Raw Wool and its Impurities

Wool is the animal fiber forming the protective covering, or fleece, of sheep or of other hairy mammals, such as goats and camels. Prehistoric man, clothing himself with sheepskins, eventually learned to make yarn and fabric from their fiber covering. Selective sheep breeding eliminated most of the long, coarse hairs forming a protective outer coat, leaving the insulating fleecy undercoat of soft and fine fiber.

Raw Wool and its Impurities

Figure: Cashmere Wool

Raw Wool

Wool is mainly obtained by shearing fleece from living animals, but pelts of slaughtered sheep are sometimes treated to loosen the fiber, yielding an inferior type called pulled wool. The wool is removed with shears similar to those a barber uses. This process of shearing does not hurt the sheep. In about five minutes the wool is shorn from the sheep in a single piece, called the fleece. The fleece is carefully rolled and tied for bagging. Most shearing is done between February and June, just before lambing. Most shearers move from ranch to ranch. A good shearer can shear from 80 to 125 head of sheep a day. A highly trained expert can shear up to 225 head of sheep in one day. Fleeces are rolled up and tied, then packed into sacks. These sacks hold between 20 and 35 fleeces (of 4-12 lbs. each) and weigh an average of 200 to 400 pounds. From this step the processing of the wool begins.

Wool fiber is chiefly composed of the animal protein keratin. Protein substances are more vulnerable to chemical damage and unfavorable environmental conditions than the cellulose material forming the plant fibers. Coarser than such textile fibers as cotton, linen, silk and rayon, wool has diameters ranging from about 16 to 40 microns (a micron is about 0.00004 inch). Length is greatest for the coarsest fibers. Fine wools are about 1.5 to 3 inches (4 to 7.5 centimeters) long; extremely coarse fibers may be as much as 14 inches in length. Wool is characterized by waviness with up to 30 waves per inch (12 per centimeter) in fine fibers and 5 per inch (2 per centimeter) or less in coarser fibers. Color, usually whitish, may be brown or black, especially in coarse types, and coarse wools have higher luster than fine types.


Figure: Sheep Wool

Impurities Present in Raw Wool

The proportions of the different components of unwashed or greasy wool are:

  • Moisture = 4 – 24 %
  • Yolk = 12 – 22 %
  • Dirt = 3 – 9 %
  • Wool Fibers = 60 – 70 %

Raw wool contains three main impurities:

  • Wool grease
  • Suint
  • Dirt

These combined make up some 30-40% of the fleece weight. A typical figure of the grease content of crossbred wool is about 6%.

Wool Grease

This is a very complex mixture, consisting mostly of esters of various long-chain fatty acids with long-chain alcohols and sterols. Technically it is a wax, rather than a fat, because glycerol esters are not present. The fatty acids present fall into four main classes:

  • A normal paraffin series, with even carbon numbers approximately from C10 to C26.
  • An iso-acid series, with the alkyl chain terminating in a [(CH3)2-CH-] group, with even carbon numbers from C10 to C28.
  • An anteiso-acid series, with a terminal isobutyl group, with odd numbers of carbons from C9 to C31.
  • α-hydroxy normal and iso-acids, like [R-CH(OH)-COOH] with even carbon numbers from about C12 to C32.
  • The alcohols are even more complex. There are aliphatic alcohols corresponding to the same series of structures as the acids, i.e.
  • Normal alcohols from C18 to C30.
  • Iso-alcohols from C18 to C26.
  • Anteiso-alcohols from C17 to C27.
  • 1,2-diols, i.e., [R-CH(OH)-CH2OH], both normal and iso, C16 to C24.

However, a major part of the alcohol fraction consists of sterols; the most important member is cholesterol, followed by lanosterol and dihydrolanosterol. Smaller amounts of other sterol derivatives are present. Clearly the esters formed from these acids and alcohols form a bewildering variety, especially when one considers that the hydroxy-acids and the diols can form di-esters.


Suint is the sweat of the sheep and is a complex mixture of water-soluble salts. The predominant cation is potassium; the anions include carbonate, bicarbonate, various low molecular weight mono- and di-carboxylic acids (succinic, glycolic, glutaric etc.) and smaller amounts of long chain fatty acid anions which may originate from wool grease. Peptides and other nitrogenous substances are minor components.


Dirt consists of all the ill-defined solid fleece contaminants. It includes mineral soil, windblown dust, vegetable matter, faecal matter (dags), skin flakes, discarded cuticle cells, and fragments of fiber broken from brittle photo-oxidized tips. In many respects, in terms of wool properties, it is the very fine mineral material, largely associated with exposed fiber tips, that is the most significant.

Muhammad Rehan Ashraf

I am a Textile Engineer, founder and editor of "Textile Trendz". Currently working in an export-oriented textile organization. I love to share my knowledge about textiles.