It was dubbed the longest experiment in prison history and was one of the most controversial chapters in the history of notorious facility Barlinnie prison.

The Special Unit was established in 1972 with the aim of rehabilitating some of the Bar-L's most violent and volatile offenders.

It was often ridiculed in the Scottish tabloid press, where conditions were likened to a holiday home rather than a prison.

The intention was to give a measure of autonomy to long-term difficult prisoners in the hope they would respond to being treated more like people than dangerous animals.

Former inmate Johnny Steele told STV News: "No other prison in Scotland would take me. You've got to bear in mind in all those years of fighting, being locked up and rioting you've got a coat of armour as a way of surviving.

"In the Special Unit, all the armour was off, through time chip, chip, chipping it a wee bit."

Before the unit was established, Mr Steele paints a bleak picture of life inside the Glasgow facility. Many prisoners became institutionalised, with nowhere to turn.

"Everybody who was in prison back then was violent one way or another," he says.

"If not, mentally they were as even if they hadn't committed a crime there and then, in their minds everybody was angry and violent."

Rehabilitation was not considered as prisons attempted to quell the regular riots and violence which characterised life in Scotland's jails.

Mr Steele says: "There was nobody you could turn to. You couldn't go and chat with governors and say you had a problem."

"They'd just send you to the doctor and the doctor would send you to the psychiatrist.

"People would be scared of that in case they got sent to Carstairs [a psychiatric secure unit]. People suffered in silence. A lot of the staff suffered in silence as well because they were doing their time along with us."

Barlinnie was home to many of the most violent inmates in the country and became known throughout the world after the longest siege in Scottish penal history in 1987 after inmates took guards hostage on the roof of Bar-L.

The five-day riot remarkably ended with no serious injuries but cemented the prison's infamy.

Despite these conditions, supporters of the unit point to successful rehabilitation of some previous violent criminals.

Perhaps the most notable success attributed to the work of the Special Unit was Jimmy Boyle.

He was sentenced in 1967 for the murder of fellow gangland figure William 'Babs' Rooney and was described as Scotland's most violent man before he was sent to the Special Unit.

When there he was introduced to art therapy and found a lifelong passion. His sculptures now sell for thousands of pounds and he credits the Unit with aiding his remarkable turnaround.

In an interview with the Independent in 1999, he explained the impact: "One day they brought in a woman, an art therapist.

"A lot of prison staff were against it, saying we'd rape her. But the fact was we were impressed with her.

"We were combing our hair and saying things like: 'Don't swear, or she'll no come back.' She was quite good-looking as well and we hadn't seen a woman in years.

"One day she brought in 7lb of clay and I just worked on it and did a portrait of one of the guys inside. It was like a dam bursting in me."

Now an exhibition is tracing the history of the unit and assessing its impact.

Mr Boyle's ex-wife Sara Trevelyan is one of those who feels the legacy of the facility is due for reassessment.

"There were a lot of success stories in the Special Unit," she says.

"Guys who were in there, and went back outside and stayed outside so they broke their pattern of the revolving door."

The unit closed without much fanfare in 1994. By then few even realising it still existed, but it played a pioneering role in reintegration of offenders in Scotland and is still seen in the current prison system.