The Black Sabbath rocker let word of the secret project slip during a recent Kerrang! interview with the Queen star, asking him, "When are we going to get to work on that album of riffs together? You know what I'm talking about."
May reveals the idea was inspired by a trip he took to Iommi's studio and all the unused Black Sabbath riffs his peer had recorded.
He says, "I thought it would be great to make a compilation out of them. The idea was to put all these riffs out in some form so that people could build their own songs from them. You could make your own music with Tony Iommi on guitar!"