“Thomas Girtin is up there with the very best British artists. It is reported that JMW Turner, his exact contemporary, said that: ‘Had Tom Girtin lived, then I would have starved.’

Back in the 1790s, Girtin was by far the more innovative painter of the two and we are showing three works from the period to prove just how good he was.

They plot his immense progression from the basic architectural draughtsman- ship shown in his picture of Crowland Abbey in Lincolnshire, where subtle blues and greys depict the ancient stonework, to the vast sublime view of the Ogwen Falls in Wales, for which he deployed a new technique of applying wet pigments to brown paper.

His picture of Dunnottar Castle was completed halfway through that development and, although it’s very small, just 16.5cm x 22.5cm, it is one of the most dramatic watercolours in the collection.

Turner’s quote was, of course, just a throwaway line but he was absolutely correct in identifying Girtin as the more adventurous artist; technically, he was just dazzling.
The two men knew each other well and dealt with similar themes of light and movement.

They were also both commissioned by James Moore, an antiquary and amateur artist who went around the country making rough sketches of particular sites before asking professional painters to copy them in watercolour and prepare them for the engraving process.

There’s no suggestion that Girtin actually visited this scene but I think that’s one of the marvellous things about it. This picture is so alive even though, in theory, it’s simply a re-interpretation of Moore’s original pencil sketch.

I spent my teenage years about 20 miles away from this site in Scotland so I know the topography quite well and I realise this is quite an impossible view of the castle.

The original sketch must have been made from the beach, but it looks here as though the artist has been caught up in the crashing waves. Look closely at the water and you’ll notice tiny dabs of paint that stand for so much in terms of technique; there’s a wonderful balance between the landscape and the fierce emotion of the storm.

The picture has very clearly been delicately drawn and the castle and the cliff beneath are picked out in pencil to give the whole image some structure.

Of course, Girtin is nowhere near as well known as he should be. He’s always been greatly admired by watercolour enthusiasts but the museum-going public aren’t as aware of his work, partly because they are watercolours and partly because they are not on permanent display.

You can visit Tate Britain and see endless Turner oils on the walls but Girtin, who died at the age of 27, deserves huge credit for elevating watercolour from topographic drawing to a very emotive medium.”

Great British Drawings is at the Ashmolean Museum of Art and Archaeology, Oxford
, until 31 August