excuse my ignorance in the area of dyes... but how is soda ash hard on silk? does it dissolve it or weaken it or make the finish less glossy....?
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Yes.
Soda ash is basic, just like bleach is, but not as strong. It's necessary for using fiber reactive dyes (it makes the "reaction" happen), but it's not great on the silk. It's just like bleaching your hair: You get the color you want (theoretically), but the strength and texture have been permanently changed. Slightly. It won't be as obvious a change with soda ash as with bleach, but you should know that you are damaging the fiber, though in a small way. (So: when possible, use acid dyes with silk, if you must use fiber reactive, know that the fabric will be slightly more delicate after.)
If it's just sizing, a fiber reactive dye *might* work, if it's from a cellulosic base. I honestly don't know much about how sericin takes dye. Theoretically, it'll be a protein compound like silk is, and would take the acid dye. But if that were the case, you wouldn't be having the problem you're having. It's honestly more likely to be a synthetic of some sort, either polyester or rayon.
The highlights:
Protein fibers (Wool, silk, hair, etc) require acid dyes, are slightly strengthened by acids, and are weakened/destroyed by bases.
Cellulosic fibers (cotton, linen, rayon, hemp) require fiber reactive dyes, are slightly strengthened by bases, and are weakened/destroyed by acids.
Synthetic fibers (Polyester, nylon, acrylic) have all their own rules, are generally resistant to acids/bases, smell like burning plastic when burned, and are difficult to dye. (Exception: Nylon takes acid dyes nicely)
Edited to add: Because you've boiled it for a good long time, it's unlikely to be either sericin or sizing. Either one should have come out by now.
