US country music star Johnny Cash was well-known for his activism on prisoners' rights - but what impact did he actually have on the prison system?

On 26 July 1972, three grizzled-looking men dressed uneasily in suits gave evidence at a US Senate subcommittee on prison reform. Two of the men were former inmates of some of the toughest prisons in the US - the third was the country and western singer, Johnny Cash.

Cash's famous live albums recorded at Folsom Prison and San Quentin are the stuff of music legend - likely to feature on any critic's list of defining albums of the 1960s.

But it's much less well-known that these were only two of many prison concerts Cash played over the course of almost 30 years.

Fitting the gigs in around his relentless touring schedule, the "Man in Black" performed for inmates all over the US, always unpaid, and in the process, became a passionate and vocal spokesman for prisoners' rights.

"He always identified with the underdog," says Tommy Cash, Johnny's youngest brother.

"He identified with the prisoners because many of them had served their sentences and had been rehabilitated in some cases, but were still kept there the rest of their lives. He felt a great empathy with those people."

The roots of Cash's empathy lie as far back as 1953, when as a 21-year-old radio operator in the US Air Force, he saw the film Inside the Walls of Folsom Prison and was inspired to write a song.

Johnny Cash Born in 1932 in Arkansas to a farming family

Worked at a car plant, in the US Air Force and as a salesman before music career began in 50s

Sold over 90 million records

Nicknamed "Man in Black" because of dark clothes - it was also the title of a 1971 album

Honoured in the Country Music Hall of Fame and the Rock and Roll Hall of Fame

Died in 2003 aged 71 More on Johnny Cash

Folsom Prison Blues, released two years later, after Cash had signed to Sun Records, turned the young singer into a star.

The song, and in particular the now-notorious line "I shot a man in Reno, just to watch him die," was sung with such raw menace that many assumed Cash knew what he was talking about.

"There's people today that you can talk to and they will believe that he actually did that," says WS "Fluke" Holland, Johnny's larger-than-life former drummer.

"Johnny Cash shot a man to watch him die. He was a mean dude," he chuckles then shakes his head. "The only time he was in prison is when we played in them."

This is one of the ironies of Cash's prison reform crusade. The very thing that made convicts connect with him, and US senators hang on his every word - the air of authenticity that stemmed from the belief he had served hard time himself - was in reality a misconception.

"Cash had been behind bars," says his biographer Michael Streissguth. "But these were one-night stays in jail after drunk and disorderly arrests."

His popularity, fuelled by the desperado image, made him a refreshing antidote to the clean-cut pop stars of the era. Cash did little to dispel the growing myth around him.

One result was that he began to receive invitations to perform in prisons. He played his first prison concert in 1957 at the Huntsville State Prison in Texas. Many more followed.

The unique energy of these appearances was captured in 1968 on the At Folsom Prison recording. The album cemented Cash's outlaw image and reignited his career, which had been floundering in the mid-60s as he battled drug addiction.

Columbia Records, which had only reluctantly agreed to Cash's request to record at the prison and then half-heartedly marketed the release, was taken aback. But Cash had undeniably caught the public mood.

The growing civil rights movement had put the thorny issue of how to deal with America's huge prison population firmly on the agenda.

"In the 1960s in America, there was a growing realisation that prisons were ineffective," says Streissguth. "They were merely training inmates to be better criminals. So the recidivism rate, people coming back to incarceration, was very high."

Cash, an ardent believer in the power of rehabilitation over punishment, became the go-to voice for the media on this new hot topic.

"I think Cash had a feeling that somehow he had been endowed with this fame in order to do something with it, and one of the ways he could do something with it was talking about prison reform," says Streissguth, who also believes Cash's deeply-held religious beliefs were a factor in his championing of the cause. "He connected with the idea that a man could be redeemed."

If American prisons were failing, nowhere was this more true than in Cash's home state of Arkansas.

By 1970, the entire state penal system had been declared "unconstitutional" by a federal judge, after a series of scandals throughout the 60s involving the physical and sexual abuse of inmates, torture, inadequate living conditions and the outdated and corrupt "trustee system".

"Trustees were prisoners guarding other prisoners," says Colin Woodward from the UALR Center for Arkansas History and Culture. "It really echoed the way people worked under slavery. Under slavery a lot of the time the slave drivers were actually slaves themselves and that's what's replicated here."

Instruments of torture The "Tucker Telephone" was used to torture prisoners in Arkansas. It was an old-fashioned phone with a battery, wires and clips attached. "One clip was always attached to the toe," says Joellen Maack, curator at the Old State House Museum. "The other was usually attached to his private parts. The warden would crank the phone and it would deliver an electrical charge."

A leather strap was also used to punish prisoners. At the time, it was thought prisoners "were beyond the pale, and they could be treated any way they wished," says Bobby Roberts, an expert on prison history. "They had no rights." Badges, Bandits and Bars

Clearly, a system where armed prisoners policed their fellow inmates in the absence of any civilian guards was massively open to abuse.

"It was the closest thing to hell this side of hell," says Daniel Merry, a folk singer in his own right, as he remembers life in Cummins, then the worst of Arkansas's prisons and arguably one of the worst in the world at that time.

