Wool is an animal fiber produce by sheep. It is similar to human hair in that it grows from a follicle in skin similar in construction to ours and is similar in construction to our hair. It is composed primarily of a protein called keratin. The major elements found in wool are carbon, hydrogen, oxygen, nitrogen and sulfur (The Alden amos Big Book of Handspinning, pg.29). Although it is very like our own hair, it differs in ways that make it much more suitable for our use as spinners and we have fostered those differences in our domesticated flocks. Wool properties differ from different breeds of sheep. It is valued for its wide range of suitable uses, from carpets to clothing.
As the individual fiber leaves the skin, it is coated with yolk, a substance made up of sweat (called suint) and grease (called wool wax). Yolk forms a film over the fiber which protects it somewhat from the elements.
Each individual fiber of wool is enclosed by an outer sheath called a cuticle. This outer sheath is composed of three layers, the outermost layer being the semi-permeableepicuticle which is a waxy hydrocarbon sheath that is hydrophobic, shedding water yet allowing water vapor to pass into and out of the fibre through small pores. This absorption of moisture from higher humidity and release to lower humidity makes wool a great temperature regulator when moving from a dry warm indoor climate to a damp chilly outdoor one.
The cuticle is made of overlapping scales with ragged tips that point away from the root of the fiber. The friction difference in these scales, created by fiber movement, helps to repel dirt, moving it outward, away from the root toward the tip. This movement can also lead to felting. Scales of fibers entangle and interlock when juxtapositioned in the presence of soap, movement and temperature change. This creates a dense fabric called felt or wool blanket.
Knowing how to create or avoid felting is integral when processing and caring for wool. Cultural textile traditions have evolved based on regional wools' susceptibility to felting. In The Knitter's Book of Wool, Clara Parkes points out that Shetland wools are not as susceptible to felting and there is little role for felting in its region's fiber traditions, yet felting plays a heavy role in Icelandic fiber tradition and Icelandic wool felts easily.
Encased by the cuticle is the cortex which is composed of two types of cells,ortho-cortical cells and para-cortical cells, which are surrounded by a cellular membrane complex (CMP). The CMP hold the ortho-cortical and para-cortical cells in place on their prospective sides. It is composed of proteins and fats and held together by weak chemical bonds that are easily broken by abrasion, heat or strong alkali. The CMP runs throughout the entire fiber and can act a conduit for dyes and chemicals to pass into and out of the wool.
These two types of cells are found in varying amounts in different breeds of sheep and account for the amount of crimp in a fleece. In finer fibers, these cells are present in approximately equal amounts, each type on its own side of the cortex. They wrap around the interior of the fiber helically with the ortho-cortical cells always on the outside of the curve. Ortho- and Para-cortical expand differently when they absorb moisture, making the fiber bend. This creates the crimp in wool. Crimp adds bulk, trapping air between fibers, furthering wool's insulation properties. It also adds elasticity to fibre.
Each para-cortical cell, encased in CMP, contains multiple macrofibrils. These each contain multiple microfibrils which are embedded in a matrix. Microfibrils are composed of pairs of twisted molecular chains of proteins that are springlike in character. It is this helical structure of microfibrils that give wool its characteristic elasticity, flexibility and wrinkle recovery properties.
The matrix surrounding microfibrils is amorphous and hydrophilic, easily absorbing relatively large amounts of water without feeling wet. It is composed of high sulphur proteins which attract water molecules. As these high sulfur proteins in the matrix chemically bind to the hydrogen in water, the heat released from the breaking of hydrogen bonds causes wool to give off warmth. Wool can absorb up to 30% of its weight in water and can also absorb and retain large amounts of dye. The matrix is also responsible for wool’s fire-resistance and anti-static properties.
Coarse fibers often have a central tube of empty space called a medulla. It is spongy tube of air pockets that runs down the center of the fiber and stiffens the fiber. Kemp is a shorter straighter fiber that grows in varying amounts in different breeds. It is mostly medulla which makes it stiff and prickly. Kemp fibers pull moisture away from the sheep's skin and is found in larger amounts in regions with wet climates. Some breeds have a heavier predominance of kemp. Kemp does not take dye well and thus the dyed wool has lighter fibers running through it. This is generally an undesirable trait, however, some designs appreciate and incorporate the naturalistic appearance of these fibers.
Wool's insulating and moisture absorbing properties make it extremely comfortable to wear. Its chemical composition allows it to be easily dyed in a broad range of values and hues. Wool's flexibility, elasticity, resilience and wrinkle recovery properties lead to a broad range of textile design design. It's versatility of uses, based on the vast differences in fiber from individual sheep breeds, have made wool products one of the most valuable commodities throughout our history.
