Why does wool shrink in the dryer?

I have the sneaking suspicion that this has been asked before, but I can’t seem to find it around. Anyone have the answer?

IIRC, it happens because wool fibers have small barbs on them, a bit like the barbs on a fishhook. When the wool is agitated, the fibers slide past each other, and the barbs lock into each other, sort of like a ratchet mechanism. So, the fibers get pulled closer and closer together, and the sweater shrinks.

So, it isn’t really the heat that’s the problem, it’s more a problem of the physical agitation - that’s why you shouldn’t put a sweater into the washing machine, even in cold water. Hand wash.

Here’s a cite, with a clearer explanation:


The relevant section:


Whenever I try to shrink wool–a sweater a little too big, for example–it gets stretched! Cotton shrinks when it gets the hot dryer treatment, but my experience with wool is quite different. Guess I’ll have to try shrinking it again.

Suppose one shrinks a wool garment that was too big, to where it now fits. Does the shrunken wool retain its insulation properties? Would it more or less desirable than unshrunken wool for any reason?

Before the introduction of hi-tech Gore Tex gloves, Dachstein Mittens had for many years been the standard for mountaineers. They were made really enormous, out of a greasy wool. They were then washed in a way to maximise shrinkage. What you get is a really thick mitten which is warm, windproof, water resistant and retains much of it’s insulation even when soaked. One nice feature is that snow sticks to the outside which actually makes them warmer! Although hard to come by these days, many alpinists still swear by them.

So do sheep shrink when their wool is agitated?

Dunno. Go poke a sheep in the butt and see if it gets smaller. :stuck_out_tongue:


Oi, leave the beastiality to the experts :wink:

The process is called felting. If you have ever looked at a piece of felt, that’s the effect. So while insulation properties are retained, it becomes poorer to handle. Frinstance, you don’t often see felt garments on the catwalk.

After prolonged aggitation you can get blocks of wool felted to the hardness of wood. These are used in industrial applications including shock and sound insulation.

Wool on the sheep doesn’t felt as much, mainly because you don’t tend to wash live sheep in hot water. However if a sheep regularly rubs itself say due to the irritation of an external parasite then its fleece will felt. In this case the wool is termed “cotted”.