Human bodies contain and give off sulphur, which is harmful to covers, documents, and photographs.
They will not go away and as time moves on may etch their way deep into the gum. For these, and other reasons, philatelists commonly house philatelic materials in one type or another mount to protect their philatelic materials. With clear mounting corners used on a cover on an exhibition or album page, no part of the item is obscure. The use of corner mounts is very pleasing esthetically, but care must still be taken. Corner mounts for covers do provide a completely reversible mounting system.