With cotton you use soda ash to soak but i don't know what i'm to use when dying silk.

like wool, you'll need an acid to help the dye stick to the silk. I"m pretty sure dharma has an explanation on their site for using fiber reactive dyes on silk

you can use procion dyes with silk - and you actually can use soda ash, tho most dyers prefer to presoak in white vinegar instead

I've done playsilks in the same dyebath as cottons, and they take the color really well. I've been *surprised* a couple of times by the colors I've gotten, but they've always been vibrant and pretty.