Renowned art conservator David Bull next to a Delacroix portrait he has been restoring in his studio/residence in New York. (Jesse Dittmar/For The Washington Post)

Not long ago, the bestselling author Daniel Silva e-mailed an important question to an old friend. How does one go about creating a credible forgery of a van Gogh?

The key, David Bull told him: Craquelé. Those are the web-like lines, similar to tiny cracks on an egg, commonly found on old oil paintings.

“The great, infamous forger, Han van Meegeren, used to bake his forgeries in the oven,” Bull explained. “Then he would get the canvas and roll it over the side of the table so it would crack the paint.”

Silva was not looking to dupe Christie’s. He had turned to his prized expert for help with his latest book, “The Heist.” It is the 14th — and counting — in a series of bestselling mystery novels that star Gabriel Allon, an art conservator-slash-assassin.

David Bull, 80, has never killed a man, but he is Silva’s secret weapon. In a way, he’s helped the author become the John le Carré of art conservation. Bull’s tools have touched Picassos, Monets and even a da Vinci. And while the conservator is nothing if not respectful of his writing friend — he would never consider meddling with Silva’s prose — there was one proposed twist in “The Heist” he couldn’t accept. Silva wanted Allon to roll up a real van Gogh at one point in the book.

“Never, never, never,” Bull protested. “Because Gabriel loves the painting. He would not damage a painting.”

As he recounts this, Bull sits in the studio and home that he and his wife, conservator Teresa Longyear, share on New York’s Upper East Side. He is trim, with a wisp of brownish gray hair across his forehead, and wears a green tie and blue sport coat on this day.

In this space, a few steps down from the sidewalk, the couple run Fine Art Conservation and Restoration, a practice with a client list that’s as passionately protected as the paintings themselves. Bull does share that, over the years, he and his wife have done work for the National Gallery, the Los Angeles County Museum of Art, Sotheby’s and Christie’s. That is about as much as he will reveal.

“Some conservators,” he says, “will have all the works they’re employed with available for anybody to see. I’m not going to name names. To impress others perhaps. But no, no, no. This is something we absolutely insist on, discretion rather than secrecy.”

One person he is generous with is Silva. The author has taken to writing one Allon book a year since 2000’s “The Kill Artist.”

“I might not hear from him in six months and then suddenly there’s e-mails and telephone calls because he’s hit a certain moment and he just wants to be sure,” Bull says. “When it comes to dealing with paintings and the restoration of paintings, he wants everything to be absolutely accurate. Tell him you use a little thing like a stick, like a Q-Tip, then he says, ‘What solvents do you use?’ He’s meticulous in his research.”

How it began

Silva, a former CNN producer, met Bull in the late 1990s. Silva’s wife, Jamie Gangel, then a reporter for NBC’s “Today” show, did a report on Daniel Shay, an art handler for the National Gallery of Art who also happened to be an artist. Gangel interviewed Bull, who collected Shay’s work, for the piece. Later, she invited him over for dinner with Longyear.

“I had come up with the idea of giving Gabriel this amazing cover job, of turning him into an art restorer,” Silva said. “I asked David whether he would be willing to take me under his wing and teach me how to do it. He was very generous.”

Now, 14 books later, he still finds Bull’s input vital. The conservator is always the first thanked in the acknowledgments section of the Allon books.

“When you’re constructing a piece of fiction, little mistakes are sort of like loose bricks in a foundation,” Silva says. “As a writer of genre fiction, spy fiction, you want to get as much right as you can. Then build your story on top of a firm foundation and platform.”

Born in Bristol, England, Bull studied painting, sculpture and interior design at the West of England College of Art. His older brother, John, had gone to art school ahead of him and became a conservator, eventually heading to the Tate Gallery in London.

David, who had never planned to become a restorer, fell into it, largely by watching John. He found the work deeply satisfying.

