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I've made about a dozen strings in my "oven", and the first couple came out a tiny bit on the short side. I'd cut the string to at least 24" long (~ 60 cm), and then have trouble getting 50 cm (20") of straight line when I was done. There's a bit on each end that needs cutting off, but only ~ 2" worth.

I measured everything carefully, and it turns out the line is shrinking by about 15%! It fattens up a bit, but by less than 10%. This was cooking it at around 400F. I backed the temperature down to around 350F, and that reduced the problem to the point where I can get the length I need, but it's still shrinking quite a bit.

I suspect they wind it up on a spool under considerable tension when it's made, and that when it anneals, all that internal stress gets relieved.