What is wool?
Wool is a natural protein fiber shorn from sheep. It is made of keratin — the same protein as human hair — with a structure of overlapping microscopic scales that give wool its unique properties: natural crimp, elasticity, moisture wicking, temperature regulation, and odor resistance. No synthetic fiber reliably replicates all of these properties together.
Types of wool by fiber diameter
| Wool type | Breed | Microns | Best uses |
|---|---|---|---|
| Merino | Merino sheep | 15–24 µm | Base layers, knitwear, next-to-skin garments |
| Lambswool | First shearing (any breed) | 25–29 µm | Soft knitwear, scarves |
| Shetland | Shetland sheep | 23–29 µm | Traditional knitwear, Fair Isle patterns |
| Bluefaced Leicester | BFL sheep | 24–28 µm | Soft outerwear, hand knitting yarn |
| Standard / Crossbred | Various | 28–38 µm | Outerwear, blankets, carpets |
| Lincoln / Romney | Lincoln, Romney Marsh | 35–45 µm | Rugs, upholstery, coarse outerwear |
What is wool?
Wool is a natural protein fiber that grows as part of a sheep's fleece. The fiber is made of keratin — arranged in overlapping scales that interlock, giving wool its natural crimp. That crimp creates air pockets that trap heat for insulation. The scales also allow wool to absorb up to 35% of its weight in moisture without feeling wet. Wool is naturally flame resistant, UV resistant, odor resistant, and biodegradable. It has been used in textiles for at least 10,000 years.
Where does wool come from?
Wool comes from sheep. The fiber is harvested by shearing — using electric clippers to cut the fleece from the sheep — typically once a year in spring before warm weather arrives. The world's largest wool producers are Australia, China, the United States, New Zealand, and Argentina. Different breeds of sheep produce different grades of wool. Merino sheep, concentrated in Australia and New Zealand, produce the finest fibers. Breeds such as Lincoln and Romney produce coarser, stronger wool.
What are the types of wool?
Wool varies primarily by fiber diameter, measured in microns (µm). Merino wool (15–24 µm) is the finest and softest, suitable for garments worn directly against skin. Lambswool (from a sheep's first shearing) is also fine and soft. Shetland wool (23–29 µm) has a distinctive character and is associated with traditional Fair Isle knitting. Standard crossbred wool (28–38 µm) is used in outerwear, blankets, and carpets. Coarser breeds like Lincoln (35–45 µm) produce durable fibers for rugs. The lower the micron count, the finer and softer the fiber — and typically the more expensive.
Is wool a natural fiber?
Yes. Wool is one of the four major natural fibers alongside cotton, silk, and linen. It is a protein fiber that grows from the skin of sheep and is harvested by shearing. No chemical conversion is needed to make it into fiber — it already is fiber. This distinguishes it from semi-synthetic fibers like viscose and modal, which start from plant material but must be chemically processed.
Is wool warm?
Wool is exceptionally warm relative to its weight. The natural crimp of wool fibers traps air in tiny pockets, creating insulation similar to the way down works. Fine wool, such as merino, is warm enough for cold weather but also regulates temperature in milder conditions — the moisture-wicking and breathable properties mean it does not overheat the way synthetic insulation can. A mid-weight merino sweater provides meaningful warmth while remaining comfortable across a wider temperature range than most alternatives.
Is wool itchy?
Coarser wool grades (above about 30 microns) can feel scratchy against sensitive skin, as the fiber ends are thick enough to create a prickling sensation. Finer wool grades — particularly merino at 15–24 microns — are soft enough for most people to wear directly against skin without irritation. People who have found wool itchy in the past often find fine merino comfortable. The itchiness of wool is a function of fiber diameter, not of wool as a category.
Does wool shrink?
Wool can shrink if washed incorrectly. The overlapping scales on wool fibers can interlock when exposed to heat, agitation, and moisture simultaneously — a process called felting. The result is a smaller, denser fabric that cannot be reversed. To prevent shrinkage: wash in cold water, use a gentle cycle or hand wash, and lay flat to dry. Never put wool in a hot dryer. Some wool is treated to be machine washable — check the care label for "superwash" or "machine washable" wool, which has had the scales chemically smoothed.
Is wool sustainable?
Wool is renewable — sheep are shorn annually and the fiber grows back. It is biodegradable, decomposing in soil and releasing nutrients. It requires no pesticides or irrigation the way cotton does, and sheep can graze land unsuitable for crop farming. The main environmental considerations are land use (overgrazing if herds are too large) and sheep methane emissions, which are a real but relatively modest part of the fiber's footprint. Certifications like the Responsible Wool Standard (RWS) and ZQ Merino address both animal welfare and land stewardship.
Every brand in The Plain Thread directory uses natural fibers. Browse brands that use wool and merino wool.
Browse wool brands