Crimp
Also searched as: fibre crimp, wool crimp, staple crimp, crimp frequency, fibre curvature, waviness
The natural or induced waviness along a fiber's length, expressed as crimp frequency or curvature, which affects bulk, elasticity, fiber cohesion during processing, and hand.
In detail
In wool, crimp originates inside the fiber. Fine wools tend to have a bilateral cortex in which two types of cortical cell, ortho and para, lie in distinct halves. The two differ in protein composition and respond differently to stress, so the fiber curves, with the orthocortex on the outer side of each wave. Coarser fibers, whose cortical cells are mixed less distinctly, tend to show less crimp. Manufactured staple fibers are smooth as extruded, so producers add crimp mechanically or build it in by combining polymers that shrink differently. Crimp helps fibers cling together in carding and gives yarns loft and resilience.
The science and numbers
Wool crimp is associated with the orthocortex and paracortex: the paracortex is richer in high-sulfur and ultra-high-sulfur keratin-associated proteins, while the orthocortex carries more of the glycine- and tyrosine-rich families, and differential stress relaxation between the two halves can generate additional crimp. Laboratories measure mean fiber curvature on short snippets with instruments such as Laserscan or OFDA, reported in degrees per millimeter; means range from over 100 degrees per mm in fine Merino wools to below 40 in coarse crossbreds, and curvature correlates well with staple crimp frequency. For manufactured staple fibers, ASTM D3937 measures crimp frequency on single fibers, fiber chips or projected images, treating crimp as a two-dimensional sine wave, and the standard warns that between-laboratory precision is poor.
A practical example
Two lots of wool with similar mean diameter are spun on the same system. The lot with higher measured curvature gives a bulkier, springier yarn at the same count, while the lower-curvature lot gives a denser, smoother yarn. The class uses the comparison to show why wool testing laboratories report curvature alongside diameter, and why a spinner planning knitwear or worsted suiting might prefer different lots.
What to distinguish
Fiber crimp is distinct from yarn twist, from texturing of synthetic filaments, and from the crimp of yarns bending over one another in a woven fabric, which is a separate use of the same word. Visible staple crimp counted along a lock correlates with instrument curvature but is not identical to it. Higher crimp is not automatically better: it suits bulky, lofty products more than smooth, lean cloth.
Origins and history
The everyday sense of crimp, to press into small folds or waves, carried over naturally to wool, where classers long judged fleeces by counting the waves along a staple. The microscopic cause was traced in the twentieth century, when researchers related the orthocortex to the convex side of the crimp wave, including a Nature study of double-crimped wool. Instrument measurement of curvature on optical and laser fiber analyzers later gave the trade an objective alternative to visual crimp assessment.
Related terms
Sources & further reading
- Nature: Relationship of ortho-cortex to the crimp wave in a double-crimped wool
- AWTA: Fact sheet, adjustments to fibre curvature measurement
- ASTM International: D3937 Standard test method for crimp frequency of manufactured staple fibers
- Proteins (Wiley): Wool fiber curvature is correlated with abundance of K38 and specific keratin-associated proteins
Technical references reviewed 2026-09-22. Examples are illustrative. Industry organizations and manufacturers describe their own fields; their references are not independent product endorsements. Figures are approximate and depend on the stated test conditions.