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Duncan, 57, who was indicted on May 18, sat slumped in his chair at the defence table as the jury announced the verdict on the second day of deliberations following a seven-day trial in Manhattan federal court. He declined comment, but his attorney, Benjamin Brafman, said he was "bitterly disappointed" and vowed to appeal against the verdict.

Court papers describe Duncan as a leading expert on Louis Comfort Tiffany, the renowned stained-glass craftsman whose windows are features of many churches and mausoleums and can command hundreds of thousands of dollars at auction.

Duncan has been a consultant to Christie's auctioneers, the Federal Bureau of Investigation and other law enforcement organisations, and has published several major works on Tiffany.

He was found guilty of purchasing a nine-foot Tiffany window stolen from the Salem Fields cemetary on the border of Brooklyn and Queens in New York City. In partnership with a middleman he then sold the window to a buyer in Japan for $219, 980, according to government papers. He was also found guilty of asking the grave robber to find another Tiffany-made mausoleum window for a foreign buyer, and with witness tampering. Sentencing is set for October 27.