After a chance meeting with Lucian Freud, art handler Ria Kirby agreed to sit for him  a huge commitment which was to last for 16 months, seven nights a week, on top of her day job. Martin Gayford tracked the work as it progressed. Photographs by David Dawson

By chance, I was present right at the beginning of this picture, when artist first met model. It was at the hanging of the small exhibition of new work by Lucian Freud and his friend Frank Auerbach at the Victoria & Albert Museum last year. On that day Freud happened  unusually  to be wondering what to do next. He had just finished several pictures, the ones that were in the show. So that morning at the museum, while casting an eye over the hanging, he was also looking for the subject of his next picture. Just then Ria Kirby, who is an art-handler for the museum, and who had helped to hang the show, came up to Freud and said that she thought his work was great. As we walked out of the museum, I remember him musing, ‘That girl I met, I think I could work from her.’

Above Freud and Ria Kirby with the finished painting, Ria, Naked Portrait 2007

As Ria remembers it, ‘After lunch we went upstairs to get our tools, and the head of painting said, “Mr Freud wants you to sit for him.” I was excited. Then I thought, perhaps it won’t really happen. But, in fact, less than 24 hours later I was lying there, being painted.’ These photographs, by the painter and photographer David Dawson, who is also Freud’s assistant, chronicle the long gestation of the picture. The painting continued from April 2006 for one year and four months, seven nights a week, during which Ria and Freud took only four evenings off. Each of the sittings lasted for about five hours, more than 2,400 hours in total. Freud will be 85 in December.

For Freud it is crucial that his sitter should be what the Italians call simpatico. He is fastidious and discerning about people. Two characteristic Freudian verdicts are ‘lively’  which is good  and ‘absolutely ghastly’. As the example of this picture shows, Freud is likely to spend a prolonged period in the company of the model, the most important ingredient in his working environment. Furthermore, he is an artist attuned to nuances of atmosphere  that is why he likes the model to be present, even if he is not painting him or her but a floorboard, a doorknob or a section of empty space. The actual presence of the model, he insists, alters everything in the picture. So for him to work from someone uncongenial is difficult, even impossible. Once, years ago, when his paintings did not fetch millions and he badly needed the money, he gave up a commission to paint a portrait of the Master of an Oxbridge college  after giving the subject a trial sitting  with the words, ‘I simply can’t work with that man in the room!’

Evidently, Ria, who is 26, passed the congeniality test. And, equally vital, she measured up  indeed, heroically exceeded  Freud’s other cruial criteria of punctuality and reliability. Freud has always been a stickler for time-keeping. In the past he has stopped a picture simply because the model  suitable in other ways  was apt to turn up an hour or two late. Kirby was always punctual.

Freud works from living people  not, as some painters are happy to do, from photographs. Given the time that a picture may take, and his liking for having the sitter present at every moment he is working, a high degree of commitment is obviously required on the part of the people he paints. If the subject loses interest halfway through the painting, that’s the end of it.

Freud’s life works according to a series of appointments  the first model will arrive at 2pm; another at 6.30. That is the only way in which he can function. The subject’s own life is a factor in these arrangements  and as Kirby was busy during the day ‘moving big lumps of sculpture’ about at the V&A, the evening was her only free period. Consequently this was a ‘night painting’, done by artificial light.

‘To start with it was quite exhausting, because I had only about 10 minutes’ break between finishing work and beginning to pose. I went through every possible emotion in my life,’ she remembers. ‘At the beginning I was very conscious of trying to be a good sitter. It took a bit of practice to feel very relaxed, but be totally still at the same time. I was conscious of getting back into the same position after a break. After a month or so, it became second nature, and felt completely natural. I realised there’s no point in trying to be anything. You just have to lie there and be yourself. But in the end I found it quite a release. It was one place where I could be where I didn’t have anyone phoning me or hassling me. All I had to do was lie still, which I’m quite good at.’

The routine was demanding but it had its compensations. Life in the studio is tranquil  personally, I found something therapeutic about sitting to Freud for a portrait three years ago  and also entertaining. He is an outstanding raconteur and mimic. That is one reason why, as his old friend John Richardson, the biographer of Picasso, once put it to me, his company is a bit ‘addictive’.

‘In the first few months the hardest part of sitting was trying to stop laughing,’ Kirby says. ‘There were so many tales and songs and anecdotes. We went to a dinner once and he made me laugh so much I couldn’t sit up. Because of working in the evenings we would often go out to a restaurant afterwards, where there are always some extreme shapes and characters to speculate about among the other diners.’

No one could keep up that level of conversation for ever. Kirby notes that, ‘There have been times when there is nothing to say, and no news, but it’s never an uncomfortable silence.’

Kirby having displayed admirable dedication, the picture carried on  and on  and on. Even by Freud’s standards, 16 months, seven nights a week is a marathon. What, you might wonder, coud possibly take so long? It wasn’t planned like that. As early as last autumn Freud began to hint this might be ready ‘In just another few weeks’. But that moment of conclusion was repeatedly postponed.

