The dense coat grown by sheep, known as fleece, is the natural source material for wool fiber. The fleece is removed from the sheep, typically in a single piece, and then processed through several stages before it is ready to be spun into yarn. Understanding the makeup of this animal fiber, from its microscopic structure to its commercial classification, reveals why it remains a sought-after natural resource.
The Biological Structure and Composition of Fleece
The wool fiber is fundamentally composed of hard alpha-keratin protein, which makes up as much as 90% of its weight. This keratin is a structural protein characterized by a high content of the amino acid cysteine, which forms strong disulfide bonds. These bonds link the keratin intermediate filaments (KIFs) and the surrounding keratin-associated proteins (KAPs), giving the fiber remarkable strength and resilience.
Interwoven with the protein structure is lanolin, a natural, waxy substance secreted by the sheep’s sebaceous glands, often referred to as wool grease. Lanolin is not a true fat but rather consists mainly of sterol esters, and its presence is important for the sheep’s well-being. This coating provides waterproofing and protective qualities to the fleece, helping the animal shed water and maintaining the health of its skin and coat in various environmental conditions.
The physical characteristics of the fleece fiber, such as its diameter and waviness, contribute directly to its insulating power. Fiber diameter is measured in microns (one-millionth of a meter) and is the primary factor determining softness and end-use. The natural waviness, known as crimp, is a result of asymmetrical growth and protein distribution within the fiber, creating tiny air pockets when the fibers are packed together. These air pockets trap heat, giving wool its loft and thermal properties.
Classifying Fleece by Quality and Type
Fleece quality is primarily determined by fiber diameter, staple length, and uniformity, with the micron count being the most accurate and standardized measurement used commercially. Classification systems categorize wool into broad types that correspond to their suitability for different products. This system ensures that the raw material is matched to the manufacturing process that will best utilize its specific properties.
Fine Wools are those with the lowest micron counts, typically ranging from below 17 up to approximately 20.6 microns. Wool in this range is highly valued for its softness and is generally used for next-to-skin apparel and luxury garments. Merino wool is the most recognized example of this category, prized for its high crimp and short staple length relative to other wool types.
Medium Wools have a larger diameter, generally falling between 22 and 29 microns, and represent a versatile category used for outerwear, blankets, and durable knitwear. Coarse or Long Wools possess the largest fiber diameter, often 31 microns and greater, and are characterized by a longer staple length and less crimp. These coarser fibers are more durable and resilient, making them well-suited for non-apparel applications like carpets, heavy industrial fabrics, and upholstery.
From Sheep to Usable Fiber
The transformation of raw fleece into a usable textile fiber begins with the initial removal of the coat from the animal. Shearing is the process by which the entire fleece is harvested, ideally in a single, intact piece. Immediately following this, the fleece is subjected to skirting and grading, which are initial quality control steps.
Skirting involves manually removing undesirable portions from the edges of the fleece, such as the short, heavily soiled wool from the belly and legs, known as tags. This step also removes vegetable matter, like burrs and hay, and second cuts, which are short pieces of fiber resulting from the shearer going over the same area twice. Grading then assesses the overall quality of the remaining fleece based on the classification criteria, such as fineness and staple length.
The next major step is scouring, which is the washing process necessary to clean the raw fiber. Scouring is performed using warm water and specialized detergents to remove the lanolin, dirt, sweat, and other contaminants that make up a significant portion of the fleece’s weight. This process is executed carefully, avoiding agitation and sudden temperature changes to prevent the wool from felting, a condition where the fibers irreversibly lock together.
Once scoured, the clean wool is dried, resulting in a lighter, fluffy material ready for the final stages of textile production. The raw fiber is then prepared by carding and combing before being spun into yarn for weaving or knitting. This sequence converts the natural protective coat of the sheep into the versatile textile material known as wool.