Two major exhibits in London this fall try to tease out an answer. “Madame de Pompadour: Images of a Mistress” at the National Gallery (through Jan. 12) and the Wallace Collection’s “The Art of Love: Madame de Pompadour” (through Jan. 5) are concerned primarily with how she transformed herself from the plain Mademoiselle Poisson first into a femme fatale and later into a power broker and patron of the arts. The National Gallery exhibit emphasizes her tireless manipulation of her image in works of art; the other is mainly a treasure trove of the lavish rococo furniture, sculpture and porcelain from Sevres and Meissen that she commissioned.

Both exhibits whisk the viewer into the colorful world of Pompadour’s imagination. Her relationship with the king began at a Versailles costume ball in 1745, to which her patrons won her an invitation after making sure her husband was abroad. Guests gossiped that a “yew tree” (the king) was paying particular favor to the “hunter goddess Diana” (Pompadour). Over the next two decades Pompadour sought desperately to keep the attention of the depressive and easily bored Louis XV and enhance her power over his advisers. She put on more than 50 plays and operettas at court. When his advisers realized how popular these were with the king they would clamor for tickets and roles, which Pompadour allocated only to those who pleased her.

She was a master of reinvention. When her sexual relationship with Louis began to wane around 1750, she knew that she could easily be thrown, penniless, into the street. So she stepped up the number of paintings she commissioned and actually increased her authority. The art works she acquired would be displayed at court, and as younger women began to supplant her in the king’s bed, the portraits would emphasize her friendship with and absolute fidelity to the king. Her favored artists, like Francois Boucher, no longer depicted her as a nubile Venus or Diana. Instead, she is shown as the worldly sophisticate, surrounded by musical instruments, globes, architectural sketches and highbrow books like Montesquieu’s “Spirit of the Laws” or Diderot’s “Encyclopedia.” Boucher’s “Madame de Pompadour at her Dressing Table” (1758) shows a woman who treats her face like a canvas, and also emphasizes the private locus of her power: courtiers seeking favor or offices from Louis clamoring to attend her toilette.

Ultimately, the fact that she had once slept with the king became the least interesting thing about her. Diderot declared that the legacy of Madame de Pompadour consisted of the art she inspired and commissioned, and “a handful of dust.” These clever propaganda paintings make it impossible to get a sense of what she actually looked like. Her eyes are sometimes gray, or blue, or green; her features shift. Nor do they allow us to comprehend her private life or her personal tragedies, which included the death of both her son and her daughter. Boucher’s flamboyant paintings crowded with decoration, her Caffieri furniture laden with opulent swirls, Pompadour’s 19-year shopping spree–these all feel like increasingly desperate attempts to fill a vacuum. These two exhibits make for a rich and fascinating glimpse into 18th-century France. But for all their sparkle and joie de vivre, they leave the impression that Diderot’s verdict was probably right.