Washing a cotton sweater on 40 degrees.

It you use hotter water to shrink it, I think it will just go hard

Hand wash it, then rinse and spin it on low spin speed.

Always best to hand wash jumpers.

I like the jumper.

I would wash it at 40 deg but don’t blame me if goes wrong ! I buy to shrink I think it’s the 46 yrs of nursing drummed me into it . Good luck . I noticed my granddaughter was bought a sailor suit style dress in cotton from M and S recently from a charity shop and recommended 50 deg wash - must have been at least 30 yrs old I would think and yes it washed fine . . 🍀

Witzend I gathered that she wants it to shrink a bit as it's too big.

Greciangirl Why? I never hand wash anything unless it says you have to. My washing machine has a hand wash and delicate setting. I use laundry bags to protect clothes if I'm unsure if they'll go out of shape.

Do you think it has probably been washed by previous owner? If so doubt it will shrink at 40°. Do you have gentle wash on your machine, if you do, try that at 40° Good Luck.

Cotton will tighten up at 40, but will iron back out if you need it to. But it will need shaping out when wet.

I wash mine at 60 with the sheets, but it is all cotton, and the thread in the seams is too. That you have to check.

Pretty much everything except wool will wash at 40 - we used to until it became fashionable to wash lower.

If I were you I would look on Vinted to see if you can find one in a smaller size, and donate it back to the charity shop.