Amid the classical elegance of the New Town, the turreted, red sandstone, neo-gothic building on Queen Street looks ecclesiastical.

The entrance guarded by William Wallace and Robert the Bruce, however, gives the first indication that the reverence paid here is to great Scots.

When the main door of the Scottish National Portrait Gallery is thrown open on December 1, after being closed to the public for the best part of three years, it will be clear that 122 years after it was first opened, as the world’s first purpose-built portrait gallery, it remains a perfect showcase for the growing collection of portraits, sculptures and photographs of notable Scots. As ever kings, queens and wealthy patrons share the space, Jock Tamson’s bairns-style, with footballers, scientists and poets. Visitors will find weel-kent faces from Robert Burns to Sir Alex Ferguson but will also come across a mask of Dolly the sheep, while the ladies who meet for coffee can be reassured that the airy new café will still serve the famous cheese scones.

Lesley Stevenson, chief conservator for the National Galleries of Scotland (NGS), looks upward with a smile on her face as soon as she enters the Great Hall of the portrait gallery. The most daunting of her conservation and restoration tasks has been the cleaning of the mural, frieze and the star-studded “astronomical ceiling” by William Hole which sets the tone for the whole gallery.

“It was clearly unthinkable to re-open the building without making every effort to ensure that these great works of art, which are such a fundamental and unique aspect of the building, were given some much-needed attention. I don’t think the ceiling had been cleaned since the 1890s”. A grant of £45,000 enabled her to bring together an international team of conservation students. Thanks to their eyeball-to-eyeball confrontation with historical characters on scaffolding two floors up, Scots who have made their mark on the world from Thomas Carlyle back through David Livingston, James Watt, Robert Burns, Adam Smith, David Hume, the Stuart kings, Robert the Bruce to Saint Ninian are striding round the walls in newly gleaming colour and the gold stars are twinkling from their dark blue background.

Stevenson’s enthusiasm for the gallery’s new lease of life is infectious. She begins a whistlestop tour on the second floor with the 16th century artist George Jamesone, a significant figure because he was the first native Scot to make an impression in a profession dominated by foreigners. In painting a variety of people: merchants and lawyers as well as his patron, and a set of Scottish monarchs, he sets the tone for the gallery’s claim to be a portrait of the nation.

The building has been relieved of internal walls added over the years to make offices and storage space. As a result the light now floods in through the arched windows as originally intended by the architect Sir Robert Rowand Anderson. The work means there are fewer hidden treasures but the capacity to surprise has been retained. Amid the predictable scenes of curling, hunting, golf and football in the section on sport, Stevenson points with delight to a banner announcing a women’s football match between Scotland and England.

Her most challenging task of the refurbishment was the removal of two very large historical group portraits which are key pieces in one of the major exhibitions now on the top floor: Imagining Power, exploring Jacobite history. She explains: “They are too large to take up the stairwell or in the new lift. We didn’t want to remove them from their wooden stretchers and roll them onto a large drum, the usual method of transporting large paintings, because they both had a complex conservation history. They had already been lined with new canvas and then strengthened round the margins. A lot of people put their heads together, resulting in an ingenious horse-like structure which the painting was draped over. That meant we only had to remove the central part of the structure, allowing the paintings to be transported across the city for restoration.”

A keen cyclist, Stevenson travels between the National Galleries on the Mound and the Gallery of Modern Art in Belford Road by bike and she says she’d like to add a portrait of Olympic Games gold medal winner Chris Hoy to the collection.

Forced to choose just one favourite painting, she nominates the portrait of Tom Derry or Duri, the fool employed by Queen Anne of Denmark, the wife of James VI, painted by the Anglo-Dutch Marcus Gheeraerts the Younger in 1614. “He has a terrific facility with paint,” she says, admiring the work which shines out in a room studded with glowing images of the Stuart dynasty. The jester’s pale, extraordinarily expressive face is a striking contrast with the rich red doublet with gold embroidery he wears.

What’s the particular appeal of portraits? “It’s history that everyone can identify with. Just looking at the faces tell us so much about society.”

New photographic portraits are also being added to the collection. Among them are some of the alumni, staff and students of the Royal Scottish Academy of Music and Drama in Glasgow (now the Royal Conservatoire of Scotland) taken by Ken Dundas to mark the 60th anniversary of the school of drama.

