The Sacred Made Real: Spanish Painting & Sculpture 1600-1700

The pale, crucified figure of Christ painted on a black background is as Spanish as bullfighting or Manchego cheese. This somber style of painterly realism is so quintessentially Iberian that no one has much concerned themselves with where it came from, presuming it a Catholic thing, with a touch of the cold hand of the Inquisition and some of Caravaggio's Italian chiaroscuro thrown in for good measure. However, an astonishing new exhibition at the National Gallery, 'The Sacred Made Real', reveals that the eye-popping contrast between light figure and dark ground - or tenebrism - achieved in these paintings most certainly came from the close study of hyperrealistic wooden sculpture.

These hand-coloured, life-sized idols, of Jesus wilting on his cross or the Virgin Mary clasping her hands in prayer, have been largely ignored by art history and consigned to kitschdom, or else seen once a year, as they are carried bobbing and swaying hypnotically up and down the candle-spattered streets of Seville or Granada, for the Holy Week, or Semana Santa, of Easter.

The first of many revelations here is that Spain's two greatest Golden Age painters of black and white, Diego Velázquez and Francisco de Zurburán were both trained in the painting of sculpture, or polychromy, itself an art form rarely appreciated or practiced outside of the country (although bad examples of such religious effigies can be found in churches from Madrid to Manila).

Not only was Velázquez trained by one of the masters of polychromy, Francisco Pacheco (an Inquisition censor), but the show's opening image is his portrait of Juan MartÌnez MontaÒés, one of Spain's best carvers at work - someone he obviously held in the highest regard. The real discovery here is the indisputable quality of some of these forgotten sculptures, hewn mostly from single hulking blocks of cypress or cedar, although always delicately enough for every bulging vein and expressive brow to be raised just so.

Detail reaches grisly proportions in Juan de Mesa's decapitated head of John the Baptist, where the spinal cord, oesophagus and trachea are recreated down to the last sinew, and in Pedro de Mena's 'Ecce Homo' depicting Christ's gouged and flayed skin, striped red raw from his 39 lashes, with his blood pooling convincingly at the loincloth.

With such emotive subject matter - Mary Magdalene genuflecting towards us in grief, Jesus variously beaten, hung and bondaged - it's perhaps not surprising that 'The Sacred Made Real' has already reduced a few squeamish or pious visitors to tears (one Time Out writer among them) but there is more than sheer theatricality on display. What could have easily become a Madame Tussaud's waxwork torture chamber experience is elevated, not just by the loans of some great Spanish masterpictures, but also by myriad subtleties beyond the gore and the gospel. Because, even the efforts of Spain's most talented modeler, MontaÒés - who shows Christ slumping down, seemingly in the moment just after his death, and Saint Francis Borgia in the 'Alas, poor Yorick' pose (although the skull has been removed as the National Gallery has no licence to show human remains) - would all be for nought without the restrained interventions of the polychromer, the make-up artist who breathes life into the wood.

The air-conditioned spaces of the National Gallery may be a world away from the dark chapels and cross-bearing penitents that usually accompany these shadowy figures in their ecclesiastical homes, and secular London could not be further removed from the kind of chest-beating Catholicism that Andalucia does so well, yet the atmosphere of the lower Sainsbury Wing is strangely apt for the funereal gloom and hush that descends the stairs with you. It is a truly compelling exhibition that can elicit such strong responses with such unknown material.