The word "Silk" refers to the excrement that comes from a "Silk" worm. The thread itself is not usually the problem, except for the laperdopus which isn't used for cloth. It is the "other stuff" that is a problem.
The "Silk" is spun, dyed, and woven into many different clothes.
If in doubt, go to a good mom & pop drugstore (we're boycotting the BBs) and get a bottle of distilled water, and litmus paper.
Place the cloth on a piece of rag mat board and drop a few drops of the distilled water onto the cloth untill you make a wet-spot (no giggling please) about the size of a quarter. Let stand about 15-20 minutes then with CLEAN tweezers, lay a litmus strip on the wet-spot and gently jam it into the spot untill wet/damp. If it changes color, compare to the chart that came with the litmus and you will know for sure about THAT piece of cloth.
Personally, I have yet to find a piece of silk that was more than 1 point off center.
For my age, that is pretty darn neutral.
As Rebecca noted, the silk, itself is not very
acidic, but the things that added to it to make
it shine and hang can represent a problem. Keeping the silk away from the art with
a layer of mat board is a good idea, as it is
with most fabrics.