he died in 1523 the contents of his studios would have been sold, and Andrea del Sarto, who by then ran the biggest and most productive workshop in Florence, would have been the most likely buyer. Having trimmed the panel to the size he wanted, Andrea turned it upside down so as not to be distracted by the earlier composition, and began work on his Sacrifice of Isaac.

Maybe. One day I hope we will know the story for sure. And I hope we will also find out why Andrea himself left another version of his Sacrifice of Isaac—now part of the collection of the Cleveland Museum of Art and a highlight of the exhibition—unfinished. Another Renaissance mystery that remains to be solved.