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Depends on what you mean by 'woollen' / what kind of wool it is / how it has been treated..
..some stuff does not shrink at all (like icebreaker merino shirts for example, other things shrink to baby-size even at 30°C).
I think washing it to shrink 'half a size' is not going to work (or work too well), I'd rather try putting it in a dryer. -
the problem is different aspects of the garment are likely to shrink differently, so sleeves are likely to lose more length than width etc so the garment may not retain proportions post shrinking
also the agitation in the washing machine contributes to shrinkage, so you may find a cooler longer wash or more vigorous spin cycle could have a significant effect
At what wash temperature would a woollen garment start to shrink?
Have a jumper or two that I'd like to drop half a size.