When David Dallas took up his post as International Head of Old Masters at Bonhams New Bond Street, it was a homecoming. The distinguished specialist who has held major positions in the trade (most recently he worked in St James's for the private gallery Johnny van Haeften) spent three years in the 1970s on the third floor of Bonhams HQ in New Bond Street as Deputy Director Old Master Pictures at Phillips. Indeed, the first painting he helped identify shortly after his appointment in 1975 was by the Dutch artist Jan van de Cappelle. It made the highest priced picture handled by the department at that date.

Since then, there have been many discoveries. Famed for his photographic memory and wide-ranging reading habits, Dallas draws information from eclectic sources to help him find 'sleepers' – the term for pictures sailing under an incorrect and less distinguished attribution. One triumph was when he bought a picture in New York painted by 17th-century Dutch artist Gerrit van Honthorst that had a central image of a boy with scarlet macaw's wings, holding a jaguar in reins. Realizing that the image must be a metaphor, Dallas worried away at it for some days when it suddenly came to him that the painting must refer to the Dutch House of Orange harnessing the wealth of South America. After more research, which established it was commissioned by the Dutch royal family, he was able to sell it to the Royal Collection at Het Loo. As he points out, "The moral is that all information is valuable. If you can pull these disparate threads together they can make, or save, a fortune."

Dallas, whose kinsman, George Miflin Dallas was Vice President of the United States, did not exactly plan to go into the art world. "I was at Eton and failed to get into Oxford – or anywhere else for that matter. My father did, however, get me in to see the managing director at Christie's. At the end of a ten-minute interview, during which he chained-smoked Senior Service, he said: 'Is it Impressionists? Or Old Masters?' I asked what Old Masters were and he told me they were 'all the early pictures'. When I replied that those were the ones for me, I was told I started in October. It doesn't happen like that anymore."

Dallas stayed at the auction house three years – "I remember cataloging by candlelight. It was the time of the three-day week" – before joining a gallery in the City. That closed unexpectedly after a year and he found himself out of work. But that time was to prove an education in itself. Dallas set himself the task of learning as much as possible about British painters. "I went to The Witt Library [at The Courtauld Institute] and I looked at photographs of every single British artist from A to H, (I'm going to finish the rest of the alphabet at some point). And I went to see all the annotated catalogs at the Royal Academy, and then traveled around provincial museums, looking at British landscapes. These became my real forte."

Clearly British landscape painting appeals particularly to Dallas's sensibilities, perhaps because he spent his childhood in a Berkshire village called Burghfield. It gave him an acute understanding of the undulations and folds that are peculiar to the English countryside. As he says, "Painters such as Turner and Constable don't depict 'a tame delineation of a given spot'. They interpret landscape."

One thing is for sure: all those years of criss-crossing England looking at works in dusty regional galleries certainly paid off. Over the years, Dallas has made a name for himself discovering paintings in this field – more than ten paintings by Constable and 50 paintings by Francis Danby. It is, as he says, a question of 'handwriting'. "Before everything was generated by computers, when you received a letter from your mother, you recognized the handwriting without opening the envelope. It's the same with pictures, you just know the brushstrokes." One of his most satisfying discoveries was unearthing a winter landscape by Irish painter Francis Danby. "It's odd but British winter landscapes are actually incredibly rare. The studio system was that you sketched in the spring, summer and autumn, and worked them up over the winter."

Old Masters – or "Masters" as Dallas prefers to call them – have to some extent been in the shadow of the contemporary art market during this past decade. But Dallas sees no divide. He believes that the current trend is for major collectors to acquire art from all epochs. As he explains, "People have their favorite moments in art, but I don't think you can extract genres and view them in isolation. Works by artists such as Velázquez and Adriaen Coorte, both of which did so spectacularly well in our last sale, are not necessarily of their time: they are timeless. Velázquez doesn't paint his portraits as typical of the 17th century. He paints a person."

One innovation that Dallas is looking at is to offer previews in which Master paintings are displayed next to 19th- century and contemporary works. "I think it's fascinating to see that these works can hang alongside one another, that they don't necessarily jar – and that they transcend their era."

Lucinda Bredin is Editor of Bonhams Magazine.

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