Abstract artists often prefer to talk about what they are trying not to do in their work. What you end up with is the equivalent of a photographic negative, a picture of someone in reverse, but this can still be illuminating, since the effect of any abstract art is so closely linked to the nature of its renunciations. The catalogues to Richard Serra's two current exhibitions at the Tate and Serpentine Galleries both contain interviews with the artist, often said to be America's foremost living abstract sculptor; and both read largely like lists of his dislikes. These turn out to be extensive.
Serra wants to avoid, in no particular order: 1) making anything that could be mistaken for ''an interesting object''; 2) ''theatrical effects''; 3) ''imagistic and metaphorical associations'' which ''lead by definition to the dismissal of abstraction by making it needlessly referential''; 4) ''all illusionistic strategies''; 5) ''decorativeness''; 6) ''allusion''; 7) shapes and surfaces that might seem ''too fanciful, too gestural''; 8) colour, except for black, which is ''a property, not a quality''. This seems fairly comprehensive, and it may leave you wondering what (if anything) is left.
It certainly does not sound too enticing. What is this art of anti-theatrical anti-imagistic anti-metaphorical anti-illusionistic non-decorative non-allusive unfanciful black objects going to look like? And what on earth might it have to offer anybody willing to take the trouble to find out?
The Serpentine exhibition, ''Richard Serra: Drawings'', is the first British showing of the artist's graphic work, although it should be said that these are not really drawings in the conventional sense and that they demonstrate little in the way of conventional graphic facility. In fact they are more like paintings, being executed in oily black paint-stick applied thickly to primed canvases. And they are even more like barriers: they are visual obstacles, these unrelieved black squares or rectangles, cul-de-sacs designed to frustrate that fanciful, allusive, metaphorical imagination which Serra clearly distrusts so much.
Serra's black canvases contain none of the spatial surprises found within the black paintings of, say, Ad Reinhardt, whose effects derive from the tiniest modulations of tone within a monochrome field. Instead, Serra's black is uneven but essentially unmodulated: it does not read as pictorial space but as solid substance. There is a kind of extremism at work here, which may have less to do with asceticism than with the ambition to turn one art form into another: Serra's graphic works aspire to the condition of solid objects, and they actually look less like drawings and more like those thick steel plates with which - precariously balancing and propping them against one another - he has often worked as a sculptor. They even look heavy, an impression which the artist achieves partly by hanging them so that their bottom edges rest on the floor - a way of suggesting that the sheer weight of them would be too much for the walls.
Serra does (encouragingly) have one or two positive remarks to make about his work, although these tend to be somewhat cryptic. Such as: ''The black shapes, in functioning as weights in relation to a given architectural volume, create spaces and places within this volume, and also create a disjunctive experience of the architecture.'' This does at least begin to suggest what his intentions might be, although it sounds a little disappointing, a little flat. The function of these dark expanses of paint becomes one of emphasis. They exaggerate or reinforce whichever walls of the gallery they are attached to. They alter (or reinforce) the character of the architecture.
So when Serra places two of his large black rectangles in the main domed gallery of the Serpentine, they anchor and stress the walls to which they are attached and make the squareness of the space peculiarly apparent (as well as unbalancing and squeezing it). And when he makes a long ''drawing'' run around a corner of another gallery, it is a way of emphasising the fact of one wall abutting another and, also, of making the architecture read as abstract sculpture - it is a way of turning the gallery itself, in effect, into a Richard Serra, an arrangement of planes propped against one another in space.
Whether an art so deliberately specialised in its effects can engage an audience remains open to question, and at least one of Serra's titles may indicate a certain nervousness on this score, as well as a desire for a rather wider expressive range than that suggested by his remarks in the catalogue to the show. Those two black canvases under the Serpentine dome are called Two for Rushdie.
This comes as something of a shock. What had seemed a fairly arid exercise in elucidating or disrupting the character of a specific building, of making it seem sculptural in an abstract way, suddenly assumes a different aspect. As a title, Two for Rushdie is a dig in the ribs: hey, wake up, this is a political statement! Those dark rectangles become windows leading on to non-existent views, and the space as a whole becomes a symbol of imprisonment, of ideological coercion. Or at least in theory: this may be a case of asking a title to do rather too much for the work to which it is affixed. But it certainly suggests that Richard Serra's multiple disavowals of intent should be read a little cautiously. Would an artist who really wants to avoid ''allusion'' and ''metaphorical associations'' go to the trouble of invoking the predicament of Salman Rushdie?
Serra is known for large, aggressive steel sculptures that threaten the viewer with the apparent possibility of their collapse: teeteringly balanced, held upright at Tower-of-Pisan angles. At the Tate, Serra has responded to an invitation to show in the Duveen Galleries with a more restrained, understated aggression. His solution to this long, tall corridor of a space, with its high rotunda and classical columnar architecture, has been to place two extremely heavy blocks of rusting steel on its central axis, separated by a distance of 140 feet. Serra's aim, he says in the catalogue, has been ''to redirect how one thinks'' about the ''overblown, authoritarian'' nature of the space.
The effect is enormously combative, and Serra's pieces have a kind of unexpected power: they are dwarfed by the proportions of the galleries which they occupy, yet they counter this with their sheer mass. They read like condensed equivalents of the architecture that contains them; it is as if all the space and airiness of the Duveen Galleries has imploded, has been concentrated in these two heavy, sullen forms. To all appearances, they might be hollow (they are not: each weighs between 30 and 40 tons) but somehow you feel their solidity, their material density in all that emptiness.
It is hard to say what Weight and Measure (Serra's title for the Tate installation) is about, exactly, but it does convey an extreme sense of confrontation between art and architecture. Where the Duveen Galleries are devoted to spaciousness, and adapt the language of the Greco-Roman architectural tradition to that end, Serra's works are consecrated to squatness and heaviness, and draw their inspiration from the less graceful and more forceful tradition of ancient Egyptian art: they are, in effect, abstract sarcophagi. Their intention, too, is vaguely funereal. They want to kill the space they occupy.
There is a photograph of Serra on the frontispiece to the Tate catalogue, and he looks remarkably like his work: in faded jeans and hooded sweatshirt, staring out at you with an expression that has ''No bullshit'' written all over it, he appears as the Marlboro Man of contemporary art. He has used the Tate Gallery show, you might say, to enhance much the same image, seeing it as an opportunity to define his own, tough, all-American aesthetic, the aesthetic of the steel mill rather than the marble quarry. Where the Duveen Galleries stand for classical reference, for grace tinged with grandiosity and a certain effeteness, Serra stands for masculinity, gravity, focus, concentration. There may even be a slight hint, here, of his political orientation: his antipathy to the ''authoritarian'' cultural space of the classical museum vaguely suggests he is more of a Democrat than a Republican. But what, beyond that, does he stand for? He seems to be giving little away. The toughness of his art, on this occasion, serves a primarily iconoclastic purpose, expressing disdain for the space in which it finds itself. Serra has defined himself at the Tate - but mostly in the negative.