An excellent explanation that i did not find elsewhere.
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Combing is a technique that has been used in the preparation of textiles for centuries. Essentially, combing is a process that helps to smooth and prepare fibers for use in spinning. Combing also helps to separate short fibers from longer ones, which also helps to make the process of spinning much easier.
While the combs used in this process of fiber preparation do function with a combing action, the actual devices have no more than a passing resemblance to the combs used on hair. For textile purposes, the fiber combs are equipped with long metal teeth. Two combs are utilized in tandem, with one comb holding the strand of fiber in place while the other comb moves across the surface of the fiber.
Combing accomplishes two tasks at once. As the comb moves through the fiber, it slowly separates the shorter fibers from the longer ones. Short fibers do not work well in a spinning operation, but may be set aside and processed using a technique known as carding. The longer fibers are arranged into flat bundles, with all the fibers facing the same direction. This bundles can then be processed through spinning, creating yarn and thread that can then be used to weave cloth.
Combing is normally used to prepare a worsted yarn that is made of natural fibers, such as cotton. In fact, combing cotton is a common process when the fiber is intended for use in various forms of apparel. The combed cotton will feel smoother to the touch, and also produce a garment that will wear longer than a garment made with uncombed cotton.
Not all natural fiber yarns are made by combing, however. Woolen yarn is one example. This type of yarn is made by carding the wool only; the additional step of combing is not performed. Carding straightens some of the fibers, but not to the degree that combing does. The result is a lighter, stretchier yarn with more air.
In general, carding is used on shorter fibers, while longer fibers are combed. Many textile companies will employ both combing and carding in the preparation of fibers for use in different types of products, such as shirting fabric, towels, sheeting, and other essential household items.