How to age wallpaper?

[murdercitygrrrl.livejournal.com]

My mom has some vintage wallpaper that she wants to make into coasters.
The wallpaper is textured and off-white/cream.
She wants some to look aged and some to look like tin.
I've looked in the memories and done a google search, but I'm not coming up with much.
Other than using black or brown shoe polish to brush the very tops of the texture, what are your suggestions for the "aged" coasters? She thought about using tea to color them, but she's afraid that will activate the glue on the back and warp the paper before she's ready to secure it to her base. I was thinking that we could put a few drops of paint in some mod podge and do the slightly darker base color and the securing in one fell swoop. Afterwards, we'd dry brush some darker colors on and mod podge again. Does that sound right?
Any suggestions on techniques for the "tin"?
I've seen a similar post in craftgrrl, but I can't find it anywhere.