Collectors are willing to pay a premium for widely hailed masterpieces like Orazio Gentileschia��s 1621 Baroque masterpiece, a�?DanaA�.a�� Sothebya��s New York expects it to sell Thursday for at least $25 million.A�SOTHEBY’S

The art market appears to be entering its cooling-off period. For the past six years, an influx of newly wealthy collectorsa��particularly from Chinaa��has pushed up prices for everything from Ming vases toA�Claude Monet,A�but now there are signs that the bidding paddles could come down hard.

Falling oil prices, volatile stock swings and Chinaa��s deepening economic woes are all taking their toll on the art market and spooking the worlda��s wealthy, who are counted upon to splurge for masterpieces and brand-new artworks. Christiea��s sold $7.4 billion in art last year, a billion less than it sold a year earlier; rivalA�Sothebya��s$6.7 billion in sales last year were flat with totals the year before.

Next Tuesdaya��as both houses kick off a winter round of impressionist, modern and contemporary art sales in Londona��collectors will find thinner auction catalogs and smaller estimates for art for the first time in years. The priciest piece up for bid this time around? AA�Pablo PicassoA�painting, a�?Head of a Woman,a�? that Sothebya��s expects to sell for at least $23.4 million. Not all Picassos are alike, but that estimate is only 13% of the value of a nearly $180 million Picasso, a�?Women of Algiers (Version O),a�? that Christiea��s sold last May.

a�?Ia��m 99% sure wea��re in the mania phase of an art-market bubble right now,a�? said Roman KrA�ussl, an economist and associate professor at the Luxembourg School of Finance. Earlier this month, Mr. KrA�ussl and two university colleaguesA�published a study in the Journal of Empirical FinanceA�that analyzed more than a million auction records from the past three decades. They determined that art values can fluctuate dramatically like stocks, and they wanted to see if they could detect the next art-market slowdown. Their conclusion: Ita��s already happening, particularly for the categories of contemporary and American art where speculation and price spikes have catapulted art values faster than other collecting categories.

Mr. KrA�ussl said his research suggests a market correction could eventually ripple to other collecting areas but will likely stop short of an art-world market crash. He said the wealthy still need places to store their casha��and art remains a prestigious outlet.

During next weeka��s London sales, Sothebya��s will ask at least $17.6 million for Henri Matissea��s a�?The Piano Lesson,a�� a 1923 painting that has belonged to the same British family for the last 89 years.A�SOTHEBY’S

A Citi Private Bank report published two months agoA�said the global art market has grown at an average annual rate of 13% since 2000, based on auction turnover, but its research suggests the art market is settling into a 9% annual growth ratea��not bad, given that over the same period global domestic product and exports grew at 3.5% and 8.1% a year, respectively.

The game-changer for the art market: Maturing Chinese collectors are still buying art, but their shopping sprees are now a�?more selective and savvy,a�? saidA�Suzanne Gyorgy,A�head of Citi Private Bank Art Advisory and Finance, who contributed to the report. (Sothebya��s sales in Beijing last year fell 69% from the year before.)

Ms. Gyorgy said collectors in China and elsewhere are also feeling a measure of sticker shock at the escalating prices being asked for blue-chip art, a reaction that may help the market reach a natural price cap. a�?Wea��ve been hoping for a slight correction to bring some sanity to this market,a�? she said. a�?The way prices were climbing, the whole thing was starting to feel artificial.a�?

In art-world parlance, artificial means choreographed, a description often hurled at auctions that feature artworks that are already essentially presold. Last year, auction houses tripped over themselves to offer financial sweeteners to sellers. These agreements guaranteed the sellersa�� offerings would sell to prearranged, third-party investors, if no one else competed during the sales themselves. This guarantee race peaked when Christiea��s extended guarantees to half its offerings during its major spring sales, but the practice now appears to be tapering off.

ArtTactic, a London firm that tracks auctions,A�said in its a�?Global Art Market Review & Outlook 2016a�? reportA�that it expects the houses to have a�?less appetite for this type of risk in the coming year,a�? a move that could discourage skittish sellers from putting trophies up for sale. Sothebya��s said it has extended guarantees to $92 million worth of art so far this yeara��including four pieces for sale in London. The house is still licking its wounds after losing $9 million on a $515 million guarantee it gave the heirs of its former owner, A. Alfred Taubman, last fall when a sale of Mr. Taubmana��s estate disappointed.

Expect shoppers to play it safer as well this year by gravitating toward burgeoning art hot spots that havena��t seen huge price rises yeta��like Singapore, India, Africa and Irana��as well as the worlda��s 50 best-known artists like Picasso andA�Andy Warhol,A�whose price levels arena��t likely to plunge as far as untested newcomers.

The auction houses are already bracing for retrenchment by filling London sales with these household names: On Wednesday, Sothebya��s plans to sell a cheery, redA�Henri MatisseA�scene from 1923, a�?The Piano Lesson,a�? which has been in one British collection for 89 years. Ita��s expected to sell for at least $17.6 million.

Christiea��s sale of impressionist and modern art on Tuesday is anchored byA�Egon Schielea��s a�?Self Portrait With Fingers Spread,a�? a 1909 black-and-gold depiction of the Austrian painter that last sold at auction nine years ago for $8.9 million. This time, the house is asking at least $8.5 million for it.

In contemporary art, Sothebya��s specialistA�Cheyenne WestphalA�said her strategy was to a�?focus on artists we know we sell wella�? likeA�Gerhard Richter.A�Sothebya��s Feb. 10 sale also includes a burlap bag fromA�Arte PoveraA�artistA�Alberto Burri.A�Stitched to a canvas and partially slathered in red paint, a�?Bag and Reda�? is estimated to sell for at least $13.6 million.

Edmond Francey, head of Christiea��s contemporary sale on Feb. 11, said the event is anchored by works of U.K. artists who still enjoy hometown hero status among Londona��s collectors. These artists includeA�Peter Doig,A�Francis Bacon,A�Lucian FreudA�andA�David Hockney.Mr. Doiga��s a�?The Architecta��s Home in the Ravinea�? is coming back to auction for the third time in a decade: The 1991 painting sold for $3.6 million in 2007 and for $11.9 million three years ago. This time, the seller is asking at least $14.2 million, but therea��s little suspense because the house has helped the seller secure a third-party guarantor, or a promised bidder.

a�?Sometimes you need to reassure a market,a�? saidA�Mr. Francey.

Kelly Crow, Roman KrA�ussl