Art: What's in a Wall?

It was strictly an assignment for an assistant curator. Workmen tearing down a tile factory in a Paris suburb had come upon some interesting old masonry embedded in the factory wall. Georges Poisson, assistant curator of the Ile de France Museum at Sceaux, traveled over to Choisy-le-Roi for a look. What he saw made his eyes pop. There, preserved under later coatings of the brick & mortar, stood the ornate facade of Choisy-le-Roi's "Petit Château"—the hideaway King Louis XV built for his mistress, Madame de Pompadour.

Modern Frenchmen had forgotten all about the Petit Château but in Louis XV's...