For anything gooey, gummy, sticky like bubblegum, or ossified like old masking tape residue, I always reach first for the citrus solvent. If the problem is minor, I use a few drops on a damp rag. If there is a real MESS, I use the stuff straight. It won't hurt ivory. I've never known it to hurt plastic keytops, but one other kind of plastic is ruined by it. (I tried to take gum from a price label off a ruler once, and it "melted". Wipe it off with a damp rag when you're finished. I would think it would make short work of tar, and it's harmless. You'll smell a lot of oranges for a little while. Susan
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