Buckle is a condition in which it is impossible for the film to lie flat, due to some part having shrunk more than another. This usually shows as the film forming an ‘arch‘ if the edges have shrunk more than the middle, or as ‘edge wave‘ if the middle has shrunk more than the edges.
If a film is placed in very dry conditions the tendency for the film would be to shrink. The dry conditions will affect the outside of the reel (i.e. the perforation area) before it will affect the middle of the frame, the resultant effect being a sort of ‘[legacy-smartlink:arching]’ or ‘cupping’ occurs. This is due to the fact that the edge of the film becomes shorter than the middle. If you again take your piece of flat film and cause the middle to shrink more than the perforated margin, then you have the condition where the middle is short and there is a greater length on the edges, thus causing the edge to become ‘wavy’. Edgewave can be caused by projector heat (concentrated on the picture area) and thus shrinking this part more than the margins.
It can also be caused by some treatments designed to reduce scratches on the base surface. Such treatments affect only the picture area and cause this to shrink, but not the margins. (See: base polishing/coating ). If a film is buckled or contains edge wave, it can create quite serious problems when it comes to printing. This is because the film is not in perfect contact with the dupe stock. You cannot make a surface that is curved in both directions touch all over a flat surface, but, because film is a plastic medium, if you apply enough force over a limited area such as a single frame, you can stretch some parts and compress others sufficiently to get the two surfaces into reasonable contact.
Brown, H., 1985, Basic Film Handling, in FIAF Technical Manual First Edition, FIAF Preservation Commission, Federation Internationale des Archives du Film, Brussels, Belgium