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This morning launched to go towards the total cost of £554,937.50 to buy a portrait of Ayuba Suleiman Diallo. He's hardly a household name, but he is important.

The painting according to the National Portrait Gallery (NPG) is the first known portrait that honours a named African subject as an individual and an equal, and thereby gives a useful insight into Britain in the 18th Century.

This statement tells you all you need to know about what makes the NPG different from and, from the perspective of social history, more interesting than other art galleries. It makes clear that the sitter is more important than the artist. The NPG doesn't want this painting because it's an exquisite example of 18th Century British art, but because the story of the sitter and its significance is so compelling.

Ayuba Suleiman Diallo, or Job ben Solomon as his English friends liked to call him, was a west African slave trader. The phrase "what goes around comes around" probably wasn't in much use in those days but, had it been, perhaps the wealthy Ayuba might have been more on his guard.

One day on a business trip to sell some slaves down the River Gambia, he suffered the indignity of being captured and enslaved himself. He was put onto a British ship bound for Maryland where he was sold to work on a tobacco plantation. An English lawyer and missionary called Thomas Bluett met him and decided he was "no common slave", so in the early 1730s he whisked him off to London and introduced him to high society.

Black, Muslim, highly-educated individuals were not common in those days, leading to Ayuba becoming something of a celebrity. His friends arranged for a portrait to be painted of him and chose the artist William Hoare of Bath, a founding member of the Royal Academy.

Ayuba was a religious man and didn't really go for the idea of a portrait, worrying that people would worship the picture. Or, as Thomas Bluett put it in his memoirs:

"Job's aversion to pictures of all sorts, was exceeding great; insomuch, that it was with great difficulty that he could be brought to sit for his own. We assured him that we never worshipped any picture, and that we wanted his for no other end but to jeep us in mind of him. He at last consented to have it drawn; which was done by Mr Hoare."

In fact it is the earliest known painting by the artist. The picture was painted in 1733 and, apparently at the sitter's request, has him in traditional dress and carrying a copy of the Qur'an around his neck.

And since then, till now, it has not been seen in public. It was thought to be lost and was only known about though Bluett's memoir, but then it turned up at auction in 2009 where it was bought by a private collector who wants to take it abroad. stepped in and now it is on show, for all to see at the National Portrait Gallery, while they attempt to raise the funds to secure it for the nation.

It seems likely they will, as they have already raised over £500,000 with contributions from the Heritage Lottery Fund, the Art Fund and the NPG's own contribution.

Back in the mid-1700s the money was raised for Ayuba Suleiman Diallo to be released from slavery and sent back to Africa. Whereupon he started up in business again. As a slave trader.