Hi Joan,
I can't speak to your strung piece since I have not seen it, but I can tell you that I have not had any discoloring of undyed silk threads when using a freshly-mixed solution of The Pearl Doctor in a clean container to clean my newer pearl jewelry.
It's important to use the dropper provided, and to follow the product directions for mixing and using The Pearl Doctor -- you do want to be sure you have the solution mixed in the correct proportions. I find that lukewarm or cool (not cold) water works best (and probably is safest for your jewelry).
Also, I would not save the used solution to use on another day -- it's important to start fresh with a clean solution and container for each piece you clean. I thoroughly rinse the container and dropper in lukewarm water after each use (and each piece).
In addition, I have found that The Pearl Doctor works beautifully and safely to clean and brighten pearls, pearls with diamonds, pearls with cubic zirconia, pearls with sterling, gold, or platinum, and untreated mother-of-pearl shell strung on white stretch thread.
If your piece is older, heavily-soiled, or has silk string that is showing signs of wear, you may wish to consult in person with a jeweler who has extensive knowledge of all kinds of pearl jewelry, and long experience cleaning them.
You also can send email to the folks at The Pearl Doctor, as they have tested the product extensively.
Incidentally, I just passed on to my niece an antique, heirloom piece (created circa 1900) that is set with several natural pearls. The estate jeweler who examined the item with me told me not to clean this kind of piece with any cleanser, or allow any jeweler to clean it, as it is just too delicate.
I hope this helps.
Best wishes,
Johanna
Johanna Lolax, BA, MA
GIA Pearls Diplomate
www.tenderbeautypearls.com