ART / The Two Gentlemen of Venice: Andrew Graham-Dixon sees 'The Glory of Venice' at the Royal Academy as a contest between two great painters, one famous, one unknown (CORRECTED)

CORRECTION (PUBLISHED 21 SEPTEMBER 1994) APPENDED TO THIS ARTICLE

When Giambattista Tiepolo painted The Finding of Moses he turned Egypt into a place that looked suspiciously like home. The Nile has been shrunk to the proportions of the Grand Canal. Cleopatra has become a courtesan in yellow silks and pearls with a brittle, yapping lapdog, attended by a grinning dwarf and procuress. She looks down at the bawling, red-faced baby Moses, not with the conventional compassion but with a distant frostiness that gives the game away. This is a satire in diguise: Tiepolo's way of insinuating, circa 1740, that innocence does not belong in Venice. Even his paint seems touched with cynicism, uncharacteristically coarse and garish, as if in mimicry of Cleopatra's own cold gaudiness.

'The Glory of Venice: Art of the 18th Century', at the Royal Academy, commemorates a self-consciously decadent culture on the verge of dissolution. La Serenissima, in the last years of its influence, contemplates its own corruption. Retrospectively, the art of the Venetian 18th century, particularly of the later 18th century, seems moved by a prescient fatalism. Domenico Tiepolo, son of Giambattista, painted scenes of carnival gaiety that now seem strained, like dreams that have begun to turn into nightmares or Watteaus that have begun to turn into Goyas. (His drawings are stranger still, the oddest of them presenting the image of five men, a clown and a dog, their backs turned to us, holding umbrellas, waiting for something inexplicable, while four cursory birds wheel in a wet sky). But it was Francesco Guardi who, disdaining the workmanlike clarity of Canaletto's potboiled souvenirs, created the most enduring images of Venice in decline.

Guardi concentrated on the dilapidation of the city and dotted his pictures with those emphatic white highlights which were his shorthand for raggedy sailcloth, the raggedy shirts of gondoliers and raggedy laundry hung out to dry - but which were above all signs for the raggedness of the place as Guardi conceived it. Guardi also imagined Venice in the future, ruined like ancient Rome, and in his The Fire at S Marcuola the buildings of the city have become a mirage suspended between fire on the ground and smoke in the air. Guardi's drawings are even more extreme, turning people into coils and wisps of line, as evanescent as smoke.

The real heart of this exhibition, however, lies elsewhere, in the encounter - which turns out to be a kind of argument - which it skilfully engineers between the two greatest Venetian painters of the first half of the century: Giambattista Tiepolo and Giambattista Piazzetta. Their work and the tensions that animate it so overwhelm almost everything else in the exhibition that the entire exercise comes to feel like a pretext for comparing their distinctive, contrary geniuses.

Tiepolo is justly famous as the giant of 18th- century Venetian painting, though less famous than he was in his own lifetime, when he earned a somewhat dubious reputation as the artist who, in the words of one of his contemporaries, could paint a picture faster than most others could grind their colours. But Piazzetta, slightly older and quieter - a slower, more morbid and more darkly imaginative artist - is almost unknown in this country. In some respects, he puts Tiepolo to shame. He has been called Venice's Courbet. That is not a bad description, since his work exhibits a kind of brutal realism, but also allied to a most un-Courbet-like spiritual intensity - neither being qualities many people associate with Venetian painting. His breathtaking Supper at Emmaus is a picture which, among other things, contains what may be the finest painting of a plate of asparagus (Manet's included) in the world. An appropriately ghostly Christ, watched tensely by two coarse-featured disciples in the tenebrous half-light that so often illuminates his pictures, is breaking bread. He looks up, his expression an enigmatic mixture of sadness and hope. The still life elements in the picture are both intensely immediate and subtly allusive: the wine in the carafe a muted suggestive red, the asparagus stalks a morbid white and green, the colour of dead flesh, the tablecloth a winding sheet in miniature. The silent, serious intensity of Piazzetta's painting is moving and also more than a little disturbing.

