I've had good luck and bad luck machine-washing silks over the years.
I've read that what causes shrinkage in silk fibers is the agitation, and the extremes of temperatures, i.e., hot or cold. One is supposed to dip silks into lukewarm water with a gentle soap, maybe swish them around a bit and not agitate. Then carefully rinse without twisting or agitating, squeezing the water out carefully by pushing against the sides of the wash tub. I use Dawn dish soap, which is very gentle, or Orvus soap paste, which is used by textile conservators. You can buy at fabric stores.
That said I have machine-washed some silk blouses with no problems with machine settings on lukewarm and handwash. Ones that are a dark dye like black have sometimes bled. Sometimes adding synthropol to the water helps. But not always.
Worst case: I bought a Joan & David silk jacket on ebay that arrived filthy. I washed it on the handwash cycle, letting it soak for a bit. I didn't put it through the spin part of the cycle, but rather carefully squeezed it out by hand.
I had taken the measurements before washing and found that after washing it shrunk about 1" all around. Length of jacket and sleeves were 1.5" shorter. Shoulder span was 1" narrower. Armpit-to-armpit was 1" narrower. I hung it outside to drip dry and tried to stretch it back to its original size as it dried. Absolutely no luck in that regard. When the garment dried it was no larger thanks to all the pulling and stretching, and the lining was hanging out from the bottom and the sleeves.
Worse, the jacket smelled fishy. This happens when water is applied to cheap silk, like the kind you find in TJMaxx jackets. When the silk cocoons were processed all the gum that held the cocoons together was not completely washed out. So when the resulting garment gets wet, it stinks of the gum, it smells heavily like fish oil. This happens when cheap silk garments are dry-cleaned too. The only way to get rid of the fish smell is to wash out the gum completely, and that can take a dozen machine washings. Dry-cleaning won't get rid of the smell. I usually end up tossing garments that smell this badly as I have not had luck getting rid of the smell even through repeated machine washings.
So I ended up with a clean, but shrinken and fish-smelling jacket.
I machine-washed a British tweed wool coat with a rayon/silk lining. I only did this because I sewed it myself and had pre-machine-washed all the fabrics first. However it still shrunk in length and now the lining hangs out from the bottom about an inch.
Lesson learned.
I take all lined wools to the dry-cleaner. When I don't care if something will shrink I machine-wash it. Otherwise all wools and silks get handwashed and drip-dryed.
