Joseph Nahmad bought his first painting two years ago — when he was 19 years old. "I saw this amazing Lucio Fontana at Sotheby's in London, and I called my father," he said recently over chamomile tea at Café Carlyle. "But I couldn’t reach him, so I bought it on a whim." It was one of the artist's sliced Concetto Spaziale canvases, hammered down at £1.8 million. "When I told my father about it, he was not very pleased." He had sold the very same painting 40 years earlier for about $2,000.

The impromptu Fontana purchase may have been an expensive gamble, but it's not so surprising if your father is billionaire collector David Nahmad, your older brother is Manhattan dealer Helly Nahmad, and your family has amassed, over 60-plus years, quite possibly the largest private collection of Impressionist and modern art in the world. The Nahmads, who now live in London, Monaco and New York, are notorious for treating the art market like the stock market. They protect the prices of the artists they own, for instance, by bidding on their works at auction. "It's called defending your inventory," said Helly Nahmad in a 2007 Forbes interview.

Joseph Nahmad skipped college — "I've been going to auctions since I was five," he reasoned — and instead has been working for the past two years at Helly Nahmad Gallery in New York (Joseph's cousin is also named Helly and has a gallery of the same name in London). Now he is going out on his own, rejecting the family's longstanding formula of buying nothing made after 1965 (his father famously once called the contemporary art market "almost a fraud"), and launching Joseph Nahmad Contemporary this week. The inaugural show, "Blind," Nov. 17-26, 2011, is a solo presentation by painter, sculptor and interior designer Roy Nachum at the 5,000-square-foot Openhouse pop-up gallery at 201 Mulberry Street.

"At first I went without telling my father. He's very hard-headed; he likes to stick to his bread and butter and he thinks conte...