The lighter the slide, the more trebly the sound. If you are using a guitar with a low action it helps to have a lighter pick though so that you don't end up pushing the strings into the frets and thus eliminating the point of the slide. To get a better sound while playing slide, try using heavy gauge strings, and keep the action as high as you are prepared to have it while taking into account how easy it is to play when playing normally. At a pinch, do the pub thing and use a beer bottle.
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