From time to time I receive questions from knitters and crocheters about gauging and yarn usage.
She had been knitting the cowl out of new (unknit) yarn but ran out. So she went back to her stash and found an a hat she had not completed knit from the same yarn. So unraveled the hat.
Her question : Should she wash the unraveled yarn before knitting with it.
My answer: Most definitely.
Without re-skeining and washing the just unraveled yarn, she risks knitting at a different gauge. Additionally, her knitting could “list” or be “off kilter.” Washing and letting the yarn soak will relax the fibers and let them return to how they would normally lay in the yarn (given its twist and the fiber content).
I also recommended that if, after soaking the yarn, it is still kinky, hang the skein on a dowel (or a hook) and attach a little weight to the bottom of the skein. (Spinners commonly do this to set the twist.)
I keep 2 or 3 inch long “S hooks” (pictured to the right) in a kitchen drawer just for this purpose: One part of the S hook hooks onto the bottom of the skein, and then I hang a weight on the bottom part of the S hook.
How much weight you add depends on the kinkiness of the yarn: attach just enough weight to pull out the kinks.
Let hang until dry, then wind the yarn into a ball and start using!