Among the best-known names are Alan Cumming, Bill Paterson, Billy Boyd, Maureen Beattie, Ruby Wax and Dawn Steele. A selection of the portraits featured in The Herald Magazine recently. All were photographed against a plain black background without any props, giving the impression all were shot at the same time.

Such a stark portrayal was not the original intention but was set because the first subject, Alan Cumming, was prevented from being photographed on a film set in New York by union rules, so simplicity became the key. “It has helped to bring out the differences between people. If I had shot everyone in their home, they might have looked very similar despite using different locations,” says Dundas.

So what’s his technique? In this series, simplicity, a plain black background lit with just one flash, was the key. “I always want to capture the moment that shows the personality, perhaps with a tiny gesture. With performers they know what they want to look like and you don’t need to prompt them. For these portraits it was just the subjects and me. There was no-one else fussing about hair or make-up. People are much more relaxed if they are not looking to see how other people are reacting. Most of them just turned up in their normal clothes, no-one is wearing an evening frock or has big hair.”

He’s reluctant to name one he regards as successfully transferring personality to paper but says the portrait gallery staff particularly like the Robert Carlyle photograph because, unusually, it shows the actor smiling.

While portrait painters usually have an opportunity to get to know their sitters through the course of a painting, photographers have to establish a rapport much more quickly. Dundas’s time with his subjects varied from two minutes 20 seconds at one extreme to two hours at the other.

Asked to choose a painting he admires in the portrait gallery, he plumps for Ken Currie’s striking painting of three oncologists. “It is different from my portraits because it’s a group but it is similar in the light and the absence of distractions or scenery.”

Dundas clearly feels photographs will add significantly to the portrait gallery’s ability to provide a window into the history of Scottish life. “Photography is a very immediate medium and people are interested in it because they take photgraphs themselves. Taking pictures places people in a certain time and, like the famous pictures of Gorbals, show what life is actually like rather than romanticising it. Thomas Annan’s old photographs of Glasgow look almost like illustrations. Photographs are a snapshot in time.”

The gallery charts changes in dress and social hierarchy but the gallery’s evolving relationship with the public is equally a mirror of a change in attitude.

Only 15 years ago, when Willie Dickson joined the NGS after 26 years in the Army, his job description was warder. His task was to ensure the safety of the collection and talking to the public was frowned on. Now the security staff are gallery attendants, Dickson is duty supervisor at the portrait gallery and interaction with the visitors is part of the training. “That’s been quite hard for staff who have not done that in the past, but now we have a lot of younger people who have training in art who see working in a gallery as the first step on a career ladder and they have knowledge to pass on,” he says.

After stints in all the galleries under the NGS umbrella, he declares with certainty that modern art is not his forte and the portrait gallery is the one he most enjoys.

“I’d never set foot in an art gallery before I came to work here. I was just amazed by the portrait gallery, especially the portraits of great Scots by Allan Ramsay.”

As a Burns man, however, he has no hesitation in naming the Alexander Nasmyth portrait of the poet as his favourite painting. “When you look at the different portraits of Burns, I think it gives you an indication of what he was trying to say in his songs and poetry.”

He also gets a buzz from seeing the portraits of contemporary Scots, particularly Sir Alex Ferguson, Kenny Dalglish and the late Rikki Fulton.

After watching over the emptying, refurbishment and re-installing process he is delighted to see many old friends hanging on the walls, but has also found himself drawn to one of the major new exhibitions. The War at Sea features paintings on loan from London’s Imperial War Museum by Sir John Lavery, who was born in Belfast but became one of the Glasgow Boys.

Dickson, now on his third stint at the portrait gallery, is full of praise for the transformation, which included dismantling and rebuilding the library cabinets “with only one three-inch piece of wood left over”. He’s a practical man but admits that when patrolling the building at night “the imagination can work overtime”. He’s convinced by the noise of a door shutting inexplicably on a certain stairway that the building has a ghost.

Perhaps some young sleuths will be able to get to the bottom of that mystery as the galleries will have a new focus on children. On Sunday afternoons, equipped with Sherlock Holmes-style deerstalkers and magnifying glasses, youngsters can uncover clues in the portraits to solve crime in the company of Detectives Raeburn and Rothko. It’s Meg Faragher’s job to encourage children to have fun with Scotland’s great and good. They will have an opportunity to “Meet the Ancestors” in story-telling sessions or workshops which will explore that sensation we all have in a room full of faces that they are watching us. What would they say to each other if they were to come to life?

