AS the summer of 1963 ripened into autumn, a train carried Joan Eardley from Aberdeen to Glasgow. It was to be the artist's last journey. The breast cancer which she had neglected to have treated for so long, had progressed alarmingly and spread to her brain, causing blinding headaches and double vision. Eardley, who had always been burly, had lost a lot of weight and was very weak. Accompanying the painter were Angus Neil, her protégé, and her mother, Irene. Her sister, Pat Black, travelled separately by car.

When the train stopped at Stonehaven, friends from the nearby village of Catterline, where Eardley had lived for more than a decade, were able to board briefly and bid her farewell. They knew, as she surely did, that she did not have long to live.

In Glasgow, she was taken to Killearn Hospital. There, she asked her friend, Audrey Walker, whose photographs make an incomparable record of the artist at work, to read her Ernest Hemingway's short story, The Snows Of Kilimanjaro, in which, ironically, a dying writer refuses to let his wife read to him. "I don't like to leave anything," says the man. "I don't like to leave things behind." Not long afterwards, on August 16, Eardley died. She was 42.

What Joan Eardley left behind was an astonishing and varied body of work, comprising more than 1000 drawings and several hundred paintings which the National Galleries of Scotland is celebrating with a major if belated exhibition that opens next month in Edinburgh. Its timing is fortuitous. Prices for Eardley's paintings are soaring and dealers, such as Guy Peploe, owner of Edinburgh's Scottish Gallery, are actively soliticiting private owners in the hope of persuading them to sell. Paintings which Eardley herself would have been happy to sell for £25, now fetch tens of thousands of pounds above their estimated price. Recently, Peploe sold an Eardley painting for £85,000. A few years ago, the same work would have fetched £15,000 at most. "Her star is very much in the ascendant," he says.

Critically, too, Eardley is at last being accorded the respect and appreciation her work demonstrably deserves. Even within Scotland that was not always the case, recalls Cordelia Oliver, Eardley's art-school contemporary and biographer. When she tried to persuade the National Galleries to host a retrospective in 1988 to mark the 25th anniversary of Eardley's death, she was given the brush-off. According to Oliver, Timothy Clifford, then director of the National Galleries, gave the impression he didn't know who she was or didn't care. In the event, the exhibition went ahead to effusive reviews at the Talbot Rice Gallery and the Royal Scottish Academy.

At this distance, when it seems obvious that Eardley's paintings are likely to survive the rigours of time, such indifference is curious not to say perverse. For even as she lay grievously ill, Eardley was able to read critiques of a show in London in which she was mentioned in the same breath as Turner, Goya and Renoir.

"Like Turner," wrote Eric Newton in the Manchester Guardian, "she paints as though the brush were an integral part of her personality that found no difficulty in expressing, in a kind of shorthand of its own devising, the way to say cloud' or tangle of grass' or mop of hair'. No slickness here, no tricks, no elegance. Just a trial and error attempt to invent the painterly equivalent of what she so intensely wants to convey."

For Eardley, says Pat Black, art was a calling above all others. "It was her life," she says emphatically, sitting in her Glasgow bungalow. "There was nothing else." Listening to her reminisce about her sister, one is reminded of Van Gogh, one of Eardley's heroes, whose letters to his brother Theo she read as if they were addressed personally to her. "What some consider working too fast," wrote Van Gogh, "is really nothing out of the ordinary, the normal condition of regular production, seeing that a painter really ought to work just as hard as, say, a shoemaker."

Such sentiments, says Cordelia Oliver, would have been cheered by Eardley. "We never thought of ourselves as artists," she says. "We were painters." By which she doesn't mean they were not people apart; rather, that they had a special talent, a gift whose origin defies tracing. That Eardley was unusually accomplished was apparent to Oliver, who met her at the Glasgow School of Art in the early 1940s. What was equally apparent was the strength of Eardley's desire.

"There is no doubt that among us, in those wartime years, Eardley acted as pace-setter," notes Oliver. "She was in another class entirely. In no sense was hers an easy, effortless accomplishment. What gave her work its power and its presence was her absolute commitment to its demands, and the sheer, dogged persistence in study through drawing, in gaining knowledge through the eye."

