After the First World War, the scholar Griffith John Williams (1892–1963) was the first to make a full study of Iolo's work, consulting original documents donated to the National Library of Wales by Iolo's descendants in 1917. Williams aimed to find out exactly how much of Iolo's output was based on imagination rather than fact. He established that the poems Iolo attributed to Dafydd ap Gwilym were forgeries. His researches led him to become a defender of Iolo's reputation as well as a critic.