Turning the limits of a medium into benefits

• Sion, Capital of the Canton Valais. Picture: Complimentary

No English artists had a place in it, not even Turner, and the Scots, of course, did not exist at all. Things have moved on, I am glad to say, but happily the public, ignoring orthodoxy and demonstrating both its independence and its good taste, never did lose its affection for Turner. This is demonstrated afresh each January in Scotland by the annual display of the Vaughan Bequest. Crowds turn out to see Henry Vaughan's marvellous Turner watercolours and probably have done every year since the bequest arrived at the National Gallery in 1900.

Leave a watercolour in a brightly lit room and in a year or two you will be left with only its shadow. Vaughan bought his watercolours from Turner before they had suffered any deterioration from exposure to light. Hence the stringent terms of his bequest: only show the pictures in January, when the light is lowest. In consequence they are as fresh now as when they were painted.

The broad narrative of art historical orthodoxy still doesn't really accommodate the grandeur of Turner, however. Where does he fit between those bookends of the Renaissance and Impressionism? Turner knew all about the great art of the past; he learned from it and constantly matched himself against it. As the brilliant exhibition in Edinburgh last year, Turner and Italy, and the rather less brilliant one in London, Turner and the Masters, both demonstrated, he had a tremendous curiosity about the art of the past. Although both shows also demonstrated that pursuing his curiosity led him to paint some pretty rum pictures, Turner knew exactly how he related to the Italian Renaissance and the great traditions of painting that went before him.

One good reason why it is difficult for us to place him is the sheer scale of his achievement and the huge range of what he did. Another less obvious one, however, is that he was so much a watercolour painter – hence the importance of the Vaughan Bequest – and the traditions of watercolour were very different from the high art of the Renaissance or the Baroque. It is good planning therefore, or at least serendipity, that while the Vaughan Bequest is on view at the National Gallery, upstairs the Sandby exhibition is also still showing.

Paul Sandby is credited with pioneering the watercolour landscape. That may not be strictly true, but Sandby nevertheless did bring something new to landscape art because he was first of all a map-maker. As this exhibition shows, map-making was still closely allied to topography; the description and record of place. Both reflected art's key role in the business of information – until the invention of photography, artists were the indispensable partners of all the sciences from surgery to surveying. What Sandby did was to bring to landscape painting the approach to information of the map-maker and topographer.

Watercolour was his chosen tool because it is so portable. All you need is a sketch-book, a few little containers of ink and paint, a brush and some water. The limited palette of Turner's early watercolours – his little picture of Rye in Sussex, for instance, or the more ambitious, but still pretty monochrome Falls of Clyde from his first Scottish trip – is much the same as Sandby's, though Turner's ability to convey light and space shows his sense of the drama these things can convey.

Sandby's career as a topographical landscape painter coincided with the beginning of modern tourism and with the corresponding growth of popular interest in landscape. It was seen as a witness to history and also increasingly as the source of poetry.

For the Scottish ballad collector David Herd, the relationship between poetry and landscape was one of direct symbiosis. Actual landscapes are an integral part of the poetry on which Walter Scott's fame was based; in his novels they are part of the action. Turner's little watercolours Melrose and Rhymer's Glen, Abbotsford – done in a minute style to be engraved as illustrations to Scott – are witness to the collaboration between the two men. It is a collaboration that also suggests an analogy: both built a marvellous poetic art on a new alliance of the imagination with hard information.

This was not the whole of Turner's art by any means, but it was the central part and it is seen vividly in this group of watercolours spanning his whole career. Almost all of them represent an actual place, or if not a place, then credibly described weather conditions.

Place, weather, light and colour are all part of a recorded reality, heightened by his imagination perhaps to convey his emotional response to what he saw, but still rooted in his extraordinary powers of observation. You see that in the detailed account he gives of the boats and the ship's rigging in his Man of War, though his drawing loses none of its fluency in the description.

Watercolour in his hands became the means to record all these things as no-one has done before or since. Who else could capture the instantaneity of a flash of lightning by a jagged scratch in the paper as he does in Venice: A storm in the Piazzetta? This picture is also a huge advance on the early work and in the sequence here, monochrome soon disappears to be replaced by the brilliant colour we associate with Turner. This was not only a matter of genius, though genius is the only word for his use of colour in paintings like The Sun of Venice or views of Thun, Heidelberg, or the Falls of Schaffhausen.

The popularity of landscape and the fashion for sketching created a new market for the colourmen. Demand brought a dramatic improvement in the paints that were available, and paper improved, too. In consequence, when Turner travelled to Venice for the first time in 1819, as we see in The Rialto, Venice, he could already deploy a much greater range of colours than he had had at his disposal at the beginning of his career. For Turner these developments were much more than a convenience. He used them to turn watercolour into a radical new art form which in turn influenced his approach to oil paint.

A contemporary critic said of his late oils that they were "painted with tinted steam", and the veils of colour that Turner used in his late work are borrowed from one of the most important characteristics of watercolour: its transparency. Combining it with new stronger, brighter papers, Turner exploited this, inspired not by his supreme technical facility, but by what he saw in front of him.

In the watercolour The Sun of Venice, for instance, most of the picture is just white paper, but he magics the brilliant pervasive light of Venice out of its plain surface with a few touches of his brush. A few years earlier in Sea View, he uses the same transparency on blue paper to quite different effect. The blue shining through the colours he lays down unifies sea and sky with a sense of all-embracing weather. He uses the same blue paper in Ehrenbreitstein from the Mosel. The result looks so natural you do not realise at first what intense colour he uses to create such dramatic grandeur in the tiny compass of a sheet of paper barely six inches across. But that is the nature of his genius.

&149 Turner in January 2010 is at the Royal Scottish Academy building until 31 January; Paul Sandby: Picturing Britain is at the NGS until 7 February (0131-624 6200)