So in various sections of a piece I want the pianist to lift the pedal slower than usual, achieving an interesting effects as the harmonics fade. In some areas the pedal lift might be slower or faster, and so it would seem that just writing 'lift slowly' won't be effective. For this reason I'm wandering if there might be a graphic solution for notating pedal lifting, such as using hairpins?
The effect you get by lifting the pedal slowly is similar to what you get when playing flagolettes, or harmonics on string instruments. You could adapt the standard simbol for that, and indicate with a line the duration of it.
As far as I know there isn't a standard notation for what you want.
I'd probably use a half-pedal marking, but make the rise or dip in the pedal line wider - and be sure to include a performance instruction explaining the marking.