There is not much that we know about the life, let alone the personality, of Francisco de Zurbarán, the 17th-century Spanish painter who is the subject of a new exhibition at the National Gallery in London from May 2. Yet two pictures that top and tail the show from the Prado in Madrid tell us a good deal. The Apparition of Saint Peter to Saint Peter Nolasco is an astonishing, shocking work, which strikes the viewer – at least, this viewer – as extraordinarily modern. The friar saint, founder of the Mercedarian Order, kneels before another Peter, the apostle. But it is St Peter as he is being crucified upside down, slanting down the canvas in a dramatic head-first plunge, supported by rusty clouds. The saint is old, his lips drawn over his teeth and his unseeing eyes staring, his cheeks reddened – this is not a dignified death but an undignified torture. The kneeling St Peter Nolasco gestures to him in reverence. In this drama the two figures are separate and distinct, drawn together by the friar’s compassionate gesture and downward gaze.