Deep-pocketed collectors have ample opportunities to pick up works by midcentury masters of abstract expressionism. Since 2006, more than 200 drawings, lithographs, and paintings by Willem de Kooning have sold, including Woman III, which billionaire Steven Cohen bought for $137.5 million. At least 30 works by Jackson Pollock have changed hands in that time, including No. 5, 1948, the most expensive painting ever sold, at $140 million.

It’s a different story with Clyfford Still. In the past four years, there have been just six public sales of his work.

Make that seven. On May 13, Christie’s will offer one of Still’s masterpieces, 1946 (PH-182), a painting that could set a new auction record for the artist. Painted in red and black at the height of his career, 1946 is a prime example of Still’s talent and the first piece by him to be seen at auction in a year. It has been in a private collection for many years and has never before been seen by the general public.

Given the rarity of such an event, one might expect a startlingly high price for 1946. Yet the current record holder, 1947-R-no. 1, which was auctioned in November 2006, fetched a relatively paltry $21.3 million. Christie’s has priced 1946 (PH-182) at $8 million to $12 million. But “it could very well exceed our estimates,” says Christie’s senior contemporary art specialist Robert Manley, overseer of both sales.

Still’s prices have lagged in part because there’s been such a limited supply of his work to fuel interest. At his death, in 1980, only 150 of Still’s paintings were in circulation—most in permanent museum collections, with as few as 25 in private hands. “They are Vermeer rare,” says Dean Sobel, director of the new Clyfford Still Museum, which will open in Denver in 2010.

Well, they are and they aren’t. Though Still counted Peggy Guggenheim as a patron and influential critic Clement Greenberg called him “one of the most important and original painters of our time,” the artist was never comfortable with the commercial art world, and by the 1960s had retreated to his Maryland farmhouse, taking his unsold works with him. After Still died, 2,000-plus paintings and works on paper were found in barns on his farm.

But Still was as careful in planning his estate as he was in controlling his work. His will dictated that the collection be bequeathed to an American city that would erect a museum dedicated entirely to him—and agree not to sell any of it. For the next 30 years, his paintings were stored in warehouses, while his widow sought a suitable home for them.

Denver won the rights to the museum in 2004 (an earlier attempt in 1999 failed), agreeing to erect a new building next to the Denver Art Museum. In March, the museum unveiled plans for the $33 million facility, designed by the Portland, Oregon/New York firm Allied Works Architecture; construction is expected to begin next year.

The museum has helped boost the market for Still’s works. Before it was announced, Still’s paintings were selling in the $1 million to $4 million range at auction. A 10-by-9-foot painting sold for nearly $8 million last year at Christie’s, and Sobel says it could have gone for more if not for its size—too big for most collections to handle. Prices in private sales are likely to have been much higher.

The Zwirner & Wirth gallery in New York has facilitated three private sales of Still’s work since the 2006 Christie’s auction, says director and partner Kristine Bell. While prices haven’t reached the blockbuster level, they have been trending upwards.

“The prices for a very good one are going to be much, much higher than for the next best,” says Michael Findlay of Acquavella Galleries in New York. Other gallerists said it had been years since they’d had the opportunity to sell one of his paintings and longer since they’d seen a good one. “We haven’t sold one in a long time,” Findlay says.

The Denver Art Museum previewed a selection from Still’s estate last summer, marking the first time many art historians and collectors laid eyes on some of his major works. The show was so popular, drawing 100,000 visitors, that the museum extended it through November.

The upcoming Christie’s sale will be the first since the Denver preview, and though it certainly won’t be the last, the sale will prove whether it whetted collectors’ appetites for more. While the 13 works in the preview, like the rest of the estate, will never be up for sale, the attention the museum will bring to Still’s work might mean that the timing is right to invest in a piece like 1946.

“Every single time his work comes up,” Manley says, “it’s a test of his market.”