Check out Charles Ives' Concord Sonata and some of his song accompaniments. At some point he indicates to the pianist to use a 12" ruler to play a mass of white keys.
Is it true that he was the first to use them regularly?
Not only this: he was the very first composer to use them, in the work The Tides of Manaunaun, written in 1912. Another astonishing use of clusters is Tiger. To anyone who is interested in Cowell music or avant-garde procedures of early 20th century music, I would refer to his book New Musical Resources. Best wishes!