He also created the Coelbren y Beirdd, an alphabet. The British Museum holds records of the Coelbren writing system dating back more than 2,600 years, in the form of a Coelbren and its runic system imprinted on a clay tablet discovered in the Ancient Middle Eastern Assyrian Library collection of Ashurbanipal, excavated in 1849 by Austen Henry Layard. It has been suggested that some of Iolo's claims were supported by oral tradition: recent research has revealed that the tale of Ieuan Gethin, a soldier in the Glyndwr revolt, might have basis in fact.