Conservationists at the National Museum of Wales were restoring Turner's Llandeilo Bridge and Dynevor Castle when they found a second canvas sealed under the original. The new picture also depicts the bridge and castle, but from a slightly different angle.
Ian Warrell, assistant keeper of the British Turner Collection at the Tate Gallery, which with more than 20,000 Turner works is the largest collection in the world, said: "It adds to what we already know about him, rather than being a complete revelation. It's very interesting and it's something that with other research going on here gives a fuller picture of the artist."
Mr Warrell, who studied and authenticated the museum's discovery in the spring, believed the water-colour might have been a draft then used as a trace for the picture that was eventually completed. He said that the artist had reached an advanced stage before evidently becoming unhappy with the result. "What's interesting is that it shows how Turner worked in a broader fashion before he introduced details and light effects . . . At that stage he was quite a young artist still experimenting to see how he could use colour to create different effects, so he would often abandon work halfway through," he said.
Turner undertook five major sketching and painting tours of Wales between 1792 and 1799. He discovered Llandeilo Bridge and Dynevor Castle in 1795 and completed his painting of the hilltop castle the following year. The new Turner, which is immaculately preserved, is currently being shown alongside three of his other water-colours in an exhibition touring Wales.Reuse content