Originally Posted by: caponi14 What Slash seems to be so good at is mixing the minor and major pentatonics.
I think Sebastien has it right... while I know what you mean about mixing scales, from what I've read about Slash and his background, I'm betting that its something he does mainly by instinct... there's points in the solo where notes just "feel right", and he may be using them without concerning himself much with the theory behind it. There's no magic formula out there that says "use this scale in a solo and it will always sound good". A lot of it just comes down to the players gut reaction to the music, drawing from an arsenal of licks cultivated by listening to other artist's music and figuring out the guitar parts. Over many years of doing this, a player may develop an instinct as to what notes will work when & where.
Think of pool players... its all physics and geometry, but you don't see good pool players taking measurements and working out equations on their calculators.. they take the shot, using their instincts which have been built up over the years.
We could look at the opening riff of "Sweet Child O'Mine" and start by breaking down the bass line, and the following chords & drums, and then say "ah, the notes used for the riff are a very logical choice with those parts considered..." But that's not how Slash wrote that part. Apparently, he was just screwing around with that riff during a rehearsal, making silly faces because he thought it sounded kind of funny... but the band latched on to it, and a song was born.
I'll bet Slash knows his scales, but I'll also bet that he "hears" much of what he plays in his head before he plays it. I think some of his playing is more tying together riffs that he has picked up over the many years of playing guitar, and he just has the knack for feeling where those riffs are going to fit in.