Shiny Science

Lillian Steenblik Hwang – Blog & Journalism Portfolio

Laundry science – exactly why do colors bleed in hot water?


White socks stained by a red shirt - Image credit -w00kie on Flickr

I am still trying to find the answer to this question, which was originally sparked by wondering how “color-catchers” work.

This is what I’ve learned so far…

Color bleeding is influenced by the following factors during a wash cycle:

  • mechanical (clothes rubbing against each other)
  • water temperature
  • and how long the wash cycle lasts.

Modern laundry detergents have a lot of additives and enhancers to help keep colors from fading and bleeding into each other. Usually these additives are bleaching agents or enzyme activators.  The enzymes are used to help to degrade protein-based stains. Bleaches de-color stains and add power to cleaning agents, and sometimes blue dyes are also added to counter yellowing.

Hot water

Hot water is bad for a few reasons. The hot water can cause the fibers in the garment to expand, and could allow the dye to be released. Or sometimes, like in the case of wool garments (or other animal fibers) the hot water combined with the agitation of the washing cycle and soap can cause felting, and often shrink a piece of clothing.

On the other hand, the heat in the water can act as a catalyst for chemicals in the detergent and helps loosen soil and oil from the clothing. Hot water has also been shown to be better at removing allergens from clothing.

Keratin structure from PubMed

Hot water (and drying on high heat) can also cause clothes to shrink. The classic example of this is a wool sweater. Wool consists of chains of keratin, a protein. In hot water the keratin changes shape, and the molecules can slide over one another. This creates shorter fibers, making the threads shorter, and shrinks the sweater (felting).
Cotton is also notorious for shrinking.


The type of fiber being washed has a lot to do with if the colors will bleed. The newness of the fabric (recently dyed jeans might not be fully colorfast) also plays a part in determining if that the garment will bleed color, since all the dye might not be absorbed into the actual fibers of the garment. The extra dye might come out in the first few washes.

Natural fibers like linen, cotton, silk and wool might not hold dyes as easily as snythetic fabrics.


That’s what I’ve learned so far. Still working on trying to find out EXACTLY why colors bleed in the laundry.

(If you’re interested – about why clothes wrinkle…)



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