This week Sotheby’s expansive 10th floor exhibition galleries were filled with artworks that appeared as staid as can be. Gilt frames, powdered wigs, Empire dresses, pudgy cupids, chateaux, and portraits of Duchesses ringed the room. Vinyl letters on the wall indicated a sale with an imposing name: “Art of the Enlightenment.” Though the material seems conservative to the modern eye – especially all that big 1780s hair – the period was one in which radical changes were brewing, including revolutions in France and America.

And these changes are reflected in the art. “Art of the Enlightenment is a much freer flowing and prettier art than the 17th century,” said private dealer Gerald Stiebel, who has been dealing in French 18th-century art and decorative arts for decades. Mr. Stiebel has seen the rise and fall of the art market for this period. In the heady days of the 1950s and 1960s, collectors such as the Ford heirs and Jayne and Charles Wrightsman snapped up everything worth having (the latter’s collection eventually went to the Met).

That time is long over, however. Tastes have shifted to more modern and contemporary work. Enlightened art hits the block today, but don’t expect enlightened prices. “We are in another slump,” said Mr. Stiebel, who says he has had trouble selling 18thcentury French portraits. But he holds out hope. “After speaking with curators and decorators at the auction previews, I thought, hey, maybe this is coming back.”

Evidently the auction houses see some demand as well in recent years. Christie’s held four “Arts of France” sales, including French 18th-century painting, decorative arts, and furniture. They discontinued these theme sales in 2001 because it seemed the theme was going stale and the collectors weren’t buying as rigidly as in the past, when buying the total French look was de rigeur. Today’s buyers prefer to diversify.

“One wants to make these sales fresh,” said Christie’s Head of European Furniture and Decorative Arts, Will Strafford. “It’s hard if we have them every year. It was also a subconscious reaction to collecting patterns. Fewer of our clients collect in a rigorous way and in grand Fifth Avenue apartments, you’ll find fewer 18th-century French drawing rooms.”

Now Sotheby’s has taken charge with its own version of the sale. The auction includes mostly French artwork from the 18th and 19th centuries. The house sees plenty of room for crossover among collectors who were traditionally focused on earlier material, according to Sotheby’s Old Master painting expert, Christopher Apostle.

Sotheby’s believes Old Master collectors may now look to later periods in their quest for quality. “This is a shifting in tastes,” Mr. Apostle said. “My Old Master collectors have realized artists from the first generation of the 19th century speak to them as well.”

Some of the sale’s highlights are strong enough to speak for themselves. The sale’s centerpiece lot, hung in the most prominent spot in the gallery, is a massive, 8-and-a-half-foot-tall painting of the Duchesse de Montebello and her five children, standing in the garden of their estate near Paris.

The wide-eyed children are adorned in suits laced with gold brocade. Their mother is elegantly dressed in a white silk gown and gold-edged shawl. They are noble, rich, beautiful, and sweet as honey. It is an early version of those annoyingly perfect photo spreads in Town & Country magazine.

“It is to me a quintessential image of the Empire style,” said Sotheby’s Mr. Apostle. “It is incredibly evocative in terms of characterization of the sitters.”

The Duchesse, in her white silk gown and flowing scarf and curls, is estimated at $2.4-2.8 million. Aside from the obvious size and beauty of the picture is the matter of its impeccable provenance. The seller is a descendent of the original sitters. The painting was handed down from the Duchesse to her eldest son, to his eldest son, and so on.

The portrait was commissioned in 1814, five years after the Duchesse’s husband, the Duc de Montebello, was killed from a cannonball wound at Aspern. Known to history as Marshal Jean Lannes, he was one of Napoleon’s best officers. The artist, society portraitist Francois Pascal Simon Gerard (who went by the nickname “Baron Gerard”) was 44 when he made the work. In 1809, he had painted a formal military portrait of the Duc.

Incidentially the auction record for a Baron Gerard was set at Sotheby’s in January 2002, when the Metropolitan Museum of Art paid $1.87 million for a portrait of Princesse de Talleyrand-Perigord.

There are other Gerards in the sale, but all with significantly cheaper estimates. A fleshy oil of two nudes, “Cupid and Psyche” is estimated at just $30,000-$40,000.It is a version of a 1798 work in the Louvre, which helps keep the price earthbound.

No Enlightenment sale would be complete without portraits of the era’s celebrities. There is a small canvas, “Voltaire Narrating a Fable” by Swiss artist Jean Huber (est.$18,000-$25,000). Huber painted a series of about 20 scenes from Voltaire’s daily life, according to Sotheby’s. This particular one captures the philosopher on a mountainside, arms splayed, expounding to a cluster of innocent country folk. They look a bit distracted, except for the family dog, who looks up with rapt attention.

A more significant work is a “Portrait of Benjamin Franklin.” Franklin went to France in 1776 and was instantly adored as the emissary from the colonies. His likeness was rendered by hundreds of artists. This portrait shows Franklin seated, clad in a red suit and fur collar, a bit dishabille. His hairline recedes, his neck is jowly, and his massive belly protrudes. He is no European dandy.

The painting is known to have sold for $1,000 (equal to about $20,000 today) to the Vice Consul of the United States in 1850 in an auction of property from H. van der Burch. He then sold it for $1,000 to the manager of the Astor House Hotel in New York City. When the manager retired in 1876, he sold it for $1,000 to relatives of the present owner. Only since then has it seen a big price jump: The painting is now estimated at $300,000-$400,000.

Understanding who actually painted the picture takes some work. “The present portrait is an autography repetition with some possible studio participation of one of the first and most important paintings made by a French artist after Franklin’s arrival in Europe, the portrait by Joseph Siffred Duplessis, exhibited in the Salon of 1779,” reads the auction catalog text.

In other words, this painting is probably not the first version of Franklin made by Duplessis, but is reckoned to be a later one. No one said shopping for Old Masters would be easy.

Franklin was hardly vain, and, according to Sotheby’s, he found sitting for portraits dull. “I have, at the request of friends, sat so much and so often to painters and statuaries that I am perfectly sick of it,” Franklin wrote. “I know of nothing so tedious as sitting in one fixed posture.” Sotheby’s notes that Franklin was pleased with Duplessis’s handiwork, however, which eventually became the model for his portrait on the $100 bill.

“Art of the Enlightenment” today at 10:15 a.m. (1334 York Avenue, at 71st Street, 212-606-7000).