For his Majorcan altarpiece, Miquel Barceló says he regarded the clay as analogous to paint. The result is breathtaking , writes Aidan Dunne.

It's a bitterly cold day in Palma de Majorca and the artist Miquel Barceló, wrapped in a dark woollen coat and a white scarf, is hunched against the chill winter air. Hunched as well, perhaps, against the questions of dozens of journalists, all of them assembled for a preview of his epic ceramic installation in a chapel of the city's fine cathedral - officially inaugurated by King Juan Carlos on February 2nd. A formidable building in warm yellow stone, still incorporating its 14th-century Gothic origins, and replete with rows of flying buttresses, the cathedral is proudly situated over the harbour, looking out on the Bay of Palma.

Thirty years or so ago, Mallorca's then most famous resident artist, Joan Miró, proposed designs for a set of stained-glass windows. Somewhere along the line there was a failure of nerve and the scheme was never pursued, which makes it one of the great missed opportunities of public art patronage: Miró and stained glass being a marriage made in heaven.

As though making up for this lapse, at the end of 2001 Barceló was enlisted to make an altarpiece. It was a brave initiative on the part of Bishop Teodor Úbeda (who sadly died before the altarpiece was installed), but it didn't come out of the blue.

The process had begun when Barceló, probably Spain's most celebrated living artist and a Majorcan, was approached about accepting an honorary degree from the University of the Balearics - it was duly awarded last Thursday. But what he would really like, he said, rather than just accepting a degree, was to do something in Majorca. One possibility was a retrospective exhibition in the cathedral.

And, it was suggested, Barceló might like to design some gargoyles - that assignment is still outstanding, but the artist doesn't think it will ever happen now. Perhaps it's become a bit superfluous. In the meantime, he realised the cathedral didn't make an appropriate venue for an exhibition. The bishop offered him the Chapel of the Holy Sacrament, a substantial chapel to the right of the high altar, as the possible sight for an artistic "intervention". Barceló proposed making an ambitious altarpiece, and his proposal was accepted.

There remained the problem of funding. In the end, a foundation was established, comprising representatives of a veritable array of civil and cultural bodies. The theme, the miracle of the loaves and fishes, was suggested by the bishop and Canon Pere Llabrés, and enthusiastically embraced by the artist. It fitted him like a glove, he said. The finished work is an organic whole but falls into a triptych arrangement: on the left the sea, on the right the land, in the centre an evocation of "frontal humanity" with emblems of mortality and a spectral figure of the wounded Christ. The walls are alive with teeming shoals of fish, with a profusion of crusty loaves and ripe fruit.

Barceló, who was born in 1957, is an engaging character, omnivorously interested in things, and forthright in his speech. Physically he is compact and contained, with a wide face and bright, lively eyes. He came to prominence at a relatively early age as a bold textural painter. He was, he has said in the past, part of a post-Franco generation that yielded relatively few artists but was central in the process of opening Spain up to the outside cultural world. His own initial popularity coincided with the international success of Neo-Expressionist paintings across Europe.

Barceló's achievement was to look both outward and in, forward and back. His hugely energetic style of painting, combining geometrical underpinning with thick, amorphous swathes of meaty pigment, often on ambitiously large scales, looked equally related to Anselm Kiefer's portentous historical statements and Antonio Tapies' grittily textured compositions, redolent of the worn facades of the buildings in Barcelona's Gothic Quarter. From there, one could trace a lineage back to the sombre strain at the heart of classical Spanish painting.

As with many artists who rose to international prominence in the midst of the 1980s revival of painting, Barceló has utilised the momentum gained to propel his career since, even though the fashion- conscious gaze of the art world has to a large extent shifted elsewhere. To his great credit, Barceló has continued to engage in increasingly ambitious projects on a wide variety of fronts, and his gutsy approach to the Palma Chapel was reputedly instrumental in winning him what may well be - though it is as yet unconfirmed - his next large international commission, for the UN in Geneva.