Perhaps appropriately for someone originally from Berwick on Tweed, Faragher has chosen as her favourite a portrait of Margaret Tudor, whose marriage to James IV foreshadowed the union of the crowns. Here, however, she is pictured with her third husband, Lord Methven, with a liveried servant in the background, pointing towards a butterfly. “Looking at this movement, you feel that a moment has been captured,” she says. It is one of numerous unexpected glimpses of wildlife among the portraits she has collected for a nature trail through the gallery. Her quest for details to inspire youthful imaginations has made her look again at familiar portraits, in the process discovering new insights: “It’s fascinating because people are holding things that are strange to us.

It’s impossible to settle for only one choice and she is also struck by a First World War painting in the War at Sea exhibition by Lavery of a man with a souwester.

Among the modern paintings, she has a special fondness for Poet’s Pub, Alexander Moffat’s vision of Hugh MacDiarmid, Norman MacCaig, Sorley MacLean, Iain Crichton Smith, George Mackay Brown, Sydney Goodsir Smith, Edwin Morgan and Robert Garioch gathered in a favourite Edinburgh howff.

Additional space in the portrait gallery will provide a new home for the NGS photography collection.

Migration Stories are linking up with the Autograph Project which aims to fill a gap in the official photographic record of Britain, so it focuses on families with an ethnic minority or mixed background and invited people to add their family photographs to the archives.

Meanwhile, the extraordinary overview of Scottish life provided by the early Scots photographers, Robert Adamson and David Octavius Hill, and their successors up to the late-20th century is celebrated in Romantic Camera, the opening exhibition of the photography gallery.

Justin Fenton of Glasgow architects Page and Park, the project architect for the portrait gallery’s remodelling and refurbishment, says the prime consideration was to remain true to the spirit of the original building.

It’s clear as he explains the task that he regards the building as a work of art in its own right. He has also embraced the idea that the gallery should itself be a portrait of the nation not only through those hanging in splendour on its walls but in terms of visitors. Ensuring everyone could get in the front door by creating a ramp there was the first step.

Nevertheless, the imposing exterior can be a psychological barrier. “In the past the building has been seen as a bit of an ivory tower and rather intimidating. The entrance hall is a main element of the building but felt very old fashioned,” says Fenton. Creating a vestibule to open up the whole of the ground floor and increasing light around there was essential to the plan to increase circulation around the building.

Key to access is the very large lift, designed to make it easy to move both school parties and large works of art around the building. “You now have options when you come into the building. Because you don’t have to pay, which is fantastic, you can decide where to go. Some people will go straight to the Great Hall, which is no longer a lobby, or they can go up to the top floor in the lift and work their way down. It’s easier on the legs”.

Opening up the gallery and “clearing out the clutter”, as Fenton puts it, was essential to improve circulation round the galleries. The cleverest architectural trick has been to insert new mezzanine floors to join up with the platforms that already existed. This has resulted in a complete extra floor to house all the administrative functions that had been carried out behind temporary walls and allows the galleries to occupy the whole space envisaged by Robert Rowand Anderson in the 1880s.

Orginally, however, it was designed as two buildings: one to house antiquities, which were eventually transferred to the National Museum of Scotland, and the other to house the portrait gallery. Metaphorically donning his hard hat, Fenton describes the differences between each side.

Structural changes had been made to accommodate different requirements over the years, including altering the form of the roof on the east side and putting in flat ceilings on the west side.

The most radical aspects of the overhaul has been focused on unifying the two buildings. The major operation was putting an entire new roof and ceilings on the west side.

Fenton acknowledges the building had “quite a lucid plan” originally. However, it became so complex once parts of it were closed off that often people didn’t make it to the second floor. His solution was to “shuffle some of the doors” to open up views through gallery spaces, allowing people to orientate themselves and getting them circulating though the galleries.

It’s obvious from his enthusiasm that his favourite item is the building itself, which combined from the beginning exquisitely detailed gothic stonework and exposed brickwork and steel. Like Dundas, he’s drawn to the Currie portrait of the oncologists, which he finds uplifting as a portrayal of compassion and wisdom, but settles for the William Holt frieze in the Great Hall. “You have a very simple entrance which goes up into this wonderful gothic interior. After the messy building work was finished the galleries organised cleaning and conservation of the friezes, which really glow now.”

As a portrait of the nation through historical figures of all sorts it’s a highly appropriate choice for a man whose mission was to make a 19th century vision work for 21st century Scotland. n