Architecture: The Vulcan has landed

It's a surreal thing. A gigantic tin man with welded seams and a Frankenstein's Monster face stalks through the core of the new national Dean Gallery in Edinburgh. Vulcan, a 15-foot sculpture by Sir Eduardo Paolozzi, is allegorical: Vulcan, god of fire and the working of metals, as well as the patron of craftsmen in mythology, shows us that post-modernism is back in fashion. Man and machine are celebrated in Modernism but the post-modernists like to put a bit of poetry and fantasy back into their projects.

Terry Farrell, the architect, shaped this gallery inside an old stone orphanage. The corridor that runs 154 feet from one end of the building to the other is tall and stuffed with objects of curiosity; the glass showcases are lit with brilliant fibre optics and every available space is used to show off the collection.

A hippo skull atop a joist overlooks busts and torsos perched above architraves on the pediments. Paolozzi's Chelsea studio is recreated in the gallery - even the magazine pages from which he cut out images to make his collages are stored here, giving a fascinating glimpse into his way of working.

Paolozzi has said that his ideal gallery would be a gutted cathedral, full of clutter and change. Farrell has sympathy with that: "There is in my own work a magpie quality in what I do. I like the mix and unlikely juxtaposition - such as the garage-like TV station [TV-AM] I did in Camden, the grand palace that is the Charing Cross office."

A shy man, Sir Eduardo deliberately turned up late for the opening on 25 March but Lord Snowdon, who opened the exhibition, was clear in his opinion that a gallery should be created for the artist in his lifetime. "Absolutely right for an artist of his standing. It was generous of him to donate his work to the nation so now it's there for posterity."

And then there are the Surrealist archives: Salvador Dali introducing Surrealism to the British at the New Burlington Galleries in 1936 in full diving gear, including a bubble mask, which rendered his speech inaudible. The shock waves of this audacious art had a profound effect in Britain.

"The British have always had a love affair with Surrealism," says Ann Simpson, curator of the archives. "From my point of view Paolozzi's work shows a seamless join with Surrealism - multiple sources, a lateral view of life. These images comes straight out of that."

You don't have to know much about Surrealism to get the message from Cloak of Secrecy by Conroy Maddox, 1940, which is exhibited against one of the yellow walls at the far end: a torso cut off above the waist, cloaked in red gauzy net with two billiard balls where the breasts should be.

The restoration is sympathetic to the building by Thomas Hamilton (1784- 1858). Farrell calls the building a masterpiece though it is a weirdly wonderful one - all facade and no content. Hamilton designed this Edinburgh orphanage in 1830 as a fantastic baroque flourish.

The two wings and each corner support clusters of impossibly high crenellated towers that are the flues. The chimney pots hide shyly behind overscaled urns that scrape the sky. The building is tiered like a wedding cake with baroque features that are out of time with the strait-laced Scottish interpretation of Regency style.

This exuberance breaks down once inside the Ionic portico. Here the rams' head bas relief on the walls explains the homage to the benefactor paid in the facade with all its flourishes and the austere interiors. The ram is the symbol of the shepherd with his flock. The outside of the orphanage pays homage to the benefactors, the interiors austere as befits a workhouse.

Architects working there at nights for the past four years say that although they have never seen a ghost, the place was spooky. Farrell has done everything to dispel that. Now the ram motif inspires the chairs in the Dean Cafe which stand on cloven-hoofed feet. Small silver studs on the black wooden seat represent the constellation of Aries, sign of the ram. Daylight from roof lights shines through glass panels in the floor above and onto intensely coloured walls, bringing the place alive. But there are pools of shadow to keep the mystery.

Sue Farrell, the architect's wife, has collaborated with him for 15 years and in the magnificent cobalt-blue corridor, sparkling under fibre optic lights, she has achieved the impossible feat of warming up blue to an Yves Klein intensity that makes the space dynamic. Blue is usually cool but this colour is not. "It's called Betty Blue Two, after a Dior dress Queen Elizabeth II wore in the Sixties," Terry Farrell explains.

Girls and boys were separated by a thick stone wall which had the effect of cutting brutally in half the gracious Neo-classical building with two wings on either side of the entrance. The windows were above fireplaces where you would expect a flue and it is possible to stand in front of the fire and look out into another room, or across into leafy Edinburgh.

What is more challenging is that there is no sign of the flues, with chimneys all gathered together on top of staircases. "I've never seen such a thing." Terry Farrell is bemused. "Hamilton continuously did it throughout the building. It was like a mad challenge, obsessive." So Farrell, of course, kept them and added a few of his own, and used glass bull's-eyes set into the floor to house some of Paolozzi's beautifully sculpted feet and hands.

There was no spatial organisation, just dormitories and hospital wards off endless corridors. Farrell's brief was to supply public galleries to house the permanent Paolozzi collection and the Dada and Surrealist art, mostly collected by Roland Penrose and Gabrielle Keiller, plus storage space in the basement and offices in the wings for the National Galleries of Scotland. The Heritage Lottery Fund found pounds 6.5m for the pounds 9m project.

First he blasted through the block that separated the girls' and boys' wings and concentrated on a double-height floor for the great hall where the Vulcan sculpture landed. Then he put in two bull's-eye windows in the floor so that you can see from top to bottom, just as you can see from one end of the corridor to another.

Now there is a legible reading of the building as a total space, the reverse of that almost blockaded 19th-century orphanage. To insert load- bearing floors, bathe it with light and create a central axis uniting the building was technically difficult - putting porthole windows into structural support walls is a real challenge - but the effect is seamless. You would never know.

Like all post-modernists - and Farrell admits we do live in the post- modern era whether you want to call it New Modernism or whatever - he has always enjoyed the allegorical.

He has respected the Classical with Hamilton's plan uninterrupted, built rooms for art that he admires and used the ideas of William Blake as inspiration. William Blake challenged the legacy of Isaac Newton in his own time and Farrell, reading through his poems, found 40 references to Newton in them.

So he took Paolozzi's great figure of Newton which was designed for the British Library and levitated a second casting among the diners in the gallery's cafe. Props winched underneath with boxes removed form a void which means Newton rises off his plinth effectively to hang in mid-air. Terra is not so firma in the Farrell lexicon.