Here's my experience with a box of Kodak Wratten Pan plates. The packaging isn't air-tight like a modern film. So most of the plate was oxidized, and thus not sensitive to light. Only an area about 2x3 inches in the middle of the 4x5 plate was still sensitive. That's because the plates are packed in pairs facing each other, so oxygen didn't get to the middle.

So compose assuming that only the middle is alive.