The massive work, created from 20 paintings on the front and back of 12 panels, is an astounding work of art. The show features 13 original van Eyck paintings shown alongside 100 artworks by contemporaries from the late Middle Ages, writes Nina Siegal.

Hélène Dubois was frustrated.

It was Friday morning, just hours before she was set to reveal the $2.4 million, multiyear restoration she had led on a panel of one of the world’s great artistic treasures: “The Adoration of the Mystic Lamb,” also known as the Ghent Altarpiece.

But the lamb’s “new” face had already been seen by thousands, online. In the previous 48 hours, a side-by-side of the panel before and after restoration had gone viral on Twitter, with Smithsonian Magazine calling the new lamb “alarmingly humanoid” and users comparing it to pouting fashionista Derek Zoolander.

“A lot of misunderstandings have been propagated by absolutely stupid tweets taken completely out of context,” Dubois said as she opened the secure chamber at St. Bavo’s Cathedral that held the altarpiece.

Inside, displayed in a huge glass vitrine, was the nearly 600-year-old masterpiece by brothers Hubert and Jan van Eyck, its details defined and its colors vibrant and joyful, as they must have been when it was painted. The massive work, created from 20 paintings on the front and back of 12 panels, is an astounding work of art. The lamb’s face, at the center of the largest panel on the front, is no bigger than a walnut.

“You see how small it is?” Dubois said. The changes to the lamb’s face were indeed quite substantial, she added.

“This is the original sheep of van Eyck, which was painted over by someone else to make it look like a passive animal, and more anatomically correct,” she said. “But this is what van Eyck’s intention was. It’s nothing bizarre; it’s just not what people are used to looking at and perhaps do not expect.”

The unveiling of the lamb panel was the first event in what is being called here “The Year of Van Eyck,” and the return of the newly restored central panel to the cathedral is accompanied by a landmark exhibition at Ghent’s Museum of Fine Arts called “Van Eyck: An Optical Revolution,” which opens Feb. 1 and runs through April 30. The show features 13 original van Eyck paintings—more than half of the 22 extant known works—shown alongside 100 artworks by contemporaries from the late Middle Ages.

The restored exterior panels, including ones portraying Adam and Eve, will be separated into pairs and presented in the museum, next to similar or contrasting works by other artists of the period, like Fra Angelico and Masaccio. Most of the altarpiece’s front, including the “Mystic Lamb” panel and the unrestored top panels, will remain in the cathedral. (One of the original lower panels, known as the “Righteous Judges” panel, was stolen and continues to be missing; it is represented by a copy.)

“The last time two panels of the altarpiece, Adam and Eve, were shown in a museum was in 1902,” Frederica van Dam, a co-curator of the exhibition, said. “When they depart back to the cathedral, you’ll have this distance again. To have an exhibition where you can see them so close is so extraordinary, to be able to see all the wonderful details.”

It’s the details that may stir the most discussion, as shown already by the recent social media excitement. The restoration of the altarpiece, which began with a study of the painting by a consortium of international experts in 2012, has revealed that a substantial amount of it was reworked in the 16th century, about 120 years after it was originally painted.

“Basically, we have been looking at a masterpiece by someone else,” van Dam said in an interview at the museum.

The Ghent altarpiece was commissioned for a private chapel in St. Bavo’s Cathedral around 1420. Although there is still debate among scholars as to its precise origins, the current consensus is that it was conceived and designed by Hubert van Eyck, who died in 1426, and completed by his younger brother, Jan van Eyck, by 1432. Both painters worked in a studio with students, and so support probably came from other painters, too.

Dubois, the restorer, said that at some point in the 1500s another painter, or perhaps a group of painters, decided that it needed a reworking. They may have wanted to change the painting for theological reasons—this was, after all, the middle of the Protestant Reformation and the Catholic Church was rewriting its doctrines at the Council of Trent—or because painting styles had changed.

The whole painting was redone, she said, “not only the lamb but all the draperies, the top part of the landscape, the sky and the city view, and on the reverse of the wings, about 70% was overpainted.”

She added: “What is peculiar about this one is that it was extremely carefully done so that all the contours of the figures were respected and most of the colors were reproduced with very high-quality pigments. It was not a botched job.”

But luckily, restorers discovered that there was also a very thick layer of varnish between the original painting and the newer version, which made it relatively easy for them to remove that layer to get back to van Eyck’s work, Dubois said. The original head of the lamb, for example, was “in very good condition,” she said. “We only had to do a tiny restoration of tiny paint losses.”

Nevertheless, Dubois said that she and her team had spent about two weeks removing the overpaint from the lamb alone, using a painstakingly meticulous process to remove the pigment.

“He is not the first one who painted the lamb in that way,” Dubois said. “We have many, many examples from the early Middle Ages and late antiquity, including Roman mosaics, which show the lamb with these very large frontal eyes, gazing at us to make it very identifiable as the Lamb of God.”

Because she could see the original image of the lamb emerging over a span of time, rather than in the instant way people look at side-by-side comparisons on Twitter, she said, she didn’t find it shocking to realize that the eyes were facing forward, and the lamb was far more alert than in the overpainted version.

“Of course, it’s more intense than I expected,” Dubois said. “It actually moved me. You’re just used to this demure, passive lamb and then you’re confronted with this very strong vision of the religious symbol of Christ being sacrificed on the altar. Here, Christ is aware of his sacrifice.”