Shrinkage can be a problem. No, not in that way, we're talking about laundry.
Received wisdom tells us that clothes tend to shrink when we wash them, and that's true, but the actual reasons why may be more complicated than you think.
As Trace Dominguez explains in this DNews dispatch, there are actually three distinct processes that trigger the phenomenon we call shrinkage. Keep these in mind next time you're flirting in the laundromat, for there is nothing so sexy as ... knowledge! (Seeker motto: Ex scientia pulchritudo)
Relaxing shrinkage refers to the inevitable but generally minor adjustments fabrics make after the manufacturing process. Natural fibers, which are generally curly, get stretched and pulled when crafted into dungarees, trousers, what-have-you. This kind of shrinkage occurs on the molecular level, is triggered by warm water, and only results in size reductions of about one percent.
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Felting shrinkage sounds like something that might be illegal in several states, but in fact refers to the shortening of individual fibers. Wool, for instance, will shrink when it contact with water, because it's actually made up of tiny scales. Moisture reduces friction, causing the scales to slide and retract.
Finally, there is consolidation shrinking, in which fibers curl up do the mechanical bouncing action of your washer and dryer.
A few other elements can factor in, as well. A study in the The Research Journal of Association of Universities for Textiles found that the specific structure of different fabrics can make a difference. Tightly-woven denim jeans, for example, won't shrink as much as a loose wool sweater.
Moisture content plays a role, too. Then there's the fact that many apparel companies add various synthetic chemicals to impede shrinkage. It gets complicated. The best approach is to just do what the label tells you, and hope for the best.
-- Glenn McDonald
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Textile Research Journal: A Comparative Study on the Felting Propensity of Animal Fibers
AUTEX Research Journal: Effect of Laundering on the Dimensional Stability and Distortion of Knitted Fabrics