How did one of Scotland’s most successful artists fall from glittering, privileged world to die in poverty?

But a curator researching the life of FCB Cadell has uncovered the sad story of how the artist’s final years were dogged by ill-health before his death in penury, aged 54.

Alice Strang, the curator working on the National Galleries of Scotland’s first exhibition devoted to Cadell in 70 years, discovered that the artist who enjoyed a bohemian life of parties and drinking in Edinburgh, died in 1937 with an uncashed cheque for £50 in his pocket from a fund for indigent artists.

In the months before his death, Cadell, who was suffering from cancer and cirrhosis of the liver, wrote to patrons offering to sell his paintings for as little as £10. He also held half-price sales of his work at his home and on one occasion turned up at a neighbour’s home in such a dishevelled state that his host would not let him in.

Cadell’s will also confirms that he left his entire estate to his loyal “manservant” and live-in companion of more than 20 years, Charles Oliver. “Except family portraits and silver which I bequeath to Jean [his sister] and Gruff [Jean’s son], I leave everything to Charles Oliver, my most faithful friend,” he wrote. It is likely to have included scores of artworks, potentially worth millions today, but almost unsellable at the time.

Cadell’s estate was valued at less than £500 and the uncashed check from the Alexander Nasmyth Fund for Decayed Scottish Artists represented a substantial share of his wealth. “It’s quite poigant,” said Ms Strang.

It’s a far cry from the glamorous world Cadell inhabited as a younger man and the success he was to achieve after his death. Last year, Cadell’s Florian’s Cafe in Venice told for £553,250, a record for the artist and one which confirmed his place among the most successful Scottish artists.

An Edinburgh Academy schoolboy, Francis Campbell Boileau Cadell was taken to Paris by his mother as a teenager to study painting after his talent was singled out by the Glasgow Boys painter, Arthur Melville.

An exhibition of impressionistic paintings in Edinburgh in 1912, after a visit to Venice paid for by Patrick Ford, a fellow Academy pupil and future politician, marked his coming of age as a painter.

He became known for his interiors of Edinburgh, with The Red Chair among his signature works, along with his trademark paintings of Iona, which he visited frequently with his friend SJ Peploe.

The Cadell exhibition opens on 22 October at the Scottish National Gallery of Modern Art. It is the first in a series on the Scottish Colourists and shows his formidable talent and range of painting styles, including gifted early drawings, impressionist works and modern, clean-lined Colourist works between the wars.

It also features previously unseen photographs from the back of a scrap book kept by Cadell’s mother of the artists on holiday in Iona.

Cadell served with the Argyll and Sutherland Highlanders and the Royal Scots in the First World War. His sister was the actor Jean Cadell, famous for her flaming red hair and appearances in films such as Whisky Galore, I Know Where I’m Going and Pygmalion.

She appeared in the premier of Sir James Barrie’s play The Boy David at its Edinburgh premiere in 1936, the year before her brother died.

FCB Cadell, born in Buckingham Terrace, was an Edinburgh character, charismatic, sociable, and famously well-dressed, who loved the company of the upper class and the aristocracy.

But his lifestyle in Edinburgh descended through the depression years from the grandeur of Ainslie Place to a more humble flat in Warriston Crescent.

Biographers have noted how his last years were dogged by ill-health, with accidents including falling down the stairs of a tram and being mugged while walking home.

On one occasion he walked several miles to the artist William Caldwell Crawford’s house, Strang notes, to ask for help.

He was so dishevelled the artist’s wife would not allow him in, though he was allowed to sleep for a couple of nights in a studio on the grounds.

The exhibition, featuring more than 80 paintings, does little to explore the issue of Cadell’s sexuality.

But new evidence has also emerged of Cadell’s close relationship, not just with Oliver but also with Ivar Campbell, a grandson of the Duke of Argyll.

The sometime diplomat and poet, who dedicated some of his writing to Cadell, was killed in the First World War. Cadell had stayed with him several times at the Argyll family seat at Inverary Castle and the two travelled to Venice together.

It is thought Cadell met Oliver during the First World War. He is believed to be the figure wearing red bathing trunks painted by Cadell in Iona. “Charles seems to have lived with Cadell from shortly after the war until Cadell’s death, but the only hard evidence of their relationship is that he is registered as an employee, as a servant,” said Ms Strang.

“You can read more into that if you want to but the evidence does not suggest anything further.”

At the time homosexuality was illegal.

The exhibition’s catalogue notes Cadell’s “radical depictions” of the male nude.

In Iona, Oliver oversaw the sales of Cadell’s paintings – with the story that those who bought works were shown out of the front of their cottage with great ceremony, and those who failed to do so, out the back.

In 1923, the two men travelled together to France, visiting the Somme Battlefields on the way, and staying at the Boileau family’s ancestral chateau, before travelling on to Cassis.

“I find this part of France most interesting to paint,” Cadell wrote. “The light is wonderfully brilliant even fierce… A very comfortable and cheap little hotel too. Charles is becoming quite proficient in the French language as he is having lessons three days a week.”

After Cadell’s death, however, Oliver drops out of sight. According to family stories, he reappeared only to ask Cadell’s beloved nephew for money. “Charles Oliver was the big mystery in all of this. He unfortunately remains a bit of an enigma,” Ms Strang said.

It is hard to know how much Cadell’s legacy is worth, but he left Charles an unknown but potentially large number of paintings.

In 1931, for example, Cadell staged an exhibition of 90 works at Parsons’ Galleries, in Edinburgh’s Queen Street, with sales of just £95.

Ironically, 1931 was the same year of the Paris show, Les Peintres Ecossais, which included all four Colourists – Cadell, Peploe, Fergusson, and Hunter – part of the story that turned them into one of the most famous schools of Scottish painting.

But Cadell’s works continued to sell for modest sums well into the 1960s and 1970s, when the market for Colourist works began to pick up.

It remains unclear to this day just why Cadell’s fortunes took such a downturn. Through the 1920s he was staging well-received annual exhibitions.

“I think it was probably more to do with the period. Glasgow was heavily involved in the general strike in 1926, and then the economic recession just deepened and deepened,” said Ms Strang.

“It also affects the west of Scotland more than the east because it’s industrial and a lot of his patrons were based in the west.”