Carbon will tone silver . as for copper heat and most anything cause it to tone. that why the 2009 P&D mint set were so late the cents were toning before the could get them packaged
Mainly sulfur is responsible for creating thin film interference on coin surfaces: http://www.google.com/#hl=en&source...ql=&oq=thin+film&gs_rfai=&fp=ff16f4b8e1473a19