Vintage thread color

[claireshaeffer]

Why does thread darken with thread? Does this happen to cotton and/or silk?
I'm looking at my photos of a Worth gown from the 1860s. The dress is blue and the thread is a dark rust where it's hand sewn. The machine stitched thread is still blue.
Many thanks.

 

[dollsntrolls]

I would guess some element of the dye has oxidized. Many older pieces I've had, the cotton or silk thread has deteriorated considerable and had to be replaced completely. I would worry that the discolored areas are much weaker and have been susceptible to mold, mildew, water - something is causing that to happen, and it can't be good for the rest of the stitching.