Short answer
Wool and silk are protein fibres whose charged chemical groups hold acid dyes strongly, so they dye to deep, clear colours at modest temperatures. Silk adds lustre: its smooth fibres with a roughly triangular profile throw directional highlights, so its colour changes as the fabric moves. Both are sensitive to light and heat, which yellow cream and white wool and weaken and yellow silk; perspiration also yellows silk. How much and how fast depends on exposure, the dye and how the fibre was processed.
Relative to other materials in this section, not a measurement of a particular product.
| Process | What you see | Driven by | Slowing it |
|---|---|---|---|
| Photo-yellowing and weakening | Cream whites turn yellow; silk splits | Light and heat attacking the protein | Low light levels; avoid storing silk in sun |
| Perspiration yellowing | Yellow underarm and collar marks on silk | Sweat reacting with the fibre | Prompt, gentle washing |
Wool (keratin) and silk (fibroin) are proteins with amino and carboxyl groups along their chains. In an acidic dyebath the amino groups carry a positive charge that attracts negatively charged acid dyes, so the dye is drawn into the fibre and held by ionic attraction plus weaker forces. This is why wool and silk can be dyed to saturated, even colours, and why the same acid dyes also suit nylon, a synthetic polyamide with similar groups. Historically, mordant dyes — natural dyes fixed with metal salts such as alum, iron or chromium — were used on wool, and the mordant changed the colour: iron 'saddens' colours towards dull greens and browns. Reactive dyes for wool exist and give better wash fastness in machine-washable wools.
Wool is usually creamy white, but some breeds grow black, brown, grey or mixed fleeces, used undyed in natural-colour yarns and in heathered blends. Because most wool is slightly cream, pure white wools are bleached, and pale dyed wools inherit a warm undertone. Cultivated mulberry silk is naturally off-white once degummed (the sericin coating removed), while wild silks such as tussah are honey to brown and harder to dye, as mineral deposits on the fibre block dye uptake. The fibre surface matters to the colour: wool's scaly cuticle scatters light and gives a soft, matt depth, while lustre wools from long-wool breeds and mohair have flatter scales and a shinier look.
Mulberry silk fibres are smooth and roughly triangular in section with rounded corners, and their flat faces act as small reflectors. In a satin or charmeuse weave, where long floats of filament lie in one direction, those reflections line up, so the fabric shows bright bands where it faces the light and deep, saturated colour where it turns away. That is the same principle as gloss on a hard surface, with a directional twist: the colour depends on how the fabric is draped and cut. Garments cut with pieces in different directions show panels of different shade. Raw silk and noil, with shorter, rougher fibres, scatter more and look matt and paler.
Protein fibres are among the more light-sensitive textile materials. Light and heat yellow cream and white wool — a problem for pale carpets, knitwear stored in sun and museum textiles — and prolonged exposure weakens silk until it splits, which is why historic silk is displayed at low light levels. Many dyes used on wool and silk also fade. Perspiration yellows silk and can alter dye colour locally; chlorine bleach damages both fibres severely. In storage, wool attracts clothes moths and carpet beetles, whose damage shows as bare patches and colour loss. Keeping light low, washing gently and storing clean and dark are the main protections.
Why: Photo-yellowing of the protein fibre by light and warmth.
Fix: Keep pale wools out of direct sun; yellowing is largely permanent.
Why: Pieces cut with the lustrous weave running in different directions.
Fix: Cut all panels in one direction ('with nap').
Why: Perspiration reacting with the fibre and dye.
Fix: Wash promptly and gently; avoid antiperspirant contact and high heat.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactModerate evidence
Protein fibres such as wool and silk, and the synthetic polyamide nylon, are dyed with acid dyes.
Source: Dyeing
FactModerate evidence
Mulberry silk fibres have a roughly triangular cross-section with flat surfaces that reflect light at many angles, giving silk its sheen; silk is weakened by prolonged sunlight and yellowed by perspiration.
Source: Silk
FactModerate evidence
Wool is generally creamy white, although some breeds produce black, brown, grey or mixed-colour fleeces; the fibre has a cuticle of overlapping scales over a cortex.
Source: Wool
ConventionStrong evidence
Conservation guidance recommends about 50 lux for textiles on display because many textile dyes and fibres are highly light-sensitive.
Source: Agent of deterioration: light, ultraviolet and infrared
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.