Was so frustrated today while preparing a piece for a commissioned order that took me far longer than I imagined, but what a great lesson it was for me as there were 3 different threads all used in the same item that was from the 1960's. It was quilted & seamed on 3 edges of the comforter with 100% cotton thread. On the top edge, it looks like perhaps it may have originally had some kind of opening (I think it might have originally been a duvet cover that someone shoved batting in & quilted) and on the inside folded edge monofilament thread was used. On the outside top, it was hand stitched with what seems to be either a poly or poly-cotton blend based on texture and the fact that the color didn't fade.
The poly/poly-cotton thread held up the best by far. It was in good condition still. The only "problem" I saw was that the rest of the "quilt" had sun faded, but the thread maintained it's original color. The rest of the comforter also had become a bit worn from the wash (she said they did wash and dry it on warm), but not the one thread up at the top.
The cotton thread had worn and faded in a manner consistent with the rest of the comforter. Some of the stitches had snapped & other had started to fray.
The mono thread was the worst of them all. It had separated into 18 different plies and just sat inside the comforter. Who knows whether more of it may have been used originally on the top side of the comforter and fallen away over time?
With all the other issues I have with mono thread, this has definitely convinced me to steer clear of it ... and I'm keeping a bit of the frayed thread in my sewing basket lest anyone suggest I use it for anything other than art quilts.
(My mom is great at giving me all sorts of "helpful" suggestions in spite of the fact that at the height of her sewing days, the most she ever did was mend a couple socks.)