Geezer Butler says he still isn’t sure what famously eccentric producer Rick Rubin brought to Black Sabbath’s final album.

He'd previously been critical of Rubin’s work on 2013's 13, which marked the first time Butler, Ozzy Osbourne and Tony Iommi recorded together since 1978’s Never Say Die!

“Some of it I liked; some of it I didn’t like, particularly,” Butler now tells SiriusXM. “It was a weird experience, especially with being told to forget that you're a heavy metal band. That was the first thing [that Rubin] said to us. He played us our very first album, and he said, ‘Cast your mind back to then when there was no such thing as heavy metal or anything like that, and pretend it’s the follow-up album to that’ – which is a ridiculous thing to think.

“I still don't know what he did,” Butler added. “It’s like, ‘Yeah, that’s good. … No, don’t do that.’ And you go, ‘Why?’ ‘Just don’t do it.’ I think Ozzy one day went nuts. He'd done like 10 different vocals, and Rick kept saying, ‘Yeah, that's great, but do another one.’ And Ozzy was, like, ‘If it’s great, why am I doing another one?’ He just lost it.”

Butler said Iommi was also unhappy with some of the things Rubin asked him to do. “He was making Tony get 1968 amps, as if that’s going to make it sound like back in 1968. It’s mad! ... But it's good for publicity and it's good for the record company,” Butler argued. He said their thought process was: “If you’ve got Rick Rubin involved, then it must be good.”

Iommi has also hinted at his own bemusement over Rubin’s approach. When asked last year what he learned from the producer, Iommi replied: “I learned how to lie on the couch with a mic in my hand and say 'Next!’”

He confirmed that Rubin tried to trace the kind of equipment used by Black Sabbath in 1968. “I get to the studio, and there are 20 different bloody amps there. He goes, ‘They’re vintage amps’,” Iommi remembered. “I said, ‘That doesn’t mean they sound good; they’re just old.’ He went, ‘Well, let’s try them.’ I tried them, and I didn’t like any of them.”

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