Felting is a popular technique for hand knitters. To create felted fabric, knitters create an item and then machine wash it in warm soapy water. The result is a dense fabric that does not have any stitch definition, can be cut without unraveling. Felting is popular for creating bags and shaped items like hats.
There are two types of natural fibers, plant and animal. Plant fibers are cellulose and animal fibers are protein.
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Protein fibers have scales that grab onto each other to create durable yarn. Cellulose fibers do not have scales.
Effects of washing
When you wash protein fibers in warm, soapy water, the fiber swells. This causes the scales to push outwards. Agitating the fiber in the water causes the scales get tangled up with each other, creating felted fabric.
Because cellulose fibers do not have scales, they do not felt.
Other cellulose fibers
The other popular cellulose fibers are linen, bamboo and hemp. None of these fibers felt.
There are a lot of protein fibers to choose from including wool, mohair, angora, alpaca and cashmere,. They all felt.