The Philadelphia Museum of Art is fortunate to be able to show many aspects of the protean genius that was Peter Paul Rubens (1577-1640), one of the artistic giants of the seventeenth century. In addition to the splendid Constantine tapestries woven after designs by Rubens (and Pietro da Cortona) that surround the Great Stair Hall, Ruben's impressive Prometheus Bound is displayed in one of the second-floor painting galleries. The focus of the present exhibition, however, is on an earlier stage in Ruben's creative process, one in which he formulated his compositional ideas--the oil sketch. Prolific as both a draftsman of the quickly rendered pen or chalk sketch that recorded an initial thought, and the detailed figure study, Rubens relied as well on his dexterous brush to work out his pictorial conceptions. But his oil sketches are more than just functionally aligned with drawing; they share with this medium a sense of intimacy and spontaneity as we observe the artist rapidly executing one passage and rethinking another. Yet the very materials of the oil sketch--panel and oil colors--unmistakably link them to the artist's finished paintings. This "aesthetic dualism," as described by Julius Held, author of the critical catalogue of Ruben's oil sketches (1980), contributes to the fascination that these hybrid creations hold for us. The panels that Rubens used for his oil sketches were commonly prepared with a smooth, light-colored ground over which a transparent gray or light brown primer was applied. This imprimatura, often brushed vertically, horizontally, and diagonally on a single surface, frequently remained partially visible, as Rubens recognized the capacity of its striated appearance to create an illusion of space around figural groups and landscape motifs. Following a cursory blocking in of the composition with charcoal or black chalk, traces of which occasionally are still visible, Rubens would brush in his forms using brown or reddish-brown pigments. Lead white would be dabbed on for highlights, and black strokes added for emphasis or structural clarity. The amount of colored pigment added and the degree of finish given to the sketch depended on Ruben's purpose in making it. Preparatory designs for prints would remain in a grisaille state. Sketches that Rubens made for subsequent use by him or by a collaborator would be carried out only to the point at which a compositional solution became apparent. If, however, a sketch were to be submitted for the approval of a patron, Rubens would devote greater attention to the details and colors that he envisaged in the final work. Rubens highly valued his oil sketches and kept them until his death. Even in the seventeenth century a market existed for copies of those sketches that he had completed himself. As a result, art historians today not infrequently debate the attributions of certain panels, and Julius Held's published opinions on two questionable sketches are examined in this exhibition. In the United States the collection of Ruben's works began slowly, but in 1914 Wilhelm Valentiner could write, "The laudable exception is Mr. Johnson of Philadelphia who has a special liking for Rubens' studies and owns almost one fourth of the works from his hand that have thus far crossed the ocean." Also exhibited are oil sketches by a number of Ruben's contemporaries in Antwerp, including Abraham van Diepenbeeck, Cornelis Schut, and Erasmus Quellinus II.