The word wool was wull in Old English, wullo in Teutonic, and wlna in pre-Teutonic days. Wool is the fiber from the fleece of domesticated sheep. It is a natural, protein, multicellular, staple fiber. The fiber density of wool is 1.31 g/cm3, which tends to make wool a mecfiurn weight fiber.
The wool fiber is a crimped, fine to thick, regular fiber. Fine wools may have as many as 10 crimps per centimeter, whilst coarse wools have less than 4 crimps per 10 centimeters. As the diameter of wool fibers increases, the number of crimps per unit length increases. The number of crimps per unit length may be taken as an indication of wool fiber diameter or wool fiber fineness.
Wool fibers may vary from off-white to light cream in colour. This variation in colour is due to the disulphide bonds which seem to be able to act as chromophores. As a result the incident light may be modified to cause the reflected light to have a tinge of yellow, giving the wool fibers their off-white appearance. When the fiber is cream to dark cream in colour, this is due more to polymer degradation on the surface of the fiber. This can readily occur, as the wool polymer is chemically very sensitive to atmospheric oxygen and air pollutants.
Fibre length to breadth ratio can be critical with wool, since the short, coarse fibers spin into less attractive yarns than do those of fine wools. In general, fiber length to breadth ratio ranges from 2500:1 for the finer, shorter wools to about 7500:1 for the coarser, longer wools.