The Vanderbilt family is selling a J.M.W. Turner masterpiece that was considered "one of the finest watercolour-drawings in the world." "Bamborough Castle"  which was believed by many in the art historical world to be lost  has been in the family's collection since 1890, when Cornelius Vanderbilt purchased it. But at a time when art prices, especially prices for Turners, are soaring, the family has decided to sell.

Click Image to Enlarge Sothebys London J.M.W. Turners Bamborough Castle, a detail of which is above, was thought to be lost. It is now on view  for the first time in 118 years  prior to being auctioned at Sothebys in London. The painting is in New York until November 7.

Sotheby's, which will auction the painting in London on December 5, is exhibiting the work at its York Avenue headquarters between Friday and November 7  making this the first time in 118 years that the Turner has been available for public viewing. Though "Bamborough Castle" is estimated at between $3 million and $5 million, there is plenty of reason to suspect a higher hammer price. A comparable work recently sold for $7.2 million, and the thrill of this sale is the picture's secret life in hiding.

Unlike some lost treasures  unappreciated in their time, only to be esteemed highly later on  there was never any doubt about this Turner's value. Shortly after the work was painted, the Graphic Society described it in 1837 as among "the finest watercolour drawings in the world." And, in 1872, the Earl of Dudley, an avid collector, validated the society's opinion by paying a record price for any watercolor to own "Bamborough Castle."

Eleven years later, the earl died. His son and heir was a teenager and not the Turner enthusiast his father had been. The second earl allowed the picture to be shown at the Royal Academy in 1889, the last time it has been seen in public. Then, in 1890, Sophia and Cornelius Vanderbilt were on a trip to Europe, and in them, the young Earl soon found a home for his father's Turners. Indeed, the Vanderbilts bought several Turners from the earl, including a painting that the family eventually donated to the Metropolitan Museum of Art.

The Vanderbilt family enjoyed its privacy, and four generations of them enjoyed the Turner as it went from being a famous work of art to a family heirloom. Perhaps because the picture was acquired so long ago  and as a watercolor, it may not have been viewed with as much importance as an oil painting  its whereabouts were unknown to the art world. The close-knit Vanderbilts didn't trumpet their possession, leaving scholars to wonder. "I was one of many who knew it existed," Sotheby's expert, Henry Wemyss, said, "but didn't know what it looked like." Art historians resorted to reconstructing the work, and its powerful depiction of a storm, from Turner's sketches, which might have made it hard for the original to live up to expectations. But Mr. Wemyss was bowled over when he finally saw it.

"The various experts have all been speechless and in awe," vice chairman of Sotheby's, Benjamin Dollar, who saw the work for the first time 10 years ago, said. "They were particularly struck by the vibrancy of the colors and the drama of the composition. Each expert has spent a number of hours with it."

"With most 19th-century artists you can admire their work, but you can see how they did it," Mr. Wemyss said. "With Turner, I can't understand how he did it. You can spend a long time looking just at the brushstrokes."

The brushstrokes show an artist who worked over the picture again and again. One must work quickly with watercolors before the surface dries, but Turner found ways to add and subtract to create the effects he wanted. There are brushstrokes on wet paper and brushstrokes on dry paper. Turner went back and scratched into the surface of the paper, creating inflections of white in the already vibrant colors.

The vibrancy of the work is a testament to the care the Vanderbilt family gave the painting. Though the later generations were not involved in the obsessive collection of Turners  or even that deeply connected to the art world  they kept the watercolor away from the light and in a dry spot.

The family may not have participated in the art world but they were savvy enough to check in every so often. Ten years ago, they approached Mr. Dollar about a valuation. He was so struck with the find that he kept the file on his desk for a decade, hoping one day the family would be willing to part with the masterful work.

And that day has come. Earlier this year, a cache of Turner watercolors sold for record prices at Sotheby's. The family called Mr. Dollar before he could pitch them that the timing was perfect.

The motivation to sell can be overwhelming for those in possession of highly valuable works. There was a time when great families gave great works to great museums. But even the redoubtable David Rockefeller sold a prized Rothko at Sotheby's this spring, choosing to donate the proceeds, instead of the painting, to charity. Why aren't the Vanderbilts donating? It could be that the family's wealth, diluted with the generations, can't ignore the gain. Or perhaps they just see the market as the most efficient way to transfer the burden of owning such a majestic work.

After all, it's one thing to care for a treasured family piece. It's another thing to be responsible for what could soon be a $10 million masterpiece. As prices rise, so does the burden of owning great works. Insurance rates go up; security is an issue; with works on paper, there is additional care. Most onerous is simply the stewardship. And the family is ready to pass that along.

So now, 135 years after it was last sold, "Bamborough Castle" is ready for the market again. The discovery was too late to have been included in the show of Turners currently up at the National Gallery in Washington, D.C., but at least there's no longer the mystery of whether the painting still exists. And, with luck, whoever spends upward of $3 million on this finest of watercolors will be a little more show-offy about it.