One of the points that not everyone grasps is that it is genuinely hard for painters to know when a picture is finished. Once upon a time, perhaps, there was general agreement on what a finished picture looked like. But those days are long gone. In modern times there have been almost as many answers to the question ‘How do you know that a picture is finished?’ as there are painters. Jackson Pollock once answered it, ‘How do you know that you’re finished making love?’

Freud’s criterion is that he feels he’s finished when he gets the impression he’s working on somebody else’s painting. You can see what he means: his own input is complete. In practice, Freud tends to get preliminary inklings that a certain work may be coming to an end. But they are not entirely accurate. Occasionally, he has been surprised by a picture suddenly reaching that point  as he was by the head of a grey horse some years ago. ‘But there it is,’ I remember him saying in a slightly puzzled way. ‘It definitely is finished.’ More often, the painting process takes longer than he anticipated. In the case of the oil he painted from me (Man with a Blue Scarf 2004), one shoulder and my shirt collar held us up, needing to be painted and repainted. Some paintings  like this one of Kirby  just take an amazing amount of time. And the length of time it has taken has nothing whatever to do with the quality of the result  as Freud himself reflected to me as he decided to abandon a work that represented countless hours of toil. He has quite frequently put his foot through pictures that had consumed months of his life and would be worth millions of pounds on today’s art market.

The completion of a picture may be a moment of relief for the model, or at least some models. But for Freud, completion is the most nerveracking point. ‘I worry,’ he once confessed, ‘in case it isn’t really finished.’

With this picture, that culminating point seemed never to arrive. For one thing, the work kept growing. As you can see by comparing the earliest of David Dawson’s photographs with the later ones, the canvas was extended on both sides, although any join would be hard to detect from looking at the final work. Freud has found an expert at sewing on canvas extensions almost invisibly. This kind of alteration sometimes happens as he finds the proportions he wants between the figure, or portrait head, and the rest of the picture. The size of this finished painting is 86x163cm.

As you can also see from the earliest of Dawson’s images of the work in progress, Freud has an un-usual method of working. Most painters begin with a rapid overall image and bit by bit  so to speak  bring it into focus. Freud, after drawing a charcoal sketch on the canvas  which, he points out, he doesn’t necessarily ‘go by’  starts in one specific place. In my case, the painting began with a patch in the middle of my forehead and then gradually moved outwards from there so that eyes, nose, mouth, chin, hair and jacket appeared. He likes to keep a little window of bare canvas open right to the end, more as a way of reminding himself that the picture is still in progress and provisional than for any technical reason. For Kirby’s painting, he also started by painting her head and hair. ‘It’s hard to tell now, because the painting seems so organic,’ Kirby says.

Often  especially with new sitters  he begins with the head even if the painting is to be one of the entire body. It’s a way, as far as he’s concerned, of getting to now the sitter (that’s how he began to paint Leigh Bowery, the performance artist, whom he painted and etched at least half a dozen times). After the rest of the body has been depicted, he may return to the head and repaint it, now that he knows the person better. That is what happened with Kirby. The final version of her face is covered with a thick, buttery impasto of chunky brushstrokes that seem to echo her thick blonde curls. But that effect only materialised quite late in the painting process. ‘I didn’t worry about whether it looked like me, or how it looked,’ Kirby says. ‘I just thought of it as a whole picture.’ As the painting neared completion, every aspect  the section of floor to the right, the surface of the cover on the bed, which over those innumerable hours of posing had taken on the shape of Kirby’s body, the radiator, the screen behind  became clearer, stronger and more closely meshed into the total image.

In a way, Freud paints just what is in front of his eyes; in another sense, he does no such thing. Every detail of what he paints is pondered, analysed, reconsidered  if necessary omitted. The painting is not simply someone lying on a bed with no clothes on. It is also the accumulation of all those months of thoughts, feelings and observation that are layered on to the canvas.

David Dawson’s photographs chart that process. For the past decade he has had a unique entrée into that place, and his methods are almost indiscernible. A camera appears in his hand for a second or two then vanishes. As the subject, you barely notice it has happened. Last year a book of Dawson’s photographs, Freud at Work, together with earlier ones taken by Bruce Bernard, was published (Jonathan Cape, £30). But the images on these pages were taken after that volume was completed.

Freud’s studio is an unusual place in which the normal bustle of deadlines and shortcuts doesn’t exist. Time passes slowly; it sometimes seems suspended. The sole objective is getting the painting right. In the end, with luck, it takes on a life of its own. Now the finished picture, entitled Ria, Naked Portrait 2007, will first go on show at Tate Modern, then on to the New York art market  to be bought, in all probability by a wealthy American collector. Thereafter it will have its own future and fate. Recently Kirby went into the studio, saw it lying on the floor and, she told me, ‘it looked as if it was breathing’. An artist could wish for no more.