It was the war that drew Eardley to Glasgow. Contrary to received chauvinism, she was not born in Scotland but on a farm at Warnham in Sussex. It was 1921 and she was the first child of Captain William Eardley, a young English officer who, while billeted at Maryhill Barracks in Glasgow during the first world war, had met and subsequently married a local girl called Irene Morrison.

Eardley's father struggled as a farmer, eventually losing his farm and with it his capital. Forced to find other work, he joined the Ministry of Agriculture, "or whatever it was called in those days", says Black, who was born 18 months after her sister. Life outside the trenches, in which he had been gassed, proved a trial for Captain Eardley. "He would have these moody fits," says Black. When she was five and Joan was seven, Captain Eardley committed suicide. It was not something the family spoke of much thereafter. Certainly, Joan didn't. "Perhaps," surmises Cordelia Oliver, "her own recurring bouts of deep depression brought the thought of her father's suicide too close for comfort, like the knowledge that she was, in great part of her being, her father's child."

Following their father's death, Joan and Pat were taken by their mother to live with her own mother and sister at Blackheath in London. It was an all-female household. Joan was especially drawn to her maternal grandmother, a cockney who boasted that she had been born "within the sound of Bow Bells". Thanks to the generosity of a great-aunt, the Eardley sisters were educated privately at a school where Joan's talent was recognised for the first time. In her biography, Oliver reproduced a picture drawn in Joan's last year at the school. Titled Fair At Blackheath, it conveys simultaneously a sense of fun, chaos and humour, and later formed part of her portfolio for admission to Glasgow School of Art.

By then - 1939 - it was clear that London would be targeted by German bombers. The following January, Eardley was evacuated to Glasgow, a city she and her sister knew well from holiday visits to their Morrison grandparents. Oliver recalls "a big, sturdily attractive, dark girl with bright black eyes, a friendly smile, and hair frizzed into an unbecoming, if currently fashionable, permanent wave. The perm she soon lost, the attractiveness never - something that surely had its source in the quiet restraint that somehow suggested hidden fires".

The Eardleys lived in Bearsden and Joan commuted daily to art school by bus. Due to the war, the student population was low; there were no more than 200 day students. As the war wore on, men became relatively scarce and the teaching staff dwindled. Eardley, however, together with her friend Margot Sandeman, was earmarked for special attention by the head of drawing and painting, Hugh Adam Crawford, a charismatic teacher in the Jean Brodie mould who encouraged his students to play the sedulous ape to painters such as Piero Della Francesca or Velasquez or Van Gogh.

If you want to be a good painter," Crawford said, "one who matters beyond the ordinary kind of success with people around you, you have to be prepared to break down some kind of barrier within yourself. In Scotland we are born with a constraint of this kind - we get it with our mother's milk. Somehow we have to enter into another plane of living, to break through, or away from, Scottish morality - the vulgar idea of Godliness. There can't be any good' or bad' or yes' or no'."

Eardley, though, was no wild bohemian. On the contrary, she epitomised middle-classness. What distinguished her from the bourgeoisie, however, was her attitude to art. Oliver, now 84, has a vivid memory of her friend standing, at her board in the life-drawing class, late one afternoon, "feet straddling the floor on either side, her gaze fixed on the model, putting no mark on paper until enough visual information was absorbed about stance, thrust, weight and balance - in contrast to most of her fellows with their tentative, premature scribblings".

Eardley left art school in 1943. Doubtless she would have preferred to make her living as an artist but the war was still rumbling on and she felt obliged to take a teacher-training course. That was soon abandoned and for a while she worked as a joiner's labourer. When the war ended she lived in London for a spell but a chance encounter on a Glasgow street with her old mentor and champion, Crawford, led to the award of a rare bursary which allowed her to travel extensively in Europe, in particular in France and Italy. "None of us," says Oliver pointedly, "got travelling scholarships in those days."

