I have two 2 knitted cardigans. One is 100% silk, the other is half silk, half cashemere. I have washed them with regular washing powder (persil non-bio) and now cannot wash out the soap residue. Both garments are dark so the white residue stains really show. I have tried washing them again with silk and wool soap, rinsed them endlessly but every time I think the residue is gone, it reappears when the cardigans dry :( please help!