Connoisseurship in the late Ming period centered on these items of refined artistic taste, which provided work for art dealers and even underground scammers who themselves made imitations and false attributions. The Jesuit Matteo Ricci while staying in Nanjing wrote that Chinese scam artists were ingenious at making forgeries and huge profits. However, there were guides to help the wary new connoisseur; Liu Tong (died 1637) wrote a book printed in 1635 that told his readers how to spot fake and authentic pieces of art. He revealed that a Xuande era (1426–1435) bronze work could be authenticated by judging its sheen; porcelain wares from the Yongle era (1402–1424) could be judged authentic by their thickness.