Hi, when I got my dorff off of ebay last year the guy gave me 20 sheets of velvia that he had no use for. It expired a few years ago and has not been refrigerated. I was thinking of doing something experimental with it. Maybe processing it in c-41, or b&w chemistry. I've never really dabbled in experimentation of slide film. What would you do with 20 sheets of old expired slide film? I hate to just throw it away...
I had a roll of it developed in C41 by mistake by a local shop. The results were pretty wild. I'd try a few sheets if it were mine.
Test it. It might still be usable as slide film. Slow films hold up pretty well.
I bought 50 sheets of Astia with an expiry date of 1999 from Freestyle, which they said was cold stored. the only thing "wrong" with the film was that it needed a warming filter (like an 81B) to make the film have a normal color balance. I think David is right, I'd give it a try as a regular transparancy, perhaps rating it at 32-40 and using a warming filter. An 8x10 on a lightbox is awesome!
BTW, I once developed a sheet of transparency film in pyro by mistake. the pryo and fix seemed to have no effect on film, when all was said and done, it looked unexposed. C-41 cross processing would be a better bet.