“First of all, just being with great paintings,” Bull said. “This is the most important part of the job. Secondly, just the meticulous work that has to be done on them. The whole range of different possibilities. The craft that you have to bring to it.”

His approach is to be patient. He can meet deadlines, but he prefers to work without them. The work is done when it is done. The painting itself is not his. It is the work of the artist. His job is to make that work fit the artist’s original intention, to “help a painting be well again.”

It is a subtle process and one that even his clients find somewhat hard to understand. The results, though, are no mystery.

The one that got away

Jon Landau, a prominent collector of French works — and best known as Bruce Springsteen’s longtime manager — has had Bull work on about 40 of his paintings. One of them is a small Titian that Bull spotted on the wall during a visit. Landau thought the painting was in fine shape. Bull asked if he could take it back to his studio with him.

“The picture, which was incredibly beautiful, was sort of quiet,” Landau says. “I put it back on the wall and the picture just jumps. You feel like you’re seeing it, something has happened where it has come into a 20-20 focus that wasn’t there before.”

Landau, like many collectors, also consults with Bull when he’s considering a purchase. He still regrets the time he didn’t take his advice.

He had spotted a Madonna and child in a landscape by the Venetian painter, Cima de Conegliano, at an auction house. It was, on paper, a perfect work for his collection. But Landau had concerns. Repairs had led to the Cima being overpainted.

Landau consulted with a series of conservators. They were also concerned.

“David was the only one who said to me, ‘If you love it, Jon, buy this picture. Don’t not buy it because of the condition because I promise you, underneath, it’s going to be great.’ ”

Landau let the painting go.

“And today,” he says, “it sits in the L.A. County Museum.”

At the National Gallery, where he served as chairman of painting conservation from 1990 to 1999, Bull worked on the only da Vinci in the United States, “Ginevra de Benci.”

“It’s fair to say he’s a legendary figure,” says Charles S. Moffett, the Sotheby’s vice chairman who worked with Bull when he served as senior curator of paintings at the National Gallery. “The guy’s range is amazing. He can work on anything form a 15th century painting to a 21st century painting. There are only a handful of guys in the entire world who are that good.”

Cloak of secrecy

Back at his home studio, Bull is asked about his own favorite painting. He walks over to a small frame containing a rocky coastal scene by Lord Leighton. He bought it in 1969. His grandmother had just died and left Bull 300 pounds. He was walking by Thomas Agnew & Sons and saw the picture.

“I often think, if we had a fire and I could only take one thing, what would it be? That,” he says, smiling and pointing.

Across from the Lord Leighton is a portrait by the great French romantic Delacroix. A client purchased it from an auction house after seeking his advice. Now the work has been in the studio for four months.

“This takes a long time and you’ve got to take as much time as the painting dictates to you,” he said. “You can not push things. If you push things, then you won’t do a very good job.”

How much did the painting cost? What is the name of the owner? Bull won’t say. He stressed that just allowing it to be seen took a special request of the owner. Normally, if a visitor were coming by, the Delacroix would be packed away.

That goes for whether you’re a museum director or Bull and Longyear’s 21-year-old son, David.

“Nobody who comes in here is allowed to see other people’s paintings,” he says. “If an owner of a painting shows up by chance, and we have somebody else’s painting on an easel, we say, ‘I’m sorry, you can’t come in.’ ”

That cloak of secrecy is about the only piece of Bull’s real life that he shares with Allon.

In “The Heist,” Silva delivers his typical brand of hard-boiled dialogue, romantic intrigue and culture-packed references, from Mozart to Bruckner, Caravaggio to van Gogh. By Chapter 2, Silva’s written of a bloody murder and a delicate restoration project in a Venetian church.

Later, readers are treated to the forgery process, as detailed by Bull, but thanks to the conservator’s protests, no rolled-up “Sunflowers.”

“We joke that David is the real Gabriel Allon, but in point of fact,” says Silva, “he is not anything like Gabriel. He shares only one thing with Gabriel Allon. He is a world class art restorer.”