HEARING Jack Johnson's voice at a distance of almost a hundred years can be unnerving. Imagine how it must have sounded at the time. A threat, a menace, an irresistible iconoclast, he would become the first in a line of totemic heavyweight boxing champions - Louis, Liston, Ali, Tyson - whose lives somehow contain a shadow history of the century.

While Johnson's voice, thrumming with self-possession and charm, rings across decades, the voices that opposed it, in particular the media, are shocking in an entirely different way. As Geoffrey Ward notes in his introduction: "Even in ostensibly objective news stories, Johnson is called the 'dinge', the 'coon', the 'stove', the 'Texas Darky', the 'Big Smoke', the 'Ethiopian', the 'Senegambian' and - more often than one can credit - simply 'the nigger'."

It is remarkable, then, that Johnson resolved to live his life "as if colour didn't exist". He was born in Galveston, Texas, in 1878, a "pure-blooded American". Both of his parents, Henry and Tiny, had been slaves. By geography and by spirit, he was essentially an immigrant, an outsider, and by that definition he was truly American, too.

Jack attacked life with vigour. His background and his colour were simply shrugged off. He could read and write, and he could talk his way into anything. He was six feet tall when men of that height were rare, and almost 200lbs without the benefit of regular food. By the time he'd had his most famous fight, with Jim Jeffries in Reno on July 4, 1910, he was one of the most photographed and written-about men in America, a figure who polarised opinion, usually against him.

Johnson invented much of boxing's schtick, too. He'd show up in hotel lobbies and restaurants frequented by opponents and announce his availability to fight them; he'd pick the round for his knock-outs (sometimes with the built-in advantage of knowing the fight was fixed); he'd lose his purses within hours of winning them by gambling on dice; he'd buy more cars than he could drive, more clothes than he could wear, marry more women than he could afford and he'd do it all in full view of America, in newspapers, on fight-films and in public.

He understood the value of hype before anyone even had a name for it. "Who do you think you are?" was the question he heard most often, and the answer was always the same: "I'm Jack Johnson."

THE FORCE OF Johnson's will was epic, sometimes beyond his control. It defined his fighting career, and it defined his sex life, too. It was that which would bring him down. Boxing was America's foremost sport, and sport was the coming cultural force. When he began, Johnson could only fight black fighters - the so-called colour line remained unbreached by successive champions, John L Sullivan and Jim Jeffries. When Jeffries retired and bestowed his title on the Australian Tommy Burns, Johnson pursued him to Sydney where he finally took the belt on Boxing Day, 1908.

Once he had beaten a white man, Johnson's preference for white women became intolerable. Miscegenation was dangerous and also illegal. Johnson couldn't have cared less. The search for a 'great white hope' to put Jack in his place became feverish, and ended only when Jim Jeffries was persuaded to end his retirement to reclaim white honour.

That fight in Reno was the most heavily publicised in the history of American sport, and it carried its richest prize - the fighters split $110,000 60-40 in favour of the winner. Johnson won easily, and his real fight began. In boxing, his blackness had been surmountable. In life, it was unforgivable.

Etta Duryea, his first white wife (Johnson might have been briefly married to a black woman while still in Galveston), blew her brains out in a nightclub owned by the fighter on September 14, 1912. Less than three months later, he was married again, to a white prostitute called Lucille Cameron. The public was outraged: Johnson had made no secret of his affair with Cameron before Duryea's suicide, and their marriage meant that Cameron would be unable to testify against him in a prosecution under the new Mann Act.

Officials found another on-off girlfriend of Johnson's, Belle Schreiber, and in 1913 he was sentenced to a year and a day in jail for breaching the Act, which prevented the "transportation" of women over state lines for "immoral" purposes. Jack skipped bail and went to Europe, where his power and skill as a boxer finally began to fade. He lost the championship to Jess Willard in Havana in 1915 and no black man would contest it for another 22 years, until Joe Louis won it in 1937.

Johnson returned to America in 1920 to serve his sentence, and once he had done so, he slipped into the afterlife of the famous sportsman, fighting exhibitions, telling his story in dime museums and taking bit-part roles in the movies. He also did the less expected - taking out a patent on a kind of wrench, for example - and more of what he always did: escorting white women, another of whom, Irene Pineau, he married.

The section of the book that deals with Johnson's post-boxing life is a little long, but until then there's a delicious detail on almost every page. When Johnson engages with Jack Jeffries, brother of the reigning champ Jim, his cornerman Frank Carillo brandishes a gun in his face, in part enraged that Johnson has entered the ring wearing hot pink longjohns.

Research this powerful gives Unforgivable Blackness a richness that rewards contemplative reading; reading that's sometimes at odds with the speed of the narrative (imposed by the breakneck pace of Johnson's life). It is a technical conundrum that Ward cannot always solve, yet his bountiful footnotes are nearly always worth the effort. Johnson's voice sounds clearly, especially from the prison memoir that Ward has brought into print for the first time.

Jack died in 1946, aged 68, after crashing one of his beloved cars, a Lincoln Zephyr, into a tree, thus becoming one of the first high-profile victims of the road, too. It was a fittingly extravagant way to go. He was buried next to Etta Duryea, and at the funeral Irene Pineau was asked what she'd loved about Jack. She replied: "I loved him because of his courage. He faced the world unafraid. There wasn't anything or anybody that he feared."

While we wait for the next galvanising heavyweight champion to lift the division back above freak-show status to its rightful place, Ward has performed a great service by resurrecting Jack Johnson in all his unyielding greatness.

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