I don't think fraying would be too much of a problem, the blocks, at least most of them, are already set in strips. I can do on a delicate wash cycle also. I'm more concerned about bleaching them. Would love to get the rust out, but not sure if that is possible. If I can't remove rust, I still have about 8 blocks that are in pretty good shape, and about 17 inch square. Should be able to do something????? with them.