This 18-pound (8-kilogram) iron ball-and-chain set was found in the mud on the banks of the Thames River and is thought to date to the 17th or 18th century. LONDON - An iron ball and chain found on the banks of London's River Thames is causing a stir among archaeologists who say the 300-year-old artifact used to restrain convicts on ships may have a gruesome story to tell. The leg irons, believed to date from the 17th or 18th century, were pulled from the mud with the lock fastened, suggesting a convict could have drowned while trying to escape.