Bather with beach ball by Pablo Picasso

Wherever there was an expressive possibility in any kind of earlier art, Picasso found it and exploited it. Interestingly, his own invention and Braque's, Cubism, initially posed a problem in this respect. Faces were swallowed up in puzzle-like compositions, their features emerging only obliquely, incidentally, encoded. The face of the dealer Wilhelm Uhde in the Cubist portrait here is reduced to a few pinched lines, while the one of Daniel-Henry Kahnweiler is nearly impossible to decipher. It took time before Picasso realized that Cubism could actually be deeply expressive: that the disconnection and rearrangement of parts of a person's appearance opened up vast new emotional possibilities for portraiture.

You see the results in Bather With Beach Ball, Picasso's sardonic remake of his young mistress, Walter, as a pneumatic plaything, a floating zeppelin with a blow-up tube for her head.

The portraits of Walter are often said to be Picasso's best. Some of them are remarkable: heraldic paeans to sex, composed of ripe, somersaulting shapes of unmodulated violets, reds, yellows and blues, bounded bp thick black contours. They are voyeuristic, erotic.

But the Walter portrait here, is not his finest. The more memorable works, it turns out, tend to be unexpected. For instance, there's one of Sara Murphy (or whoever) painted with sand. Braque painted with sand, so this portrait is partly a response to him, but what makes it special is the tender effect: a kind of low relief of soft forms fading into shadow that creates an evanescent image, a face seen in the half-light or as if through a scrim.