Actually, silk IS washable, but only if you wash and dry it several times BEFORE you sew.
Silk begins its life as a cocoon. The process of unwinding the L-O-N-G silk thread from the cocoon begins by dropping the cocoon in boiling water, then spinning it (in China they do this with an UNPROTECTED hand) until a thread comes loose. The thread is attached to and wound on a spool. Eventually, it is woven into cloth. The cloth may be left in its natural state (raw silk) or dyed and printed.
After it is woven, it WILL shrink, and it WILL water spot. If you want to use it in a quilt, prewash and dry it several times so it shrinks. I do this in a washer and dryer, because I give quilts away, and I want to be sure the giftee won't be able to ruin it. It may shrink a little or a lot, depending on the weaving process. Soaking the entire piece of fabric all at once prevents spotting. Spritzing would make water spots - which could be an interesting texture, if that's what you want. In either case, be sure to serge or pink the non-selvedge edges, or you'll have a LOT of raveling.
Cut template pieces from lightweight fusible, fuse them to the wrong side of the silk, then cut the silk. This solves the ravel problem while you are piecing.
Good luck. I'm sure the quilt will be gorgeous.
JoanneS