In 1917, Varèse boldly announced that he longed "for instruments which are obedient to my thought and whim, with their contribution of a whole new world of unsuspected sounds, which will lend themselves to the exigencies of my inner rhythm." From 1918 to 1936, Varèse abandoned tradition almost completely. Hyperprism of 1923, for example, provoked a riot at its premiere. During his quest for new sonorities and effects, which climaxed in Ionisation, Varèse incorporated new musical instruments wherever possible. Ionisation, composed between 1929 and 1931, attracted the greatest interest and divergent critical opinions of all his works. The score introduced the electrical siren as a musical instrument for the very first time, and denoted the start of Varèse' increasing interest in electronic music. Nicolas Slonimsky conducted it at Carnegie Hall, on March 6, 1933, and the composer later dedicated the work to him. Carlos Salzedo, Henry Cowell, Paul Creston, and William Schuman performed as some of the 13 instrumentalists required. Its effect was likened by a critic at the time to receiving "a sock in the jaw." Varèse argued in his defence that "in music we composers are forced to use instruments that have not changed for two centuries....Composers like anyone else today are delighted to use the many gadgets continually put on the market for our daily comfort. But when they hear sounds that no violins, wind instruments, or percussion of the orchestra can produce, it does not occur to them to demand those sounds for science. Yet science is even now equipped to give them everything they may require."
The title is derived from the ionization of molecules, as electrons are dispersed through the process of atomic change. In Ionisation, rhythmic cells are expanded, varied, and contrasted against one another. Their timbre keeps them identifiable as each cell becomes more involved and larger, and these cells grow in such a way that renders them independent of one another. The dramatic contour is in the degrees to which these rhythmic cells, which evolve into recognizable blocks of sound, seem to not be working together, continually growing and expanding the soundscape with the friction of their coexistence. Occasional relief is found in rhythmic unisons, that bind the separate blocks of sound into a single, propulsive rush, but there are not many. Varèse loved the unmanageable aspects of nature, the things that humans have no control over, and reveled in the way that, from our perspective, nature does not run smoothly.