It’s happened to the best of us: we throw a new cotton shirt in the dryer without thinking, and voilà – now we have a shirt fit for a toddler.
Cotton is susceptible to this type of laundry error in a way that synthetic fibers, such as polyester, are not. Much of this vulnerability comes down to the individual fibers of the cotton garment, Jillian Goldfarban associate professor of chemical and biomolecular engineering at Cornell University, told Live Science in an email.
“Cotton fabric is made by weaving together fibers from a cotton plant, which are themselves made mostly of cellulose, a natural biopolymer,” she said. “Cotton … is prone to shrinkage because its fibers swell when wet and then shrink as they dry.”
If you’ve ever sweated in cotton clothes, you know how well they can absorb moisture. On the other hand, synthetic fabrics – such as polyester, nylon and spandex – are more resistant to sweat and shrinkage because their tightly woven threads do not swell in water.
On a chemical level, weaving cotton fibers into clothing brings tension that creates a network of hydrogen bonds, Erika Milczek, a chemist and CEO of the biotech company CurieCotold Live Science.
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When variables such as heat and water are introduced, this network of hydrogen bonds can transform, causing the fabric to either relax or contract. This is also the science responsible for wrinkles in your clothes, Milczek said.
The science of contraction
When it comes to accidentally shrinking your cotton clothes, not all items are created equal, Goldfarb said.
“Even when they’re made from the same material, some cotton fabrics are more prone to shrinkage than others depending on how the fibers are gathered in a fabric,” she said. “Woven cottons, while they will certainly shrink, see significantly less shrinkage than knitted cottons.”
Think of the interwoven cotton fibers as a hashtag, where some fibers are woven under others, Goldfarb said. Threads woven horizontally are called “warp”, and threads woven vertically are called “weft”.
“As the fibers swell when wet, they push the tissues closer together, shrinking in one direction,” Goldfarb explained. “When moisture is removed from the fabric, the fibers shrink.” This means that shrinkage actually begins before the clothes ever hit the dryer. Shrinkage is a dual consequence of fibers immersed in water and high heat.
Exactly how much your clothes shrink is determined by a number of factors, Milczek said. For example, it depends on whether you wash your clothes in water alone or add detergent – detergent further breaks the hydrogen bonds – and whether you dry your clothes on high heat or low heat or hang them to dry.
“The temperature [when line drying] is significantly lower, so evaporation occurs much more slowly and the fibers aren’t ‘stressed’ by the heat of shrinking,” Goldfarb explained. A line-dried shirt also experiences more consistent moisture between the outdoors and your closet, which it could result in less shrinkage, she said.
Saving a shrunk shirt
For some, this knowledge may come a little late. But don’t worry; there may yet be hope for your shrinking wardrobe.
One obvious answer, Milczek said, is to start by looking for clothes that are shrink-resistant. These include cotton fabrics with synthetic blends or cotton fabrics that have been pre-shrunk.
If that doesn’t work, there’s a science-backed way to try to “undress” your clothes.
“Depending on the quality of the yarn and the weave … if we blow the yarn and let it dry under tension, it is possible to ‘disappear’ some cotton fabrics, at least temporarily,” Goldfarb said.
One way to do this at home is to use a steam iron, she said. This reintroduces moisture into the garment to expand the fibers while applying mechanical force to stretch them. But tread lightly—this method can also swing too far in the opposite direction.
“Of course, it’s easy to ‘stretch’ cotton this way, and if it’s done unevenly, you can be left with a rather misshapen garment.” said Goldfarb.