If you don't know the 18th century portraitist Jean-Etienne Liotard, you are in very good company.

Idiosyncratic, highly skilled and with a penchant for marketing himself that very few artists of the 18th century could rival, the studiedly eccentric Jean-Etienne Liotard created portraits that were uncompromising in their likeness of the sitter and had an intimacy that marked them out from the formal magnificence of public portraiture.

And yet the artist, dubbed 'The Turk' by a marvelling British society when he first arrived in London in 1753, and now the subject of a major exhibition of works at the National Gallery in Edinburgh, is largely unknown, particularly in Britain, outside the small coterie of private individuals on whose walls his works hang and the art historians who admire him.

"He is undoubtedly one of the most impressive artists of the 18th century," says National Portrait Gallery Director, Christopher Baker, a specialist in 18th and 19th century drawing whose own passion for Liotard helped kindle the idea for this fascinating joint exhibition (part of the Edinburgh Art Festival) with London's Royal Academy some five years ago. "He was a brilliant technician, a brilliant maker of portraits."

Born in the republic of Geneva in 1702 and trained there and in Paris, originally as an enamellist and miniaturist, Liotard was refused entry to the French Royal Academy. Liotard sought, thereafter, for recognition not by the artistic establishment, which was remarkably snooty to those who did not fit the accepted mould, but in the purses of the European gentry.

And it was a policy that worked remarkably well. Reinventing himself as a "Turkish gentleman" after four years in Constantinople spent painting the British expatriate community, Liotard donned a red fez and grew a lavish beard, arriving in London in the mid 1750s, where his exotic appearance and fluid way with a crayon excited the interest of some equally interesting society figures, from actors to royalty.

That idiosyncrasy is nowhere more evident than in his own Self Portrait, Laughing (c.1770), one of many he produced over his considerable lifetime. As we stand in the lower galleries of the Royal Academy in Edinburgh, contemplating the string of luminous images of the titled and fashionable members of Georgian society, it is this curious portrait, perhaps produced to underline the eccentricity of Liotard's public image, that seems key.

The unusually laughing demeanour, the pointing hand with protruding, almost comically long finger and thumb, the blind curtain behind. One cannot view this portrait without feeling some sympathy with Joshua Reynold's biting observation that there was "something of the quack" in his appearance.

Certainly Liotard, concerned with drawing what he saw, was an artist who was not afraid of a double chin or a prominent nose, depicting his subject's visages with uncompromising, if usually sympathetic veracity.

If his rivals accused him of being something akin to a copyist, replicating his subjects 'warts and all', then it appeared to put off no one but the French Philosopher Jean Jacques Rousseau, the only person known to have rejected one of Liotard's portraits.

"Many of his works were never intended for the public sphere," points out Baker. "They are all very private images for family consumption and use." His chalk likenesses of the children of the Austrian royal family travelled everywhere with the Grand Duchess Maria Theresa, a family album. "He travelled all over Europe in search of patrons, from Paris to Rome, Vienna to Constantinople."

As such, in preparing for this exhibition, the National Gallery has been door-stepping the gilded families of Europe, much like Liotard himself. And the unusual thing, at least for those unfamiliar with the vagaries of 18th century portraiture, is that his palette was not filled with oils, but with thick sticks of pigment and gum - pastels - a medium which was hugely fashionable in the 18th century.

The luminosity is immediate, from the staggering fuzzy surface of a peach to the lustre of a velvet ribbon. Everywhere there is blue, from the pale eyes of the Austrian children to the royal blue of a watery silk coat in his 1754 portrait of the Marchioness of Hartington or the trim on the dress of his loving portrait of one of his own daughters.

His depiction of lace on the shawl of Harriet Churchill, Lady Fawkener, the wife of one of his key early patrons, is staggeringly realistic, the diaphanous, fragile nature of the material more fabulously realized even than the sitter herself. If sometimes, then, the realization of the clothes can dominate our immediate perception of Liotard's work, the effect is ravishing. His portraits have long been used by historians to study contemporary dress.

But it is in the intimacy and apparent honesty that Liotard's off-centre appeal lies, such as the wide-eyed gaze of the five year old Princess Louisa Anne (1754). Baker tells me that on the back of the series of portraits in the Royal Collection at Windsor, commissioned by Augusta, Princess of Wales and the mother of the future George III, is a child's drawing of Liotard himself. The genial fez-wearing portraitist, faced with the difficult task of encouraging small children to sit still whilst he sketched their likeness, got his young subjects to draw him in return.

Jean-Etienne Liotard, 1702 - 1789

Scottish National Gallery

The Mound

Edinburgh

www.nationalgalleries.org

0131 624 6200

Until 13th September 2015, then Royal Academy of Arts, London until 31st Jan 2016

Daily, 10am - 5pm (Thurs until 7pm)