Quote Originally Posted by mark
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.
Most likely it is your set up. Different tissues need different temperatures. Tissue this old most likely requires a very hot press somewhere in the 250 F or more. Having said that, sometimes the glue becomes brittle and as you say goes bad.