I have been experimenting with pretzels on and off, and finally broke down and bought some lye to get a beautiful deep brown crust on them. Last weekend I baked the pretzels on my silpat-lined sheets, then put the sheets away without bothering to wash them (I am exceptionally lazy). Last night I washed one of the silpats off, and what do you know, I have pretzel-shaped clean spots! Turns out the lye cleaned the greasy feeling (that I've been unable to remove otherwise) form the mats. I'm still terrified of lye, so I may experiment with drain cleaner first, but I think that this is the way to get rid of the greasiness that silpats develop over time!!!