Why do some clothes shrink when washed in hot water and why can’t they stretch back to their normal size?
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There are different reasons depending on what your clothes are made of. When we wear our clothes we stretch all the little fibers that they are made of, and putting them in hot water helps them “remember” their original shape. That’s why most clothes shrink a little bit after washing. Clothes made from plant fibers like cotton will do this.
Some animal fibers, like wool, are covered in tiny little scales we can’t see. Your hair is like this—you can feel like scales by feeling the difference between pinching a strand and running your fingers away from your head (in the direction of the scales) vs. toward it (against the direction of the scales).
When you heat up the fibers the scales relax and lift up like little hooks. That alone won’t shrink your clothes much! It takes some agitating to do that. When the fibers get jostled around in the washing machine or dryer those hooks start grabbing on to each other like Velcro. The fibers get more and more tangled and closer together until eventually there’s no way you could untangle them. When the clothes cool down, the scales flatten back down while still hanging on to each other. Then the fibers are really locked together. This is why after you shrink wool it feels thicker and stronger than it did before.
We can remove the scales from the hairs with chemicals, and then they won’t shrink. But the fibers will be a little weaker as a result.
Fun fact: clothes shrink top to bottom, generally never from side to side.
**Bought a long shirt?** *You can shrink it shorter.*
**Bought a wide shirt?** *Learn to live with it, muffin top.*
Some clothes will shrink because they’re made from animal fiber, i.e. hair. Hair is made from the same stuff as our fingernails, keratin, which is surprising because hair is much softer. But if you put an animal hair (including a person’s hair, and even silk!) under a microscope, you’ll see a core covered by scales of keratin. So the type of material is the same–hard keratin–but by putting it together in a different way, the body has made it soft. (For a look at the scales, try this illustration from [Tricksy Knitter](https://tricksyknitter.com/blogs/learn-to-knit/why-wool-felts-and-how-to-felt-your-knitting-on-purpose) or [Knit Along Club](https://knitalongclub.com/why-does-wool-shrink/).)
These scales are larger or smaller depending on the type of animal, the health of the animal, and just individual variation. Just look at people hair–we have flat hair and poofy hair and everything in between.
When making clothes, the hairs are organized by combing them and bundling them together as strings, then the strings are woven or knit together to make fabric. Each hair still has scales, but there are so many hairs that the structure of the strings and the strength of the fabric will determine how the material behaves.
This can change, though, if the hair is exposed to heat, movement, and water all at once. Water causes the scales on hairs to open up, movement gets hairs near each other to latch onto each other’s scales, and heat both keeps the scales open longer and strips off any oils that keep the hairs from latching. Soap makes this process go faster, since it’s designed to strip away oils. When this happens, it’s called **felting**. Sometimes people do this on purpose, and sometimes it’s accidental. Light felting can sometimes be reversed, but it takes a lot of time and care to do so.
When a plant fiber shrinks, that’s a different story! A lot of shrinkage happens with cotton. This is because it’s a short fiber with a spiral shape. Take a look at the cotton plant [here](https://en.wikipedia.org/wiki/Cotton#/media/File%3ABALLS_OF_COTTON_(ANDHRA_-SOUTH_INDIA)_READY_FOR_HARVEST.jpg) and you’ll see that it’s a tiny ball of fluff. You can get fibers up to 2 inches (5cm) long from a cotton plant if you’re really lucky or if you have a very good type of plant. Because the length of these individual fibers (called “staples”) are so short, a lot of layering and spinning has to be done to make sure that the thread made from the cotton will be useful. The fibers also look like little spirals when placed under a microscope ([see this image from Wiki](https://en.wikipedia.org/wiki/Cotton#/media/File%3AC21a.jpg)).
Heat and agitation will cause cotton to shrink, just like with animal fibers above. But the cause is different! The staples of cotton try to shrink into tight spirals when exposed to heat, and because so many staples have to be used to make a string of cotton, the effect is larger. In a pure cotton fabric, this adds up to shrinking up to 20% over the life of the fabric. This shrinking is generally permanent, though like with felting, it can be reversed with time and patience.
The clothes that shrink are made of natural materials. Think cotton.
The problem with natural stuff is that it has proteins. Think like meat.
When you “cook” those proteins, they contract. Think like a steak
The catch is, with a steak, it takes a lot of heat to cook it through because it’s thick.
Your clothing on the other hand is very thin.
Tldr: Clothing made with natural materials can cook in heat, and cooking makes stuff shrink.
To avoid shrinkage with clothing, wash in cold water and let it air dry
The short answer is entropy. Both natural and man-made fibers are composed of long molecules that are preferentially oriented in the axial direction. Weak chemical bonding between adjacent chains maintains the alignment. Heating is sufficient to disrupt these bonds, allowing the molecular chains to relax into a less-oriented configuration, thereby contracting in length. The temperature at which these transitions occur is called the glass transition temperature, which is different for different materials.
It is theoretically possible to re-stretch the fibers, and maintain them under tension while cooling, to reset the extended length, but practically difficult, once in fabric form. Materials that have more permanent cross-links exhibit much better thermal stability, but they also are hard, stiff, and not suitable for making fabrics.
It’s generally the dryer that does the shrinking, not the water.
Some fibres shrink, others don’t, and some are pre-shrunk (yay!)
I know it’s not the answer you looking for, but … I started buying t-shirts at least one size bigger and after washing and drying couple of times they fit me perfectly.
Protein in the fibers condense. That’s why wool can shrivel to half their size. Many items are pre shrunk like jeans. But you can still buy shrink to fit Levi’s and boiled wool is a thing.
On the subject of OP, at wich temperature in °C cotton start to shrink ? I have a few cotton jeans wich i really loved who unfortunately got loose overtime
Fabric is made of tiny curls of stuff. If it’s plastic, it usually doesn’t change much if you wash it unless you melt it, like polyester or nylon. If it’s organic, like cotton or wool, the little tiny curls were first straightened and then twisted together to hold in long threads. When you get them warm, the straightened twisted fibers become curly bent fibers, so they shrink.
You can relax the fibers back sometimes. Hair conditioner and cool water, plus gentle stretching, can repair minor shrinkage of wool, for example. But nothing can help if you’ve “felted” together the fiber, where the curls are all so tangled now they can’t ever be made to lay in straight twists again.
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