(Royal Academy, London, until 31 January 2016)

This is the first exhibition devoted to Liotard in the UK and it’s long overdue. He’s an artist I’ve always particularly liked, for he seems to represent the most appealing aspects of the 18th century: its increasing informality and its new interest in the individual as a worthy object of study. Born in Geneva, he had an unusually peripatetic life which took him not only to the usual artistic centres of Paris, Rome and London, but also to more exotic regions: after joining the entourage of a couple of British Grand Tourists whom he met in Rome, he spent four years in the Ottoman capital in Constantinople. For the rest of his life his art would be flavoured by the textures and patterns of the Turkish world.

Some of Liotard’s sitters, enchanted by this foreign glamour, would have themselves painted wearing elaborate eastern dress. Most famously, the artist himself took on the colours of the Levant. After his return from Constantinople he continued to wear Ottoman costume and to tend a long, flowing beard: a conscious assertion of ‘otherness’ that succeeded in attracting attention and cementing his reputation. But of course that reputation would have been nothing without talent; and Liotard’s portraits, which were the mainstay of his work and the focus of this show, proclaimed him one of the most astute, humane and honest painters of the time.

Liotard’s warmth is evident throughout his career. His self-portraits are just as frank as his pictures of other sitters, and he doesn’t hesitate to show his wrinkles and snub nose; indeed, he used self-portraits as a way to show prospective patrons how true-to-life his pictures were. The most famous of these pictures, the Self Portrait Laughing from Geneva (one of the few oils in the show), depicts Liotard in his late sixties. He’s pointing at something outside the picture plane, glancing at us with deep crinkles around his eyes and gaps in his teeth, inviting us to share the joke. (I’ve always thought he looks like a jollier version of Voltaire; or perhaps a modern Democritus, laughing at the folly of the world.)

That same humour and gentleness is echoed in the portraits of his own family. That of his wife Marie is very restrained, in a subdued palette of black, white, pink and beige, with her hair ribbon offering a sole flash of startling red. It doesn’t flatter: Marie was no great beauty, but you can feel Liotard’s affection for her. He even made the great sacrifice of shaving off his Turkish-style beard after their marriage in 1756, because she didn’t like it; he only grew it again thirty years later after her death. For me, though, this is a rather careful portrait: there’s more verve and character in Liotard’s chalk drawing of Marie with their eldest son Jean-Etienne on her knee; but the most delightful of all Liotard’s family portraits is that of his daughter Marianne.

I’m not usually the kind of person who melts in front of cute pictures of children, but this really is a stunner: you get a sense of Marianne’s mock sternness as she urges an unseen companion to silence, lest he wake her doll. The silks and trimmings of both doll’s and girl’s dresses are beautifully rendered and, as usual, Liotard sets Marianne against a plain neutral background, so that her personality sparks more vividly off the sheet.

Many of my favourite portraits in the exhibition showed women and I often felt – in contrast to Goya – that Liotard is particularly good at suggesting the complex emotions of his female sitters. His luminous finish, achieved by the use of pastels on vellum, famously captures the texture of fabric, hair and skin, and it’s that blend of polished accuracy and characterful spirit that make his best works so memorable. His 1755 portrait of the fourteen-year-old Lady Anne Somerset is particularly gorgeous. It was recently sold at auction as ‘attributed to’, but it’s shown in the exhibition as a genuine Liotard and I see no reason to doubt it. Photographs don’t do it justice.

Foregoing the traditional staid court portraiture of the time, this young debutante is shown in Turkish costume, with a daringly plunging neckline. Her blue-trimmed, flower-embroidered bodice was evidently a studio prop, because Liotard also used it in other pictures. Her auburn hair is a tumble of ringlets and – although she has the pert nose and rosebud lips of a Kneller or Lely, her air is entirely different. She doesn’t simper out of the picture: she watches us watching her, withdrawn and wary, with the faintest hint of a smile, hesitating on the threshold of adulthood. She would die eight years later of tuberculosis, in Naples, alongside her young husband.

More animated than Lady Anne, but equally charming – and probably my favourite picture in the exhibition – is Liotard’s portrait of Julie de Thellusson-Polyard, made to celebrate her marriage in 1760. There is a pendant portrait of her husband Isaac-Louis de Thellusson, but for all its informality and the gorgeous rendering of his lace and blue silk robe, he just doesn’t match up to his wife (I can’t quite figure out what’s going on with his hairstyle either: is he wearing a roller?). But Madame is a vision: resplendent in white silk dress and blue bows which match her husband’s robe, she glances askance at him with an expression that’s full of affection and shared secret amusement. Her gesture, tweaking the collar of her lace cape, made me think of the way you see Roman matrons in sculpture adjusting their veils, but it also allows Liotard to show off her wrist-band with a miniature portrait of her husband. He wears her miniature in turn, in a ring on his little finger. Taken together, these two (for all my qualms about Isaac-Louis’s coiffure) are a hugely appealing picture of marriage, founded not on dynastic calculation but on mutual respect and admiration.

