A wool sweater through the laundry shrinks. But why don’t sheep shrink in the rain? It is all about friction. Wool fibers are covered in overlapping scales that run the length of each fiber. These scales make it easier for the fiber to slide in one direction than the other. When a wool sweater gets thrown into the washing machine and tossed around, this one-way resistance becomes a problem.
As each fiber rubs against its scales, act like little ratchets. It only allows the fiber to move in one direction. The water worsens by causing the fibers to swell, bringing them closer contact. By softening the scales, they floppy enough but not so floppy that they can give way and allow the fibers to move past each other.
Why don’t sheep shrink when it rains? (Wool Facts)
The structure of wool also complicates things. It has little jagged structures called cuticles all over its surface. They like to latch onto things like other wool fibers. The cuticles don’t bother the sheep, and they hold the fiber to stay attached to its skin but turn them into the fabric.
When sheep get caught in the rain, the fibers in their thick coats swell, and their scales soften too. As a result, their wool doesn’t get tossed around enough for the ratchet effect to cause it to tighten. Heat also exacerbates the ratchet effect. Throughout the wash and dry cycle, the millions of tiny ratchets on the thousands of individual fibers in the wool sweater draw the fabric into a tighter and tighter configuration. As a result, it shrinks the overall size of the sweater.
- The shrinking of woolen clothes is a result of a process known as felting.
Wool is made up of amino acids, which are the building blocks of protein. Like human hair or fingernails, wool that grows on a sheep is keratinized, meaning hardened. Wool fibers on a sheep have flat, overlapping scales pointing away from the sheep’s body. However, when these fibers get processed and turned into clothes, they get stretched out, and the scales can then point in any random direction as it all gets mixed up.
When we wash clothes, the fibers start rubbing against each other. The scales touch and often interlock. It does not allow the fibers to slide back to their original stretched-out position. Imagine hundreds of fibers doing this. That’s called felting. That’s why you end up with a smaller cloth that looks shrunk.
As sheep have unprocessed wool on their bodies, the scales in the fiber are all pointing in the same direction. Which means they don’t get tangled into each other interlocked in one place. No felting, no shrinking. Furthermore, wool on sheep is almost unrecognizable if you compare it to the fiber in our clothes. Their wool is covered in dirt, sweat, field debris, and whatnot. All of this comprises around 70 percent of the raw wool’s weight.
Sheep also produce a natural oily substance called lanolin. It covers their coats, acting as a lubricant, preventing fibers from locking together. Also, it repels water, so sheep are also more or less waterproof.
For the same reason, dogs, cats, and humans with a lot of hair don’t shrink when they get wet in the rain. Again, it is because the fibers all run in the same direction.