He spends much of his time living in Mali - work made there will feature in a show at Imma next year. His chapel in Palma drew attention from the press, and from television and radio networks, in Spain, Germany, Italy, the US, and indeed Ireland, partly because of his international profile, and partly because a church commission on such a scale, and of such aesthetic ambition, is these days extremely rare. Hughie O'Donoghue's Passion cycle, for example, had a secular patron.

At first hand, the enormity of what Barceló took on in the Chapel of the Sacred Heart is breathtaking. Early on, when a friend asked him how he was planning to do it, he replied simply: "By doing it."

Inevitably, comparisons with Michelangelo and the Sistine Chapel come to mind. Barceló's chapel is not on that scale, or in that league, it has to be said, but it's audacious and impressive.

It's also, strikingly, as a documentary on the project makes clear, a solo effort. That is, inevitably he worked with ceramics experts, led by Vincenzo Santoriell in Italy, and the five stained-glass windows were made in the studio of Dominique Fleury in France. But he didn't just devise a scheme, make a maquette and hand it over to the technicians. Far from it. It's fair to say that everything about the work was shaped by his vision of both the process and the desired result.

To a remarkable degree, what we see in the chapel - some 300 square metres of ceramic sculptural façade - is the product of his dogged, day-in, day-out, hands-on effort. He says he regarded the clay as analogous to paint, and in practice he treats the whole piece as a painting, albeit one of daunting magnitude.

Established in a huge studio space in a village close to Naples, he worked long days for four months continuously during the spring and summer of 2003. A cold spring gave way to a sweltering summer - so hot that he had to work by night rather than during daylight hours. Footage of him negotiating elaborate scaffolding and grappling with clay, clad in a sleeveless vest and dripping with sweat, recalls Bruce Willis's character in Die Hard.

Early on he decided, with a stroke of genius, that he would not structure his ceramic in conventional, regular panels of clay. Instead, he wanted it to form a continuous skin cladding the interior of the chapel. Hence the enormous, sloped scaffolding seen in the documentary, clad with a clay membrane. A gamble was involved here.

Barceló watched anxiously as the clay dried and cracks and fissures opened up across its surface. Isn't art all about observing nature, he observed wryly. In effect, the clay was dividing itself into irregular panels, bounded by lines and spaces that form part of the finished work. Paradoxically, they pull the whole thing together, as well as providing an intimation of time and mortality.

The process of creating his imagery was exceptionally physical and demanding. He improvised a whole battery of tools and effects, using his bare hands, kitchen implements, air compressors, even boxing gloves, the latter employed to punch rotund mounds into the surface from behind - for he worked the clay from both front and back, as he explained in gleefully ribald terms. He even threw lumps of clay at the surface. Ultimately, he is a spontaneous artist, which meant he was under huge pressure, working with clay that dried out with every passing minute.

As you encounter the installation, at eye level, you are treated to a lavish feast of three-dimensional imagery: packed masses of myriad fish species and crustaceans, jumbles of clay amphora, heaps of loaves and fruit. Barceló clearly fell in love with one particular device, that is: poking through a bulge in the clay, by which means he evokes, variously, the gaping eyes and mouths of fishes, the incisions etched into the tops of baked bread, and the splitting of over-ripe fruit. He does all this with a conjuror's flourish that is hardly surprising when you remember that he has been consistently interested in, and excited by, the transformation of materials entailed in the artistic process.

There are resonances of Spanish Romanesque art, and of Roman murals, for that matter, and much more besides, in what he has made. In terms of religious iconography, the installation might be viewed as an iconoclastic departure from convention, but it relates strongly to the Spanish tradition of vast sculptural altarpieces or retablos. As John Moffitt notes in his history of the arts in Spain, these monumental architectural schemes of densely packed, painted sculptural reliefs derived their names from the Latin retro tabulus(behind the altar table).

They date from the Middle Ages and the Renaissance, and were "for a long time the main theatre for collective Spanish artistic endeavour". They were not peculiar to Spain, but the Spanish made them their own or, if you prefer, went to town on them. Some things have changed. The medieval retablosembodied a fierce level of religious commitment.

Barceló is considerably less committed when it comes to describing the religious status of his own retablo.

"It is not a religious work of art," he says flatly. "It's a work of art located in a cathedral."