“It wasn’t a fun, easy record to make,” says producer Bob Rock, speaking of Metallica’s self-titled fifth studio LP, popularly known as the ‘Black Album.’ “Sure, we had some laughs, but things were difficult. I told the guys when we were done that I’d never work with them again. They felt the same way about me.”

When Rock began sessions with Metallica in the fall of 1990, the band (guitarist-singer James Hetfield, drummer Lars Ulrich, guitarist Kirk Hammett and their then-bassist Jason Newsted) had already hit platinum with 1988’s …And Justice For All, which catapulted them from cult stars to arena headliners.

“They had broken through to one level, but they still weren’t on mainstream radio,” says Rock. “When they came to me, they were ready to make that leap to the big, big leagues. A lot of people think that I changed the band. I didn’t. In their heads, they were already changed when I met them.”

Not that Rock didn’t help the group up their game any: Sonically, he gave them a rich, low-end punch that had been lacking in their previous recordings. Part of the process involved addressing Ulrich's drumming. “I noticed that Lars played to James’s guitar,” says Rock, “much like the way that Keith Moon played to Pete Townshend. That’s fine for some bands, but not every one.

“Lars wanted Metallica to groove more. AC/DC’s Back In Black was a big reference point as a rock record that grooved. I told him that in order to get that feel, he had to be the focal point musically. So on certain songs, the band played to Lars. They followed him. It made a real difference.”

Meanwhile, Hetfield was going through an even bigger transition, one which would hold significant ramifications. “He wanted to go deeper with his writing,” says Rock. “He wanted his songs to really matter. We talked about the great songwriters, like Dylan and Lennon and Bob Marley, and I think he saw that he could write for himself but still touch other people. It was a struggle for him, but he had a tremendous breakthrough as a writer.”

Released on 12 August 1991, the Black Album was immediately hailed as an artistic triumph. Debuting at number one, it spawned the singles Enter Sandman, Sad But True, The Unforgiven, Nothing Else Matters and Wherever I May Roam - songs that would annex radio at multiple formats. Commercially, the album was a colossus, selling a staggering 22 million copies and firmly establishing Metallica as worldwide superstars and an essential part of the cultural landscape.

On the following pages, Bob Rock (who would eventually work with Metallica again – on five different projects, no less) takes a track-by-track look back at the Black Album, 20 years after its release. “It’s probably the biggest accomplishment of my professional life,” he says. “I think the band would say the same thing."