Long live the Mexican revolution; EXHIBITIONS

David Alfaro Siqueiros tried to kill Trotsky. But the Mexican revolutionary was a far better muralist than murderer. And, a new show reveals, he was a pretty good easel painter too

The Whitechapel Gallery has a winner of an exhibition in its account of David Alfaro Siqueiros: this painter is a man of legend yet is rarely seen by the gallery-going public, especially in Europe. Now we have the opportunity to understand his dark, impulsive character. The Whitechapel show covers only 10 years of the artist's life, from 1930-40, but this was obviously his crucial decade and one could hardly hope for a better introduction to a South American revolutionary career.

Only an introduction, however, since the murals cannot be represented. Together with Jose Orozco and Diego Rivera, Siqueiros was a leader of the Mexican mural movement. This was an important part of his contribution to the renaissance of Mexican art. Most of us know such murals only by repute and in reproduction. At the Whitechapel there's a helpful video about the muralists' political projects. Perhaps murals look more convincing when you've got a camera panning over them than they do in still photography. Anyway, in the exhibition proper we encounter Siqueiros as an easel painter - and surely his portable pictures give us a sense of the man who painted wall-sized decorations.

They seem to wish to burst the bounds of their canvases. Siqueiros was a painter who railed against constraints, and the more he protested the more desperate his expression became. In fact he was one of the desperadoes of modern art (who form a smaller band than some people imagine). His violent desire for justice and progress meant that he took up each of his paintings as though he were attacking an enemy. Colour, touch and drawing were separately assaulted. The palette is bituminous, the handling scruffy or forced, the outlines crude or desultory. Siqueiros is one of the least cultured of painters. Yet he was not in any sense a "primitive" and he produced effective art while loading his paintings with faults of taste and execution.

Primitive or naive painters never wish to change the world. Siqueiros as man and artist was driven by the thought of revolution. He was born in 1898, went to a Mexican art school and by 1919 was in Paris, where he met Rivera and must have had some contact with modern art. In this exhibition there's just one painting, Head of a Man Smoking of 1930, that looks Parisian, so I conclude that his European contemporaries meant little to him. Next we find Siqueiros in Barcelona in 1921, where he published his Manifesto to the Artists in America. In 1922 he was back in Mexico, issuing more manifestos, now widening his address to include peasants, intellectuals and the proletariat. As the muralist movement gathered more adherents Siqueiros worked as union organiser. He served his first term in prison in 1930 and in 1932 had his first one-man show.

Siqueiros's political activities include service in the Spanish Civil War and a horrific attempt to assassinate Trotsky and his wife. The idea of a murder for Stalinist political ends appears not to trouble the curators of the exhibition, Olivier Debroise and Mari Carmen Ramirez, whose catalogue-cum-book (pounds 32) contains much valuable material but lacks a liberal horizon. The exhibition broadsheet says that "unfortunately" Siqueiros failed in his attempt to blame the shooting on Rivera, who by that date (1940) had become his enemy. A less complaisant attitude towards the mind of a professional revolutionary would have been helpful. What might Siqueiros have done to his opponents and former comrades had he achieved power?

Without knowing the murals at first hand, I think it possible that they differ in tone from the paintings at the Whitechapel. After all, it is in the nature of mural art to be public and to convey positive messages. The easel paintings, though they relate to the murals, are surely of a different sort. They are by Siqueiros the incompetent murderer. A private and limited imagination feeds on images of suffering and repression. Bondage of one sort or another is a common theme. The big Proletarian Victim shows a nude woman, head bowed, with ropes around her fleshy body. In The Torment a prisoner is hung by his thumbs. Nobody is a free agent of their own destiny, except General Emilio Zapata, assassinated in 1919 and thereafter a permanent hero of the popular movement. So the Zapata portrait belongs to the muralistic side of Siqueiros's art.

Peasant Mother is an impressive attempt at a national icon. A mother and child between cactus trees are no doubt victims of the deportation of Mexican workers from America during the depression. Siqueiros himself was often in America and influenced Abstract Expressionism. American painters were interested in his uncommon use of materials: Siqueiros painted on burlap, utterly unsuited to niceties, and used commercial paints mixed with alien additives like honey and sand. Jackson Pollock, a desperado on his bad days and an angel on his good ones, used these materials too (though not burlap). I think he was also gripped by the sort of churning rhythms that may be discerned in Siqueiros's underpainting.

The difference is that the angel in Pollock triumphed. He came to a serene painting style that is the antithesis of the Mexican's tumult. I see Pollock as a classic, Siqueiros as a phenomenon. Whatever the power of paintings such as Proletarian Mother and Down But Not Defeated Siqueiros never achieved a permanent level of fine art. There are a number of curiosities in this exhibition, including garbled attempts at Hollywood portraits. They are trying to be strong and at the same time are sentimental - a mixture in the top of the head of many a murderer.

! `David Alfaro Siqueiros: Portrait of a Decade 1930-1940': Whitechapel Art Gallery, London E1 (0171 522 7878), to 2 Nov; Tues-Sun 11-5, Wed 11- 8. The gallery will be showing complimentary films, including `Mexico: the Frozen Revolution' and `Night of the Iguana', at 6pm on 10 & 24 Sept and 8, 15 & 29 Oct.