In the early 20th-century, Beijing-based artist Xu Beihong gained renown for his ink paintings of horses and birds that blended Eastern and Western techniques.
An example of his work in the collections of the Yale University Art Gallery depicts a majestic horse with flowing mane standing in tall grass. The painting, “Horse,” which dates to 1935 and is mounted on a silk brocade hanging scroll, was scheduled to go on display earlier this year but had a small but easily noticeable flaw: The scroll was missing a knob.
The conservators faced a choice, said Cynthia Schwarz, senior associate conservator of paintings at the time. They could either replace both knobs or make a replica of the remaining knob. Believing the existing knob may be original to the scroll, they decided to pursue the latter option.
“Horse,” by Xu Beihong is an ink painting mounted on a silk brocade hanging scroll. One of the scroll’s knobs had gone missing.
The project to replicate the knob, while small, demonstrates the kinds of challenges Yale’s conservators encounter — as well as the ingenuity they apply to solving them at the university’s shared conservation lab at West Campus, where objects from Yale’s museums receive tender love and care.
The existing knob is glued to the scroll, meaning it could not be removed to make a mold for a replica without risking damage. Instead, the conservators decided to image the knob using photogrammetry, a method that stitches together overlapping photographs of a three-dimensional object to produce an accurate 3D digital model, which could then be printed into a suitable reproduction.
Richard House, senior photographer at the Gallery, performed the photogrammetry. The knob’s dark glossy finish, which produces reflections, posed a challenge. At first, House doubted his chances of success.
“I wasn’t super optimistic, he said. “But it was worth a shot.”
Photogrammetry was used to create an accurate 3-D digital model of the existing knob. The image at the left shows the camera positions used to capture the knob’s shape. The image on the right is the 3D that was used to print the replica.
One thing I loved over my 20 years at Yale is how when you’re faced with a challenging project, there’s always an expert somewhere on campus who is willing and able to help you with it.
Ideally, House would have stood the scroll vertically and photographed the knob from all angles. Due to the object’s fragility, however, he laid it down on a table with the knob hanging over the edge. He used a handheld camera in his first attempts to take shots of the knob from a variety of angles, which didn’t work. But when he placed the camera on a studio stand and used cross-polarization techniques to reduce reflections, he was able to produce a series of photographs suitable for forming a 3D model.
Rachel Heyse, a laboratory assistant at Yale’s Institute for the Preservation of Cultural Heritage, used a desktop 3D printer to print six versions of the knob at the highest resolution using black polymer resin.
The knobs were arranged in various angles on the build plate to reduce “stair stepping,” the appearance of step-like grooves on the knobs’ convex ends.
Heyse varied the size of each knob slightly to account for any sanding that might be needed to achieve the desired surface finish. She printed the knobs at different angles to reduce “stair-stepping,” the appearance of step-like grooves on the knobs’ convex ends caused by the software’s slicing process and the limitations of printer resolution.
“The first knob had bad stair-stepping, so I had to experiment with the orientation a little to get it right,” she said.
She also used as few supports as possible to stabilize the knobs during the printing process to reduce scarring.
Schwarz selected the best of the six replicas and finished it to match the existing knob.
First, she sanded the knob to remove any stair-stepping. Next, a hole slightly larger than the scroll’s dowel was drilled into one end. Then the knob was painted, glossed, and waxed.
The replica scroll knob, right, was painted and waxed to match the original.
“It was nice to have the extra knobs that Rachel printed to practice on and see what finishes worked best,” said Schwarz, who left Yale in June to start a private art conservation practice.
The hole in the knob was lined with a foam material so it would fit snugly on the dowel but could also be easily removed. The knob was affixed to the scroll while the painting was on view in the museum’s Asian galleries, but it will be stored separately from the scroll and painting until it can be tested for long-term stability, Schwarz said.
“It probably would be fine, but sometimes polymers off-gas dangerous chemicals, so we’ll test it just in case,” she said. “Conservators tend to be conservative in these cases.”
The project is a testimony to the collaborative atmosphere that pervades campus, Schwarz said.
“One thing I loved over my 20 years at Yale is how when you’re faced with a challenging project, there’s always an expert somewhere on campus who is willing and able to help you with it,” she said. “I was very lucky to be able to rely on Rich’s and Rachel’s expertise to make this replica so that this beautiful painting could be exhibited without a noticeable flaw.”
