As wool grows on a sheep, it will likely come into contact with dirt and mud, clay, dung, pollen, vegetable matter, and other things you probably don't want to wear. Additionally, sheep's wool is coated in lanolin, a protective greasy substance that sheep skin excretes. And while it's usually at least possible to spin a fleece with all of that still in, it's much more difficult and has no particular gain, except a lower chance of felting.
I do encourage everyone to try several ways of washing, to try spinning raw fleece, to give it all a shot. How else will you learn your preferences ? So I've included a few different ways to wash a fleece, including my own way. If you are particularly afraid of felting your fleece, I advice starting out washing just a handful at a time--that way if you do felt it, it's not a great loss.
This is a very gentle method of fleece washing, and
To wash a fleece, you need a dedicated wool pot (as in, don’t cook in it again), a bit of dish soap, and some time. Put the fleece in–don’t crowd it, just work in batches if your pot can’t easily fit all of the fleece–add cold water and a squirt of dish soap, and let it cook on the stove for about 45 minutes, without a lid. Don’t let it boil–ideally it should be steaming but not quite simmering. You can use a dedicated wool spoon/tongs to gently and infrequently stir the wool. The water should get pretty gross. After 45 minutes, start the tap running (you need to rinse the wool in very hot water–if you let temperature shock happen, it could felt), drain the water, and rinse the fleece. Then repeat–filling up the pot with hot water now–until the water stops looking dirty at the end of the 45 minute cooking time. Rinse it one more time, and then let your wool dry, ideally on a clothesline but over a vent/spread out flat on a towel is fine too.
Yes, it really is that easy. If you’re worried about felting or otherwise ruining the entire fleece, you can always start by washing just a handful, so that way if you ruin it there’s not much waste. But I’ve washed at least a couple dozen fleeces that exact way, and I’ve never ruined one.