Wool is a resilient and flexible fibre. The Purdue University Sheep Extension cites the strength of a single strand of wool as equal, if not greater, than steel of the same diameter. It is an extremely elastic fibre, which means that a wool hat will shape itself perfectly to your head. Wool does not soil as easily as other fabrics so it doesn't require washing nearly as frequently as cotton. If you need to wash your wool hats, avoid hot water. You can wash them by hand or in the washing machine.
Fill the sink with cool or lukewarm water to wash hats by hand. Never use hot water, as this will cause the hat to shrink.
Add either a mild detergent meant for wool, your favourite unscented, gentle detergent, or even shampoo. Yes, because wool is actually the hair of an animal, you can use shampoo to wash it. Baby shampoo is very gentle and a good choice for wool. Agitate the water by hand to make suds.
Submerge your hat and gently squeeze it repeatedly to absorb and release the water. Let it soak for about 5 minutes. If the hat is extra dirty, drain the sink and repeat the washing process.
Drain the wash water. Refill the sink with cool water. Submerge the hat and gently squeeze to rinse away the wash water. Repeat if necessary.
Lay the hat flat to dry on a towel. If the hat is not too heavy, you can also use a drying rack. Do not wring out the water.
Machine-wash a wool hat in cold water, on the gentle/delicate cycle, using a gentle detergent.
Choose the slower spin cycle, if your machine has that option. If your hats are pure wool and handmade, and you are nervous about spinning them, lay them flat to dry instead. If you choose to air dry the hats, do not wring them out first.
Lay it flat to dry or use a drying rack. Never put a wool hat in the dryer.
Wool baseball caps should not be machine washed. Wash a wool baseball cap by hand in cool water with a mild detergent meant for wool. Over time, a baseball cap has shaped itself to your head, but it will shrink up a bit during the drying process. If this concerns you, consider wearing it while it dries and it will dry to the right shape and size.