The grandfather of rock'n'roll: The devil's instrument

Even now, a full 70 years after it was recorded, the guitar-playing of bluesman Robert Johnson leaps from the speakers. Just listen to the "turn-around" that opens Kindhearted Woman Blues, its delicate finger-picking steady and measured. Or else listen to Crossroad Blues, a song subsequently covered by Eric Clapton and Cream, and hear the bottle-neck slide screech and hiss like summer rain falling on a sweltering Delta highway. Sweet Home Chicago chugs along in a chunky 12-bar blues.

Along with the legend of how he sold his soul to the devil in exchange for his talent, the musical legacy of Robert Johnson has entranced later generations of rock and blues musicians such as the Rolling Stones and Led Zeppelin. Fans and experts alike call him the "grandfather of rock and roll".

Now, a New York-based company that specialises in selling historical artefacts is offering for sale what it says is an extraordinary piece of Johnson memorabilia with a direct link to that music: an acoustic guitar purportedly used by Johnson on some of the 42 recordings he completed before his untimely death at the age of 27. The asking price is a blues-inducing $6m.

As with so much in the life and music of Robert Johnson, there is no small amount of mystery about this six-string guitar. The website of the company selling it, Moments in Time Manuscripts, claims it is "unquestionably" Johnson's instrument, saying "a once-in-a-lifetime opportunity now exists to acquire this legendary instrument owned and used by Robert Johnson to play and record some of the most significant blues music of the 20th century".

However, the company's curator, Gary Zimet, said he was not permitted to provide any additional information about the provenance of the instrument or the person who is selling it. "All I am authorised to tell you is that it was purchased in America, in the south, and that it is currently owned by a fellow living abroad," he said. "The owner is playing his cards very close to his vest." Asked as to how someone could be sure the guitar was genuine, he added: "Anyone potential purchaser is welcome to examine it in person and have his or her own expert go over it with a toothcomb."

News that one of Johnson's guitars may not only exist so many years after his death but is being offered for sale has shocked many fans and experts. It has even shocked Johnson's own family. The musician's grandson, Steven Johnson, vice president of the recently opened Robert Johnson Foundation in Mississippi, said he knew nothing about the instrument and had no idea it was being offered for sale."This is the first I have heard of it," said Mr Johnson. "[The foundation] has been trying and trying to find anything. If this is legitimate I need to get all the information as to what makes it legitimate and who had it and how was it traced."

The Johnson foundation is located in Crystal Springs, just ten miles from the town of Hazlehurst where the musician was born in 1911. During his short life he composed just 29 songs, captured on 42 separate tracks that were recorded during two sessions in Texas - one in a Dallas warehouse and the other in a room at the Gunter Hotel in San Antonio. He died in 1938, apparently succumbing to pneumonia after drinking whiskey that had been spiked with strychnine by the jealous husband of an illicit lover.

For many years there was even a mystery about where Johnson was buried and Sony records arranged for a marker to be placed in what is probably the incorrect graveyard. It is now generally accepted that his remains lie in the grounds of the Little Zion Missionary Baptist Church, two miles north of Greenwood. Another marker was placed at that location in 2002.

Such posthumous attention to Johnson is fitting. While he was alive he was just one of many black musicians making their living performing in the Mississippi Delta, that wedge of land located between the towns of Clarksdale, Greenville and Greenwood and bordered by the silt-laden waters of the Yazoo and Mississippi rivers.

Some historians say he was no more than a pop-singer, who made his money not from performing his own blues compositions but singing whatever was popular at the time. It was only after his death, they argue, that Johnson's small catalogue of work was seized on by a new generation of musicians and fans who elevated his role in the development of blues to its present heights. In particular they point to the influence of the 1961 release by Columbia Records of 16 Johnson recordings under the title King of the Delta Blues Singers.

"The blues was pop music, it simply wasn't folk music. It was reinvented retroactively as black folk music, which brought a new set of standards to bear on it and created a whole new pantheon of heroes," says Elijah Wald, author of Escaping the Delta: Robert Johnson and the Invention of the Blues. "Blues musicians such as Johnson were not moaning field labourers, they were Sam Cooke, they were Snoop Dogg, they were Aretha Franklin. That's what we've forgotten and that's what a lot of white blues fans don't want them to be."

