Refer to this forum (collectors universe) Theres actually some good points to read and some good questions asked there.
http://forums.collectors.com/message...hreadid=802186
Quote:
Rough numbers:
cardboard will start to burn around 250 F, amd I'm betting it shows heat damage (singed) even lower.
plastic will melt somewhere between 300 and 450 depending on the type.
Kiinda different, but heres a SLABBED COIN that got melted:
http://www.cointalk.com/t209641/