There is a tremendous sense of weight in Piazzetta's pictures, almost of an obsession with gravity, which is an important part of what makes them so compelling. His St James Led to Martyrdom is one of the most remarkable pictures of the 18th century (partly because it feels much closer in spirit to the 17th, to the art of Rembrandt and Caravaggio) and much of its power derives from the sheer grounded substantiality of the figures: St James, painted as if he were an ageing but powerful hobo, pulls against the rope with which his loutish executioner has lassoed him, as if determined to walk away, up out of the picture and into the light of God. Transcendence is a struggle, however, and no one flies in Piazzetta: even his levitating St Francis is a terrible, heavy burden for the supporting angel. The mystery of Piazzetta's painting is sustained and deepened in his last and most affecting pictures, which seem to look forward to the 19th century: strange, melancholy pastorals, in which vagabonds and heavy- limbed women, prostitutes perhaps, loiter among rocks and trees under skies dark with threat.

Tiepolo can easily seem a frivolous artist, when compared with Piazzetta, and in some respects that is what he was - although he was so brilliantly frivolous, and his art was so perfectly adapted to the tastes of the age in which he lived, that this hardly counts as a criticism. In contrast with his rival, Tiepolo was a painter of weightlessness: the inventor of vast and teeming airborne worlds whose splendid unreality can only be hinted at, in any exhibition, in the form of sketches for his enormous decorative schemes. And where Piazzetta was a solemn, melancholic realist, Tiepolo was a daring fantasist, a painter of risque mythologies rustling with the sound of shot silk whose point is often no more - see his Rinaldo and Armida in Her Garden - than the perfect flash of the perfect thigh.

The most awkward and head-on collision between the two artists in the present exhibition, however, occurs between between Piazzetta's St James Led to Martyrdom and the picture which Tiepolo painted, for the same church, in an evident attempt to upstage it. His Martyrdom of St Bartholomew is a bizarre, low-toned pastiche of Piazzetta's masterpiece, which replaces its sinister chiaroscuro and gravitas with gaslit melodrama: Tiepolo's saint, a flailing geriatric, is held down by one executioner while the other, preparing to flay him alive, feels his flesh with his fingers. But even when his theme is this morbid, Tiepolo finds it impossible to give due weight to his figures: overstressing their anatomies, he produces curious expanses of weightless, too-fleshy flesh, which predict the work of Salvador Dali with amazing if quite accidental accuracy.

But the exhibition also restores to Tiepolo some of the seriousness that two centuries of being labelled a Rococo artist (with all the disapproval which that implies) have taken away from him. His secular allegories are not always entirely light-hearted and they become, perhaps, more morbid with age: there is pain in the gaze that the wizened Time fixes on the young and indifferent figure of Venus in his Allegory with Time and Venus. The most moving Tiepolos in the exhibition, though, may be his two small late paintings of The Rest on the Flight to Egypt and The Entombment. Mary and Joseph sit slumped under a tree in a rocky landscape while a vulture circles above them; the body of Christ is lowered unceremoniously into the tomb in a dark cave. They are religious pictures but they feel too personal to be only religious: one a painting about waiting to die, the other a painting about the event itself. Piazzetta painted like a man who could not forget about death, while Tiepolo painted like a man who could not bear to think about it. Or not until it was almost upon him - perhaps here, finally, we can see the great Promethean artist of his age, the artist who could create his own worlds on any wall or ceiling, no matter how large, finally conceding his own mortality.

Royal Academy, London W1, until December 14. Admission pounds 5. 071-439-7438 for further details.

CORRECTION

In yesterday's art review I referred to the finding of Moses by Cleopatra, when he was actually found by the Pharaoh's daughter. If any reader can lend me the use of an asp, I would be most grateful.

(Photographs omitted)