Medulla
Also searched as: medullation, medullated fibre, kemp, hair core, objectionable fibres
The central core of some animal fibers, made of loosely packed cells and air spaces, found in coarse wool, many hair fibers and kemp but usually absent from fine wool.
In detail
Inside the cortex of certain fibers runs a medulla, a network of hollow, air-filled cell walls that may be continuous, interrupted or fragmented, and that in some cases collapses into a hollow tube. Medullated fibers tend to be coarser and less dense than unmedullated fibers. The many air and protein interfaces scatter light, so heavily medullated fibers look chalky white and stand out against dyed neighbors. Kemp is a short, thick, brittle fiber type in which the medulla fills most of the width; its stiffness makes it prickly, and manufacturers treat highly medullated fibers as a quality problem in apparel wool.
The science and numbers
In kemp, the medulla occupies more than about 60% of the fiber diameter. Because medullated fibers contain air, they are less dense than solid fibers, and the refractive mismatch at air and keratin boundaries raises light reflection, producing the chalky look. Measurement methods include projection microscopy, in which medullated fibers are classified and counted while diameters are measured (IWTO-8, with ISO 2647:1973 as the ISO method for wool), and IWTO-57, which determines medullated fiber content from opacity measured on an OFDA instrument. Demand for rapid methods came partly from the South African mohair industry, which needed to detect objectionable medullated fibers at a few per 10,000, and from New Zealand crossbred wool assessment.
A practical example
A mill dyes a navy knit from a crossbred wool lot and finds scattered pale, stiff fibers on the fabric face. Testing the retained raw wool reveals a small percentage of heavily medullated fibers. The class traces the fault back to raw material selection and notes that, in dark shades, even a few objectionable fibers per ten thousand can be visible, which is why medullation is tested before purchase.
What to distinguish
A medulla is not automatically a defect. Medullation is common in alpaca and llama fleeces, where researchers distinguish visually objectionable from non-objectionable medullated fibers. A medulla also does not identify a species on its own. Keep kemp and guard hair distinct: guard hair describes a coat's coarse outer fibers, while kemp is a highly medullated, brittle fiber. The medulla of animal fibers is also unrelated to the lumen of plant fibers.
Origins and history
Medulla is Latin for marrow or pith, the same word applied to bone marrow and to the soft center of plant stems, and it was extended to the fiber core by analogy. Wool classers and carpet makers have long dealt with kemp and medullated fibers. Standardized measurement took shape with the ISO projection microscope method published in 1973, and later international round trials led to IWTO-57, which uses optical fiber diameter analyzers to measure medullation more rapidly.
Related terms
Sources & further reading
- SGS Wool Testing Services: Info bulletin 5.4B, medullation
- ISO: ISO 2647:1973, Wool, determination of percentage of medullated fibres by the projection microscope
- SGS Wool Testing Services: OFDA (Optical Fibre Diameter Analyser) bulletin
- PubMed: Hair medulla morphology and mechanical properties
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.