It’s no wonder that Liotard also attracted the attention of Europe’s royal families. Three sets of princely children are shown in this exhibition: four chalk portraits of the children of Maria Theresa of Austria, whose home at Schonbrunn I was exploring only last weekend; three of Louis XV’s offspring, overseen by their surprisingly understated father; and Augusta, Princess of Wales with four of her brood, among them the future George III. Each family group has its own different appeal.

The most formal of the lot are the Austrians, where the half-length format still has a slight stiffness about it, and where all the children are shown engaged in appropriate and improving tasks. Maria Christina, her mother’s favourite, is in the middle of painting; while Peter Leopold carefully plans out the foundations of a fortress; and the seven-year-old Marie-Antoinette, wearing a heavily ruffled pink dress, winds thread while staring out with a knowing detachment far beyond her years.

In the interests of gender equality, I should quickly mention a portrait of a man which I liked very much: Liotard’s picture of Garrick, executed in Paris in 1751. Like most of the other pictures I’ve mentioned it’s a pastel, but Liotard has depicted the famous actor with greater flair and vigour than many of his sitters. Garrick is caught in the act of turning to point (a theatrical gesture, but perhaps also one that Liotard remembered when he came to paint his laughing self-portrait). The actor’s mobile face flickers with humour and the pastel becomes broader and freer, giving a wonderfully fresh sense of life. I know little about Garrick as a man but, based on this portrait and that by Hogarth in the Royal Collection, I get the feeling he must have been a rather warm and attractive personality.

The French princelings are older, and it’s easy to read more into their expressions (perhaps only the romanticised fantasy of the exhibition-goer). It’s tempting to see reserve and distrust in the face of Princess Henriette, who at twenty-two was miserably convinced that she was on the shelf and would never find a husband. She was right, as it happened, though perhaps not in the way she anticipated: three years later, she would die of smallpox. Her sister Princess Victoire gives a similar impression that she’s only sitting for her portrait because she has to: her unwilling resignation doesn’t take away from the beauty of the picture with its soft mint-green gown. This was Liotard’s second attempt: his first portrait of Victoire had been deemed too honest by Louis XV.

I mentioned that the Austrian portraits were in chalk. Liotard is most famous for his pastels but his chalk studies are magnificent examples of draughtsmanship, sometimes exquisitely detailed. The majority of drawings in the exhibition date from his exotic period, showing Ottoman costume and occasionally named figures from the court, such as the dignified dwarf Ibrahim, but also portraits from his trip to the court in Muscovy. His drawing of Ekaterina Mavrocordato, consort of the ruler, is a superb exercise in texture, the black chalk suggesting the deep nap of fur around her shoulders and on her hat, while the embroidery on her dress is picked out with remarkable finesse.

Look too at the Maid serving tea from 1740: we’re in a wealthy Ottoman home, where the mistress of the house reclines on a divan in a tight, rather revealing bodice. The interest in costume is almost archaeological, and the maid’s striped, plain dress with its full sleeves is the part that caught my eye; but Liotard’s gift for characterisation also gives a lively sense of the relationship between these two women. As a draughtsman, he owes much to Watteau, who pioneered the trois crayons technique blending black, white and red, and who developed the precise, slightly sharp manner of handling the chalk that Liotard adopts.

But I was also struck, in portraits like the drawing of Liotard’s wife and son, by the fact that his style almost anticipates that of Ingres in some cases: the faces drawn carefully, sensitively, and the clothes dashed in with vibrant, suggestive, darting lines.Up against these dignified Austrians and the sophisticated, many-layered French, the English royal portraits do come as a bit of a shock. They’ve often been celebrated as an expression of the greater informality of the English court, where the royals were already trying to give their children a more natural, normal kind of family life.

And it’s true that the children, in their smart but plain clothes, have a pleasing immediacy about them: especially the much-loved portrait of George III’s younger sister Louisa Anne. Hers is perhaps the only portrait which genuinely reflects the excitement and curiosity that a young child would have felt on having her picture painted. (She died thirteen years later, of tuberculosis, just before her nineteenth birthday.) But is it terribly unpatriotic of me to stand in the centre of this room, and to look at the elegant Europeans, and to wish that perhaps the Brits were slightly less plain, slightly less round-faced, pink and white?

If you know nothing of Liotard beyond the ubiquitous image of the prim maidservant carrying a tray of breakfast chocolate (represented here by a version which is wisely described as ‘attributed’), you must make the time to see this. Indeed, make a day of it and see Goya too. Liotard is an artist well worth getting to know and his work is full of the Enlightenment spirit: warm, curious, honest and fascinated by new, unfamiliar cultures. There is playfulness too, in the trompe l’oeils included in the final room, the most effective of them a portrait of Maria Theresa of Austria half-obscured by a wooden panel that one itches to slide aside.

In many cases his technique doesn’t translate well to photography: the portraits come across as too dull, losing some of their light and sheen when reproduced in books. If you can, go and look at them; stand in front of them; and feel, for a brief moment, a sense of connection with this bright, lively man and his sitters.