heating bag question..

[kimmercake.livejournal.com]

this is for those of you who have made the rice heating bags.

i know you heat them up in the microwave but my question is:
is there certain types of fabric or ribbon you can't use because they aren't microwave safe?

i'm planning on using a silk on the outside/flannel on the inside fabric. AND ribbon around the top.
but i'm pretty sure the ribbon has gold threading through it.

this isn't liable to catch on fire or anything, is it?
i'm scared and don't wanna make the before i know.
anyone else have a similar experience?

thanks!