The necessity of cleaning away the duff (any part of the plant that is *not* the seed) is that: 1) it will rot in the dampness of seed-sowing medium and may promote "damp-off", a fungus that kills seedlings; 2) the duff may hold moisture in whatever container you store the seed and cause mildew or mold which will kill the seed; and 3) cleaning is a courtesy and will help you know if you really have ripe seed. Some flower heads will look like they are full of seeds, but after cleaning there will only be two or three ripe seeds. You need to know how many seeds you have to be a fair trader.
It's a good idea to limit the amount of duff as you collect seed heads and pods; snip away leaves, cut off excess branches, etc. before you put flower stalks or pods into a paper bag to dry. In many instances, you can simply shake the seeds loose inside the bag when they've dried sufficiently, then remove the stalks or emptied pods. With more stubborn pods or those which are more easily opened and emptied while still leathery, use your fingers or scissors to roll, crush, or pry the pods open to release the seeds. Once the seeds have been separated from the stalks or pods, they can be cleaned in a shallow bowl; one with a light-colored, slippery surface makes seeing and moving the seeds around easier. Use tweezers, a knife or straight edge, your breath, and/or a wire sifter to separate seeds from duff. Commercially-made sifters of increasingly small sizes of mesh are available for this purpose, but probably unnecessary for cleaning small amounts of seed. With larger seeds, you can simply try tossing the seeds and duff up from the bowl, and allow the seeds to drop back while gently blowing the duff away with your breath.
After the seeds are cleaned of all debris, you may notice that some of them are still a little green. If so, it is a good idea to let them ripen and dry for a few more days before packaging them.
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