high temperature processes
Certainly without a separator the glass will stick to the shelf. My advice is that you should not use your friend’s shelf. There already is concern about future use, so keep her as a friend and spend a little more money or use your own.
You can get ceramic pot bases, paint them thoroughly with kiln wash and drip into them. This keeps the glass away from the shelf and leaves you with a round object, sometimes thicker than wanted. But look at the tools and book sections on this site. Paul has just published a book on pot and mesh melts. It would be worth looking at it before committing youself.