Here is the problem: if you have a rock group and an orchestra you have several options: you can integrate the group with the orchestra, treat them as just additional members of the orchestra; you can integrate the orchestra with the group, making them an extension of the group; you can try to create some sort of hybrid entity, melding the two together, or you can simple juxtapose them in opposition. I suspect that Jon Lord couldn't quite decide so he did all of the above.
Think about what was going on at this time: popular musicians like the Beatles had just succeeded in becoming such a huge commercial success that their economic power was starting to overwhelm that of classical music. We are at the end of this road that started in the sixties and now classical music sales amount to something like 2% or 3% of popular music sales. But in the sixties, the disparity was not so great. One account of the classical music business points out that some of the first big-selling artists were singers and conductors:
Tenor Enrico Caruso was the first recording star. His 1904 performance from the opera I Pagliacci became the first record to sell one million copies; and several other artists had top ten hits in the years between 1900 and 1920. Superstar conductors like Arturo Toscanini, Eugene Ormandy, and Leopold Stokowski were successful enough to become household names. Although accurate sales figures are hard to come by, Ormandy and Toscanini are reported to have sold more than 20 million records each over the course of their careers.But by the late sixties, it was the rock musicians, especially the "British Invasion" of the Beatles, the Rolling Stones and, yes, Deep Purple, that were the new powerhouses in music. The Beatles casually hired an entire orchestra just to create that wash of sound in "A Day in the Life". They could afford it. But classical music still had more prestige. That may have been part of the idea Jon Lord had: what if we put a rock band on stage with an orchestra? How would that work? Could they stand up as musical equals or, even better, the soloists, with the orchestra accompanying? The only problem was, they needed something to play. This was what brought forth the very ambitious Concerto for Group and Orchestra. Let me very quickly say that this was a very good thing to do. I had a similar idea myself, but not nearly enough savvy or willpower to actually make it happen! All that being said, let's listen to the whole of that first performance in 1969. Nothing like this had ever been tried before.
If you compare this to all the other concertos for rock group and orchestra... wait, there were no other concertos for rock group and orchestra. So what do we do? The piece seems to work in the sense that everyone had a good time, especially the audience. The musicians seemed happy. But while the third movement seems to work pretty well, you can't say the same about the first two. The first movement can't quite seem to decide where it is going and the guitar solo seems both too long and too detached from the rest of the movement. The second movement just seems amorphous with undistinguished themes. The entrance of the singer doesn't seem to fit and at the end, we are wondering what that was all about. But the third movement is better. It has a rhythmic urgency throughout and seems more unified than the other movements. Perhaps by this point Jon Lord was getting the hang of writing for this odd combination.
There is a famous concerto, not for rock group and orchestra, but for a trio of violin, cello and piano with orchestra. We might compare that and see what it tells us. Here is the Triple Concerto by Beethoven. Itzak Perlman plays violin, Yo-Yo Ma, cello and Daniel Barenboim wears two hats, playing piano and conducting. This concerto goes up to about the 36 minute mark in this clip.
Of course, Beethoven did not have to wrestle with the problems of how to meld totally different playing styles and musical genres together, but still. Actually, now that I think of it, the second movement of his 4th Piano Concerto is a wonderful example of how to contrast two totally different things:
But back to the Jon Lord piece: apart from the compositional problems of how to work with the two different ensembles, how to structure the movements harmonically and melodically and so on--apart from this, there is one big problem: the piece is fifty minutes long, but it only has enough ideas for maybe a ten or twenty minute piece. He wanted or needed something bigger and more impressive than that, so he wrote it. But at the end of the day, not much more than an interesting experiment...