Merry, now in his 60s, was sent to Cummins for eight months in 1969 for the crime of marijuana possession. "I'd say life down there on a daily basis was pretty scary," he says.

"I seen a lot of inmates get killed. One of their favourite ways was to give you what they call a 'blanket party'. When you go to sleep, a bunch of old things run up and throws a blanket over you and sticks a bunch of homemade knives in you."

When Johnny Cash and his touring roadshow arrived to perform at Cummins in April 1969, it must have seemed like a rare ray of light to inmates such as Daniel Merry.

Cash came at the invitation of Winthrop Rockefeller, a scion of the wealthy American family, who had been elected state governor in 1966 with a mandate to overhaul the corrupt and abusive prison system.

Image caption Governor Rockefeller working the crowd at Cummins

Rockefeller was the first-ever Republican governor of Arkansas. Cash was from a family of Democrats, but saw something he liked in Rockefeller and joined him in 1968 on his re-election campaign trail, playing concerts and promoting their mutual call for reform.

By the time of the Cummins concert, Cash was close to the peak of his career. In February 1969 he had recorded the follow-up to Folsom Prison, Live at San Quentin, which would go on to sell even better than its predecessor, and ABC had just commissioned the weekly Johnny Cash Show which would turn him into a prime time TV star.

As well as singing for the prisoners, Johnny used the platform of the concert, which was being filmed for local TV, to put his money where his mouth was, offering to donate $5,000 (£3,000) of his own money to build a prison chapel - and publically challenging Governor Winthrop Rockefeller to match his donation.

"Old Winthrop, that day he wrote a cheque for five grand and put it in the chapel fund," smiles Daniel Merry, savouring the memory.

Cummins today is a very different place to the "dark and evil world" a federal judge declared unconstitutional in 1970. It's a modern prison focused on reforming inmates and preparing them for a return to society.

The chapel for which Cash had campaigned so passionately is one of the lasting legacies, not only of Cash's visit to Cummins but of his entire prison reform crusade. It is durable concrete evidence of Cash's interest in and commitment to the men he met behind bars.

Wilson Banks, the present day chaplain tells me there are still two or three lifers at Cummins who were there over 40 years ago when the Man in Black came to visit. "They say that he touched their lives."

Cash's conversations with the men he met at Cummins clearly touched him too. At the 1972 US Senate hearing, Cash relayed stories of some of the worst abuses he had heard of on his prison visits, including the harrowing tale of a 15-year-old boy who died of injuries caused by his rape at an Arkansas prison.

What's the picture in prisons today? 2.2 million people are in prison in the US

It has the highest prison population rate in the world - 716 per 100,000. The rate has risen sharply since the 1980s

More than $50bn (£30bn) is spent a year to cut the rates of re-offending

More than four in 10 offenders return to prison within three years

Recidivism rates vary substantially - they are falling in some states and rising in others International Centre for Prison Studies Pew Center on the States

Former inmate Daniel Merry says he remembers the boy and there were "many more like him".

Cash not only outlined to the senators on Capitol Hill what he thought was wrong with the American penal system, he also told them how he believed it could be improved.

His proposals included the separation of first-timers and hardened criminals, the reclassification of offences to keep minor offenders out of prison, a focus on rehabilitation rather than punishment, and counselling to prepare convicts for the outside world and reduce the possibility of them reoffending.

At a time when countries around the world are still wrestling with the question of how to handle those they incarcerate, many of the issues Cash raised that day feel just as relevant today.

The fact that we are still debating them 40 years later suggests Cash failed. But did he?

Image caption Cash met President Nixon after testifying to the Senate on prison reform

The sweeping reforms he advocated to politicians may not have come into force in his lifetime, and some have still not been achieved even now, but, using the power of his celebrity, he took the debate from politicians' offices and the inside pages of broadsheet newspapers, into public hearts and minds.

Cash himself said during the US Senate hearing, "people have got to care for prison reform to come about". His greatest legacy is that he made people care, through his public statements, but most powerfully and effectively, through his music.

For the average person, listening to one of Cash's prison albums, with their songs of prison life, interspersed with convicts' shouts, warders' tannoy announcements and rattling keys, was to be transported into a world that was, until then, largely unknown or ignored.

It's unlikely the changes at Cummins and other American prisons - and the push towards a view of imprisonment that tempers punishment with rehabilitation - would have come about without this shift in public perception.

Media playback is unsupported on your device Media caption Johnny Cash on stage at San Quentin State Prison

Cash successfully humanised the prison population and gave them a voice. He had a unique ability to get inside the heads of these forgotten and ignored men and understand the problems facing them - the roar from the inmate audience that can be heard on Live at San Quentin when he launches into the provocative angry title track is testimony to this.

Perhaps Cash's impact is best summed up in the words of Senator Quentin N Burdick, one of the men he addressed at the US Senate in 1972.

Having heard Cash's testimony, Burdick said, quite simply: "We need more Johnny Cashs, I guess."

Danny Robins was reporting for a two-part Your World documentary for BBC World Service: Johnny Cash and the Forgotten Prison Blues. You can listen to the part one, and part two on iPlayer.

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