Title says it all.

I inherited a couple of boxes of mounting tissue that are about 30 years old. Is there any reason I should not use them, and will they still work? I tried to mount something and it popped right off. I was using a mickey moused set up so might have done something wrong.

Process:

Heavy sheet of metal heated to 225 in the oven
Piece of card stock under that
Photo
Tissue
Matt
foam core

I let the metal sit on top for a few minutes. When I took it off, the photo popped right off and the mounting tissue peeled right off the photo.