I just found out something really interesting last weekend. If you have a costume piece that is silk and you need to wash it, use horse shampoo. Yes thats right horse shampoo.
I had lent one of my mishiyuki (kimono jacket) to a friend and she got something all over it so Storm and I had to get it out and Storm brought out mane and tail horse shampoo. It worked really well. Just make sure that when you wash something thats silk you do it in a flat space, like a bathtub, with just enough water to submerge the garment. Gently work the shampoo into the areas that need it the most, then let the water drain and rinse the garment thouroughly to get all the soap out. Let it dry in the tub for a while then wrap it up in one or more towels to get most of the excess water off. Then place it somewhere flat where it can dry.