Rachel, this is a common problem--look over at www.glassattic.com for more
info. Consensus over the years seems to be that there's some sort of oxidation
going on/grease inside the rollers themselves. I tend towards the oxidation
theory. Are you using Sculpey? It has a waterbased plasteciser AND a oil based
one, the others have mostly oil based plasticisers. It would oxidize more....
Sarajane

Sarajane's Polymer Clay Gallery
http://www.polyclay.com