This might be silly, or it might be super interesting. I don't know.
I found some old cotton fabric in my basement and thought I'd start a quilting project. The cloth smelled mildewy, like cloth kept in damp basements does.
I soaked it in a vinegar solution (about 150mL white vinegar 5% in 40 L tap water, in the washing machine tub (metal), for about 2 hours. When I opened the lid, I was hit with a sulfur-ish odor. Also a little petroleum-ish.
I've aired the fabric for 24 hours, and it still has that smell. Is it just my nose unable to combine two odors? Or did a reaction take place? I treat stinky clothes with vinegar all the time, and this is a first.
No mothballs were involved in the storage of the cloth.
Obviously this is not urgent. I'm just curious! Thank you!
As you noted, other uses of vinegar with cloth do not have that effect. Mildew itself would not cause a different smell. The vinegar is just killing the mold with the high acidity. Regular soap washing after treatment should remove any residual dead mildew. It IS possible that because it was just an old found piece of fabric that you can't know what it might have been used for previously. It may have been used as a wipe rag for just about anything, including solvents or things that might give you that petroleum and/or sulfur odor.
In terms of time and materials cost, it really will be much safer and quicker to simply discard that piece permanently and move on to another one.