Of all the great 18th-century portraitists, Thomas Gainsborough (1727-1788) is the most appealing and the most elusive.
It's not even as though he was that good at the portrait. Although in painting his friends, he can be perceptive and sympathetic, all too often he shows little interest in the sitter, retreating to well-worn head-and-shoulder or full-length formats that tell you little of the subject. It's his way with fabric, lace and hair which makes him so special: his feathery light brush and his suffused light. And, of course, the manner with which he brings in tree and landscape to frame the figure. You can look at a Reynolds or Ramsay and sense the man or woman pictured. You view a Gainsborough and feel the artist.
The Holburne Museum in Bath provides an explanation in a revealing show of his landscapes, the first in 50 years. Gainsborough drew and painted them all through his life. Indeed, that's how he started, drawing the nature and foliage of his native East Anglia. To the end he was always careful to include at least one landscape in his exhibitions and his contributions to the Royal Academy. His landscapes had a powerful influence on Constable and the painters of the next generation. Yet Gainsborough never really tried to sell himself as a landscape artist. For him landscapes were a pursuit of interest, more important but more private than his portraiture.
The fascinating thing about Gainsborough's landscapes is that they weren't real but were always imaginary. He certainly drew from nature, obsessively. But he used his nature drawings not to recapture a place but as a means of constant experimentation in composition and mood. Again and again he would rework the same formal idea to try out different variations and subtle changes in positioning. Always a great experimenter with perspective and with the washes and finishes of his drawings, Gainsborough's landscapes were an exercise in artistic trial, the expression of an endlessly questing artist at work. The grouping of objects "in friendship" was his constant aim, and when he looked back on his early works he criticised them not for the facility of the draughtsmanship but the lack of concentration on the composition.
The Holburne show is not a huge display, nor a comprehensive one. But it is a concentrated one. Intelligently, the show's curator, Susan Slonam, has decided not to pursue a chronological narrative. Instead she has chosen to concentrate on a series of themes, each centered on a substantial oil and accompanied with drawings which elaborate the composition both before and after the date of the paintings.
The approach yields not only a fascinating insight into Gainsborough's methods but also demonstrates just how good a graphic artist he was. He started with pencil on paper and, as he matured, he turned more to crayon and washes. If nothing else, the exhibition encourages you to look at the graphics, many of which have been lent from private collections. Gainsborough was quick and careful in his drawing, with a wonderful eye for people of all ranks. There's a delightful sketch from 1751-52 of two women walking arm in arm, their skirts swept in by the wind to reveal the shape of their bodies, which he used in his first major landscape painting of the period, the Dutch-influenced River Landscape with a View of a Distant Village from 1750 (also displayed).
Some of the later landscapes, where Gainsborough reaches for the feel of time and nature, are masterpieces in their own right, without a hint of the sentimentality or striving for sensibility which he sometimes showed in his paintings. Most intriguing are the later works in which monumental rocks take the place of trees (in one drawing he actually works over an initial tree to make it into a rock). Why? Was it a nod towards the English taste for the "sublime" then becoming fashionable? Was it a reflection of age (Gainsborough was strongly religious), the man coming to terms with eternity? Or was it the artist pushing at the boundaries of vegetable and mineral, feeling for the point at which all nature merged into one?
Whatever the impetus (and I tend to the latter view), it produced a painting of Mountainous Landscape with Shepherds and Sheep from the early 1780s which takes the Italian landscape tradition of the 16th century and throws it forward into the full glory of the English Romanticism that succeeded Gainsborough and his contemporaries.
The one disappointment of the exhibition is that the Holburne doesn't try and tie in the landscapes with the portraits. It's not as though the museum doesn't have them. While Sir Thomas Holburne, whose collection provides the basis for the gallery, never showed much interest in English art – and what he did buy is apparently pretty execrable – directors since his time have managed to build up a formidable display of English portraits by the masters, many on loan. They include several top-quality works from the major working periods of Gainsborough's life, including a recent loan of a splendid unfinished full-length portrait (of Henry Beaufoy from 1785) from late in his career, which shows what a master of impressionist brushwork he was, and a sickly half-length of Louisa Skrine, Lady Clarges, from his London period, in 1778, which illustrates just how saccharine a figurative artist he could be.
Landscape entered Gainsborough's portraits from early on. It's intriguing to see how he used it as a frame for the figure or figures, arching over the heads and merging with the colour of the clothes or the frill of the lace. But what also strikes the viewer is how he makes the landscape so naturalistic and earthy. Artists such as Arthur Devis or George Stubbs inserted backgrounds to show possession and wealth. Gainsborough doesn't. In one of his best works in the museum, The Byam Family from the 1760s, husband, wife and child come out to view not parkland but countryside. A client who begged Gainsborough to make the background a rendering of his precise property got very short shrift from the artist, who told him he was in the habit of doing the landscape as the figures in his own way. "With regard to real Views from Nature, in this country," the artist explained in his scathing reply, "he has never seen any Place that affords a Subject equal to the poorest imitations of Gaspar or Claude."
Alongside the Gainsboroughs, the Holburne are showing a half-dozen specially commissioned works in homage to the master by the photographer Mark Edwards. They're large scale views taken with a 10x8ins Victorian style plate camera and digitally printed. The most striking is a light-box illumination of a transparency, imitating Gainsborough's own experimentation with transparent oil on glass, lighted by candles in a specially-designed box. A few, too fragile to be moved, survive in the V&A. The effect, in Mark Edwards' view of a river taken on a grey day, is startling for the way it gives the viewer the sense of light itself.
Gainsborough, who painted his transparencies with night and twilight scenes to create a more dramatic effect, would have been pleased by this homage. Photography as a means of recording the specific landscape he might have found useful but not creative. But playing around with translucent film, light boxes and digital printing would have fascinated him. In that sense, he was the true child of his age, an endless experimenter with form and substance.
Gainsborough's Landscapes: Themes and Variations, Holburne Museum, Bath (01225 388588) to 22 January. The View from Here: New Landscape Photographs by Mark Edwards, to 8 January