Okay so I play {Blacker Lotus}. Is it within the rules to only rip off a small corner, or do I have to rip it all up?
Hahaha, you actually played a {blacker lotus}? You're my hero. Anyway, no matter how you ripped it, I don't believe you'd be able to use it again, because you'd know which card it is in your library, due to the ripping, provided it's not in protectors. I suppose you could still rip off a small piece, because technically speaking, it is in pieces, just one really big and one really small piece.
Any rules questions for silver-bordered cards should be directed to Mark Rosewater.
But yeah, you'd need at least two definable pieces, and you can't continue to play cards that can be identified without sleeves.