Lisa Cain of Mid Cornwall School of Jewellery, has just posted this excellent video on You Tube on how to reconstitute metal clay. Basically you save all your projects that haven't worked in a jar. Then grind them to a fine dust in a coffee grinder. I always have used a mortar and pestle, but a coffeee grinder would be so much easier. Then put the powder in container and add a little water, mix till the powder and water begin to clogg together. Then put the lump of metal clay in some cling wrap, roll very thin and add a spray of water if necessary.
Last year I visited Lisa at her Jewellery School in Cornwall UK. I took her Diploma Course. Lisa is a really good tutor and has a great sense of humour.