If you've ever lost a favorite wool garment to an accidental hot wash, you know that hot or even warm water may cause wool to shrink. The sheer weight of cool water on wool fibers, however, may be enough to cause the wool to stretch -- which is useful if you've purchased a wool cap that's just a bit too small and want to make it fit. There are limits to how much you can stretch any cap without deforming the stitches or the fabric itself, but this procedure is worth at least one try before you give up on that new cap.
Things You'll Need
Sink or large bowl
Round drying form
Fill a sink or large bowl with cool water. Dunk the wool cap under the water until the cap is thoroughly saturated. Don't twist, wring or rub the cab; you might accidentally felt the fibers together and actually shrink it.
Remove the cap from the cool water and roll it in a dry towel to remove excess moisture. You can let the cap sit in the towel for a minute or two or unroll it immediately, but don't leave it for more than a minute or two.
Retrieve the wool cap from the towel and carefully place it on something clean and round -- a melon, a soccer ball, even a balloon will do -- and about the size you'd like the cap to be. The wool will stretch relatively easily at this point because the fibers are wet. If you don't mind the smell of wet wool, you can even place the hat on your head so that it stretches to exactly the right size.
Leave the wool cap in place, in a well-ventilated area out of direct sunlight, until it's completely dry. The cap will "shrink back" to fit whatever you placed it on.
This procedure is most effective on knitted or crocheted wool caps, as they tend to have more give than woven fabrics.