Now that I'm studying tailoring, I've noticed things I hadn't even looked at before, like how some stitches seem to squeeze the fabric, particularly on knits. It got me thinking, and that lead me to a question that sounds a little stupid; do thread manufacturers shrink the thread before shipping it?
So, follow me on this; you pre-shrink fabric because you know it will shrink when it's washed but, if the thread isn't pre-shrunk, wouldn't it shrink and cause the thread tension to tighten? That said, how the hell can someone pre-shrink thread? 🤨🤔
Yarns are held in tension on a loom and they remain in that state once the cloth is woven (almost like a knotting effect). Most of the shrinkage in cloth results from this tension relaxing upon contact with water/steam.
However, shrinkage can also occur in individual yarns. The weight of water/dye/bleach can stretch yarn during its processing. If dried quickly, that stretch is 'set' and only when wetted again does the thread revert to its natural state.
Thread, particularly if cheap, can shrink, but quality stuff tends to be mercerised, which is less prone to this problem. Especially if manufactured by a brand name.