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JACKI LYDEN, host:

So, your mother spent weeks knitting you that big, comfy wool sweater for Christmas, you toss it into the wash and the next thing you know it's not even big enough for a cat to wear. How did that happen? It's Science Out of the Box.

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LYDEN: Margaret Frey is an associate professor at Cornell University's Department of Fiber Science and Apparel Design, and she joins us on the line now. Welcome, Margaret Frey.

Dr. MARGARET FREY (Professor, Cornell University): Thank you.

LYDEN: I think, you know, a lot of us have wondered why it is that when we throw a big sweater into the wash it comes out tiny, I mean exactly why? We know it will, but why?

Dr. FREY: OK, so wool or any of these other, you know, cashmere, angora, mohair, these are all the hair of an animal, and like the hair on our heads these hairs have scales on them. So when the wool gets wet and warm the scales on the side tend to stick out. And then, as you agitate these and rub the fibers against each other, they lock together, so as you continue to tumble this and agitate the fibers they get stuck together more and more, and the fabric continues to get thicker and thicker and shorter and shorter. So, you can start out with, you know, very large full-grown person's sweater and have it come out like a baby sweater.

LYDEN: What is this process called, if you want the wool to shrink? What makes felt, for example?

Dr. FREY: Yeah that is exactly the felting process. If you do that on purpose and then control the way you can make a nice thick felt, so if you have some felt slippers or maybe a boiled wool jacket those are where the wool fibers have been felted in a more controlled and more desirable way.

LYDEN: So, what about clothes that aren't made of wool? Because other things shrink. What makes them shrink, say a cotton shirt?

Dr. FREY: The process for most other fabric shrinking is just what I call a stress relaxation process. As the yarns and the fabrics are made, they're constantly being stretched and stressed out and pulled along their length, so when you go ahead and throw those in the wash, particularly at a high temperature, some of that stress can relax out, and the yarns will shorten up again and make the fabrics become shorter. Most things that you buy from any reputable retailer are going to be pre-shrunk so they shouldn't really shrink very much, if at all. And this really shouldn't be a problem if you're following the washing instructions.

LYDEN: But let me ask you this science question. Normally we associate heat with things expanding and so it seems illogical that hot water makes things shrink.

Dr. FREY: Maybe imagine that as, you know, a yarn in your fabric is almost rubber band. It's constantly getting pulled out and pulled out, and pulled out to a longer shape. And then when you release the force that's pulling on that, it will shrink back to a shorter shape, except in this case the yarns won't go to the shorter shape just automatically. They need a little bit of heat, possibly water, and possibly a little agitation again to allow them to return to their shorter state.

LYDEN: Well, you're an expert, Margaret Frey; do you ever accidentally shrink a sweater?

Dr. FREY: Oh, certainly.

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Dr. FREY: And then I'm extra mad at myself, because I really should know better.

LYDEN: Margaret Frey is an associate professor at Cornell University's Department of Fiber Science and Apparel Design. Thanks very much, and happy new year, Margaret.

Dr. FREY: Thank you.

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