Two varieties of thread are spun from wool, one is known as "worsted," the other as "woollen" yarns; from these yarns two kinds of cloths are woven, distinguished as worsted and woollen cloths; the former are in general not subjected to any milling or felting process, while the latter invariably are.
Physical Properties.--When seen under the microscope the wool fibres show a rod-like structure covered with broad scales, the edges of which project from the body of the fibre, and all point in one direction.
Fig. 1 shows typical wool fibres as viewed under the microscope; the sketch shows very well the scales.
The shape of the scales varies in different breeds of wool. The outer scales enclose inner medullary cells, which often contain pigment matter. A transversed section of the wool fibre shows the presence of a large number of cells. Sometimes wool fibres are occasionally met with which have a peculiar white horny appearance; these do not felt or dye well. They are known as "kempy" fibres. See figure 2. The microscope shows that they are largely devoid of structure, and are formed of very horny, impenetrable tissue, which is difficult to treat in the milling or dyeing process.
The curly or twisted character of the fibre is caused by the unequal contraction of the outer scales, and depends in a great measure upon the hygroscopic nature of the wool. It may be entirely removed for the time by wetting the wool in hot water, then drying it in a stretched condition, or the curl may be artificially induced by unequal drying, a fact which is turned to practical account in the curling of feathers and of hair.
The amount of curl in different varieties of wool is very variable, being as a rule greatest in the finer qualities, and diminishing as the fibre becomes coarser. The diameter of the wool fibre varies from 1/2000 to 1/5000 of an inch, and the number of curls from about 30 per cent. In fine wool as little as 1 or 2 per cent. in the thicker fibres.