A drawing of Piazza SS Annunziata in Florence, which was given to Oliver on Eardley's death, demonstrates the degree to which her talent was beginning to flourish. One feels for a moment transposed to Florence and the Renaissance; to a moment in time when artists were reinventing their craft and reimagining the world around them. For Eardley, the experience of Italy cannot be understated. She saw beauty everywhere: in the crumbling relics, the shape of the hills, the people and the constant allure of colour. Like Miss Brodie, she became entranced by Giotto and unlocked the key to his genius.

"It's the thing that makes the beauty in everything outside," she wrote in 1948 in a letter home, "here, that is the way a dark figure - a peasant, say, dressed in a suit of dark clothes - appears, in the sun and the shade, all in one, black. And a peasant dressed in clothes washed and bleached by the sun appears, against the white stone, all in one, almost as white as the stone. They are single unbroken shapes of dark and shapes of light ..."

In April 1949, Eardley returned to Glasgow buoyed up, brimming with energy and ideas and with a renewed sense of purpose. She found a studio in Cochrane Street, off George Square, before moving to St James's Road, Townhead, a typical four-storey tenement building in the heart of a community teeming with rude life. It was here that she first encountered the Samson family, who were to figure in so many of her best-loved and most accomplished paintings.

One of her favoured subjects was Andrew Samson who, after a career in the army, is today semi-retired and living in Barnard Castle in the north of England. His father, he tells me, came to Glasgow in the 1930s from Stirling, his mother from Dundee. "We lived in Rottenrow in an old tenement," he recalls. "My father was in and out of jobs all his life. At one time he worked with the Hydro Electric in the Highlands. He did a lot of work on the ships when they came out of the Clyde."

There were 12 Samson children in total, though not all of them lived concurrently in the cramped two-bedroom flat. "There were probably just six of us at the one time," recalls Andrew. He first met Eardley when he was about 12. "She used to come round with a pushchair with easels and paints." One day, as he watched her paint, she invited him up to her studio.

When Pat Black visited her sister, she often encountered the children, and would take them sweets and comics. It may well have been her, she says, who provided Andrew with the comic he is reading in the painting Andrew With A Comic. In it, a boy, carrot-topped, dressed in wellingtons and shorts and evidently from a poor background, sits on a chair - a piece in one hand, a drink in the other - engrossed in his reading. It is the kind of scene many painters would have found irresistibly sentimental. Not Eardley. She portrayed Andrew as he surely was: undernourished, impish, intrigued, self-contained, the kind of spindly-legged, knock-kneed boy Bert Hardy captured in his famous photograph of two boys crossing a Gorbals street. Andrew recalls Eardley painting his siblings on numerous occasions. His little sister Pat was the "squinty-eyed" girl in the picture of the same name, shown on our cover. Joan, apparently, was horrified later to learn that the squint had been corrected courtesy of the National Health Service. "Oh hell!" she exclaimed, revealing that splinter of ice in the heart of all great artists. "She won't be as much use to me now."

Eardley, says Oliver, was a realist, not a propagandist. Like Van Gogh, she painted life as she saw it, in all honesty. The Samson children were her muses and she repaid them by rendering them an indelible place in Glasgow's history. Several of Andrew's siblings still live in the city, and are tickled pink to realise that even once they have gone, Eardley's depictions will remain and remind people of those far-off days when they played peevers in the raucous, odiferous streets and made mayhem in the closes.

As he helped the artist to tidy up, Andrew sometimes queried whether she really wanted him to discard pieces of paper on which she had drawn. "She'd say, Put them in the fire!' and I did. You didn't know then when you were 13 years old, that she was going to become famous."

What was Eardley like? For much of the time, she was in pain, the result of a slipped disc in 1956, which made it necessary to wear a surgical collar. She could be difficult and she knew it. She was, says Andrew Samson, "a big woman, not to say butch", who in her chilly studio invariably wore a sailor's pullover and heavy cord trousers. Outdoors she wore a duffle coat.

Though physically strong, she employed Angus Neil, a former joiner, to make frames