You gonna finish that? What we can learn from artworks in progress

How does an artist know when a work is finished? Sometimes it's a deliberate decision. Other times, the decision is made by fate or circumstance. Now, an extensive exhibition at The Met Breuer Museum in Manhattan is exploring great works of unfinished art.

The Unfinished show has an intriguing subtitle: "Thoughts Left Visible." The exhibit showcases works made over some 600 years, which offer glimpses into the creative process and sometimes reveal artists' anger or despair.

Curator Andrea Bayer says that unfinished works can still be masterpieces. She cites a small, exquisitely detailed drawing Jan van Eyck made in 1437, in preparation for a painted panel.

Saint Barbara sits on a hill near a looming Gothic tower. She holds a thick book and long, graceful palm leaves. The young woman is drawn in black, on a pale background. Van Eyck paints in just a few birds against a blue sky. "And then he stopped," says Bayer, and declares, "It's a masterpiece." No one knows why van Eyck didn't apply paint to the rest of the panel. But he signed and dated it, which usually means an artist thinks it's finished.

Rembrandt was once asked why so many of his works look half-finished. He replied: "A work of art is complete when in it the artist has realized his intention." Rembrandt implies that it's up to the artist to decide, not to critics, who may say a work appears raw, lacking a complete appearance.

Paul Cezanne, who was never satisfied, rarely signed his works. In a letter to his mother, he wrote that finishing things was a goal for imbeciles.

When you simply get fed up

There are all sorts of reasons for not completing a work: the artist dies, the commission dries up, the artist gets frustrated — that last one is what happened to Peter Paul Rubens in 1628 while painting a big battle scene for the French Queen Marie de Medici.

"We can see him using a liquid brushstroke to set out the horses, the men, some of them falling, some of them riding," Bayer observes. It's clear from the animated canvas that the piece was ambitious. Yet after three years of working on it, Rubens stopped.

He was having too many problems communicating with the Paris court. They gave him wrong information about the size of the work; also, Queen Marie was in trouble. "Basically," says Bayer, "the artist said basta and put down his brushes."

When you are interrupted

Centuries later, war left another picture unfinished. In 1965, the American painter Alice Neel was working on a portrait of James Hunter, an African-American draftee who was about to go to Vietnam.

"She did the first sitting with him, capturing this beautiful, somewhat melancholic head, leaning on one hand," Bayer says. On the rest of the large canvas Neel outlined Hunter's arms and legs, the chair in which he sat, in quick, precise strokes. "He was supposed to come back for a second sitting, and he never did," Bayer says.

Neel waited and waited, much as her pensive model in the painting sits waiting, knowing that his life is about to change. "This is a work in which not only has art been interrupted," the curator observes, "but you get that sense that life has been interrupted."

Maybe Hunter was sent to war, but the museum has not been able to find any record of his death. Perhaps he changed his mind about posing. Ten years later, for her first show at the Whitney Museum, Neel decided the work was finished. She put a title on the back, and signed it.

"It's magnificent," Bayer declares. "And it is a great example of so many of the works in this exhibition, in which you're happy that the artist did not add another touch."

When you can't bear to finish

"Unfinishedness" is Bayer's word for the state of some of these pieces. Sometimes, as in the case of the Neel work, it's a virtue. At other times, an artist doesn't want to finish a painting, but can't bear to part with it.

In 1867, Édouard Manet's friend, poet Charles Baudelaire, died. Scholars think Manet's dark, brushy landscape of a small funeral procession was painted after the poet's burial. "It clearly had personal significance for him," Bayer says. "It captured a moment in which he was filled with grief."

Manet never finished The Funeral. But he kept it in his studio until his death, 15 years later. Bayer posits the piece simply brought up too many emotions for Manet to want to finish.

When the viewer finishes for you

There are pieces in the exhibition that artists left unfinished deliberately. After World War II, many of them stopped trying to create perfect, completed canvases. Instead, work became about restlessness, flux. Co-curator Kelly Baum says they made paintings that looked "unstable, ongoing, boundless, impermanent."

Baum points to Robert Rauschenberg's White Painting from 1951. Four square panels are hung close together to form a big square, each panel completely covered in nothing but white paint. You can see it here. Is this an unfinished work? It's hard to tell.

But when a viewer walks in front of it, he or she casts shadows on the panels. The visitor becomes a silhouette. "And because the paintings are always changing," says Baum, "they can never truly be finished."

It also means that the viewer participates in the making of the art, and finishes it at least for that moment of shadow-casting.

When unfinished is an invitation

A canvas by Andy Warhol is like a signature for this Unfinished exhibit. Do It Yourself (Violin) salutes the paint-by-numbers kits that were popular in the 1960s. Warhol paints the outline of a violin, black, on a white background. He breaks the violin down into many numbered spaces, and fills in just a few of those spaces with numbered colors from the kit -- brown, yellow, a bit of blue. Again, says Baum, it seems to be an invitation to the viewer to finish what Warhol has only begun.

Over the centuries, artists have been asked how they know a work is finished. Warhol may have had the best answer. Bayer says that Warhol's response was: "when the check clears."