Is that Bosch on the wall of the Philadelphia Museum of Art a Bosch by the Bosch, Hieronymus by name? Or is it a faux Bosch by a devoted follower from later in the 16th century? Was it completed by Bosch's studio? Or was it created by a trickster seeking to cash in on the Flemish master's popularity?
You can ponder such mysteries and puzzles starting Friday at the Art Museum's major fall exhibition, "Old Masters Now," a selection of more than 100 paintings from the collection of John G. Johnson, collector extrodinaire, who bequeathed about 1,300 works to Philadelphia when he died in 1917.
Alas, you won't be able to employ handy aids like dendrochronology — an important technical tool for understanding what may seem like a straight-forward canvas. And you'll have to check your x-ray machine at the door.
Nonetheless, "Old Masters Now" encourages you to consider how the understanding of the museum's Johnson Collection has evolved and changed — even as the collection remains the historical and aesthetic foundation of the museum's entire collection of Western art. Johnson acquired paintings from early Italian and northern masters, like Robert Campin and Rogier van der Weyden, all the way to 19th-century artists, like Édouard Manet and John Singer Sargent.
But not everything he acquired turned out to be what it purported to be.
What makes this exhibition more than a selection of top-40 hits is the museum's decision to highlight collection as an evolving organism, displaying the almost biological changes over time. What was once a Bosch is no longer believed to be a Bosch. Why? How are such determinations made?
In the case of Bosch, Johnson bought at a time when the artist was not of great interest to collectors, and the shrewd Philadelphia lawyer was able to acquire 10 paintings by the master. He thought.
In the intervening decades, however, scientific analytical techniques have gained traction and authority, particularly dendrochronology, the dating of wooden panels based on study of their tree rings.
One by one, Johnson's beloved Bosches were found to be, maybe beloved, but not by Bosch.
Among the others, however, are paintings that have been regularly on exhibition at the museum.
"This is one of these issues," said Christopher D. M. Atkins, associate curator of European painting before 1900. "Just because it's not a big name doesn't mean it's not a quality work of art. It's just been our job to figure out what it is and to share that. But they're still beautiful objects, regardless of who painted them."
One of the most interesting aspects of the exhibition involves such changes wrought by scholarship and conservation.
"Over time, our appreciation of Johnson's extraordinary gift continues to grow, and it remains a source of endless fascination, with many discoveries still to be made," said Timothy Rub, museum director and chief executive. "We are delighted to open a window onto our work, offering visitors a fresh look at the process of scholarship and conservation that we bring to the care of our collection and an insight into the questions, puzzles, and mysteries that continue to occupy our staff."
Perhaps no work in the collection has received more intense scrutiny over a more prolonged period than van der Weyden's great work on two wooden panels, The Crucifixion, with the Virgin and Saint John the Evangelist Mourning (1460).
When Johnson acquired the paintings in 1905 and 1906, they were not well understood. Perhaps they were a diptych or shutters of an altarpiece, or even a single painting, split.
But in the last 25 years, the paintings have undergone a visual and art historical sea change.
First, conservators determined that the gold background was a later addition, not applied by the artist. Van der Weyden had painted a stark deep-blue background.
Then, considering strange small holes at the bottom of the paintings, and considering them over a great deal of time, museum director of conservation Mark Tucker came to a startling realization.
They were dowel holes used to attach carved wooden tracery to the picture panels. Johnson's diptych was no diptych. Rather, the large panels belonged to the covering case of a mammoth sculptural altarpiece measuring more than 26 feet in width.
At the same time Tucker was formulating that idea, a Belgian curator discovered two paintings that had been on the backs of the museum's van der Weydens. These recent discoveries suggest the lost altarpiece from which these panels came was one of the largest and most important of the 1400s, the museum maintains.
All such revelations are presented in this exhibition. Conservators and curators will also frequent the galleries to answer questions sure to arise as visitors ponder the walls.
The museum also plans a digital publication that will be available free and online in February.
"Our understanding of the Johnson Collection is constantly changing," said Jennifer Thompson, curator of European painting and sculpture. "This exhibition marks the first significant assessment of how our thinking on it has evolved over the years. While the careful study we have given to objects in the collection is rarely presented to the public, we are quite pleased to give visitors a behind-the-scenes look at the work we do."
Old Masters Now: Celebrating the Johnson Collection
Hours: Tuesday-Sunday, 10 a.m.-5:00 p.m. (evening hours
Wednesday & Friday until 8:45 p.m.) Closed Monday.
Tickets: $20 ($18 in advance online), $18 seniors, $14 youth 13-18 and students with ID, free for children under 13.
Information: 215-763-8100 or philamuseum.org