Correctly or not, Johnson is now considered one of the most seminal blues figures. Clapton, who in 2004 recorded an entire album of Johnson tracks and restaged one of only two known photographs of the musician, described him simply as the "the most important blues musician who ever lived".

The noted US music critic Greil Marcus wrote of Johnson in his 1975 book, Mystery Train: "Johnson's vision was of a world without salvation, redemption or rest ... Johnson's music is so strong that in certain moods it can make you feel that he is giving you more than you could have bargained for, that there is a place for you in those lines of his."

For some fans, much of the allure is doubtless connected to Johnson's fast-living, hard-drinking and womanising lifestyle. One of the most famous stories of his life concerns how he obtained his playing skills by walking out to the junction of US Highways 49 and 61 in Clarksdale and selling his soul to the devil in a Faustian pact.

The mundane truth, as authors Barry Lee Pearson and Bill McCulloch discovered when they examined all the available newspaper clippings about Johnson dating back to 1937, is that the story about this deal with the devil first appeared in 1966, in an interview given by fellow bluesman Son House. Indeed, the legend was first told about Johnson's namesake, Tommy Johnson, another contemporary blues musician who may or may not have been a distant cousin. (Nor is the story limited to blues musicians: the 19th-century violinist and composer Paganini was also rumoured to have obtained his virtuoso technique by way of a Satanic deal.)

Tommy Johnson's brother, Le Dell, once told University of Memphis ethnomusicologist David Evans, that he had once asked Tommy how he had learned to play. "He said, 'If you want to learn how to play anything you want to play and how to make songs yourself, take your guitar and go to where the road crosses that way, where the crossroad is ... Be sure to get there a little before 12 that night so you'll know you'll be there ... A big black man will walk up and take your guitar and he'll tune it. And then he'll play a piece and hand it back to you. That's how I learned to play anything I want'."

But many of the musicians who became fans of Johnson were simply entranced by his playing, fascinated by how he managed to play both the chords and the riffs at the same time. Keith Richards of the Rolling Stones said that when he was first introduced to Johnson's solo recordings by bandmate Brian Jones, he asked about the "other guy playing with him". "I was hearing two guitars, and it took a long time to actually realise he was doing it all by himself," said Richards.

Others have commented on Johnson's unique rhythm, pointing out that the strumming on tracks such as Preaching Blues gives the impression of being on a freight car - a suitable inspiration for a man who travelled from town to town by hitching a ride or else stowing away on a freight train.

The blues and roots musician Ry Cooder, another Johnson fan, has speculated that the musician obtained his comparatively clear results during those primitive recording session by facing into the corner of the room and placing the microphone between himself and the wall to get a better acoustic.

It appears Johnson played more than one guitar in his lifetime. Johnny Shines, another blues contemporary, said Johnson often played a Kalamazoo, a budget model produced by the famed guitar manufacturer Gibson. He also played a Stella guitar, which reportedly sold for around $12 at the time.

The guitar being offered for sale by Mr Vamit's company is a Gibson L-1, an acoustic model made by the Nashville-based company between 1926 and 1930. It is this model that Johnson is holding in one of the two photographs that exist of him, a staged "studio portrait" in which he is wearing a suit, posing with the L-1 and apparently playing an A seventh chord.

The company says that by comparing their guitar with the one in the picture it is clear that both guitars have the hole rings and binding, the same unbound neck and the same "dot" markers on the instruments' third, fifth, seventh, ninth, twelfth and fifteenth frets. They describe the instrument, which comes with its own plywood case, as "well played and marked but structurally perfect". It has "deeply routed wear on the finger board from a great deal of use".

If this is Johnson's guitar there are clearly a barrage of questions that remain to be answered. Where, for instance, has it been all these years? Who is the "fellow living abroad" and how and when did he buy this instrument somewhere in the Deep South? And if he obtained this guitar, did he also obtain any other Johnson artefacts? Until these questions are answered, this instrument is destined to remain the latest mystery in the life and death of Robert Johnson.