You could put some hot water in a bucket, one teaspoon of sunlight liquid or 1 in 10 mix of clean green, and about 100 pieces. Shake it up for a while, then rinse and lay them mouth down on a cloth in the sun. When the outside is dry, turn them base down, and let the inside dry out.
That should get the loose stuff off and out the cases.
You'll find that it's a royal P.I.T.A , so it's worth investing in a tumbler. They come up often in the classifieds for around R1,000. Sometimes you'll get the media with it as well.
Get one, you really will need it. And you'll need a media separator.