Great Works: A Wall in Naples circa 1782 (11.2 x 15.8cm), Thomas Jones

National Gallery, London

Good poems, it has often been said, transfigure the ordinary. They rinse the eyes so that we see the familiar anew, rendered wonderfully revitalised. The same thing could be said of a great oil sketch, such as this one by Thomas Jones. There could be few things more humdrum and insignificant from a thematic point of view than this woebegone and decrepit stretch of late-18th-century Neapolitan house-wall, with its meagre display of attention-grabbing smalls.

It almost seems to exist in order to be overlooked – or to be hurried past in the company of a snarly, scurvacious dog. It looks like the aggressive anti-type of a fashionable history painting. Which is precisely, of course, why Jones found it so fascinating.

Jones finds it so enthralling that he is clearly urging us to contemplate it for itself alone, as if it were not so much a wall as a segment of abstract patterning. There are no other considerations here. The wall practically blocks out the entire view. (Well, there is a tidy rectangle of blue sky and, beyond our wall, what looks like yet another example of its kind, sun-splashed, fairly smooth, and of a smudgy, creamy hue.) This work has no evident audience appeal. It is not destined for the august, soaring walls of the Royal Academy. Jones is making it for its own sake, and, as with Constable's cloud studies over Hampstead Heath, it is so fresh and so good precisely because it looks snatched, impromptu, unpremeditated, uncalculated to please any potential purchaser. It reminds us that the oil sketch – which was usually a small-scale preparatory work (though seldom as small as this) – often possessed a vitality that its worked-up finished version has often seemed to lack. This is especially so in the case of Rubens.

Unlike some of the other paintings of equally modest dimensions (this is scarcely bigger than a seaside postcard; in fact, it is almost engulfed by its gilt frame) which flank it in Room 42 of the National Gallery by the likes of Corot, Eckersberg and Fleury, this wall does not form part of a landscape tricked out with tragically grandiose and sweetly poignant classical ruins. It has no such pretensions. What age is this wall? Goodness knows. It could either be newish or fairly old. Its age is beside the point. What excites Jones is the status of its objecthood, its oddly various textures, its singular decrepitude, the fact that the elements have punished it so badly, that various house beams have been punched into it and then, later on, rotted or fallen away. All this is evidence of what it may once have been, which is somewhat greater than it now happens to be. And yet, for all that it seems haunted by its past, here it still stands, like those thistles once written about by Ted Hughes which, year on year, came fighting back over the same ground.

It is a wall which is wholly rebarbative and without pity. It does not invite us in. It is nothing but the pockings and pittings of its surface, and that surface somewhat reminds us of work by the Boyle Family – those slices of reality (manhole covers, edges of pavements) that they have often chosen to replicate as free-standing, wall-hung artworks in order to prove to us quite how extraordinary these things that we habitually walk over can prove to be when we bother to look at them at all.

This wall exists to exclude us, to turn away our prurience. There are windows of a kind set into it, but it is not quite clear to us whether they are shuttered against the sunlight or not. The left-hand window may be glazed, but even if that were to be true – and it is by no means certain – we are not being permitted to see through it. No window opens into a picturesque interior scene. This wall offers us nothing but an unbudgeable, unshakeable blank stare.

The wall itself is not in good condition. It needs re-rendering. We see it in the company of nothing other than a rather lame and skeletal display of vegetation, which looks like a sickly, pale-green silhouette of itself. Part of us wants this wall to be more than it is, to be more than mere context, but the wall itself, soldiering gamely on, shouts back at us: "I am that I am. Love me for the fact that I have endured at all. Admire my wounds. Who needs the slick, sad, characterless smoothness of the new?" And then there is the no-holds-barred comedy of that washing, of course. What other artist has pushed such washing to the centre in this way? Who has made equal claims for such sad, wind-thrashed fripperies?

ABOUT THE ARTIST

Thomas Jones (1742-1803), born in Radnorshire and a pupil of Richard Wilson, is best known as a landscape painter of his native Wales and for his studies of Neapolitan buildings. His extraordinary art came back to public attention in the 1950s with the publication of his memoirs.