http://www.newadvent.org/cathen/04721a.htm
It's mentioned in the first paragraph.
You think this guy was tough? Saint Lawrence got grilled alive, and he taunted his torturers by saying, "Turn me over, I think I'm done on this side."
Saint Bartholomew got skinned alive, and usually displayed holding his own skin. Talk about gruesome.
Its shown on the Sistine Chapel too.
They say that Michaelangelo painted his own face on the flayed skin.
Go here to see it:
http://www.christusrex.org/www1/sistine/40-12-Skin.jpg (the image was too big to post.)