Fabric guide
Wool does not shrink. It ratchets.
What happens to a jumper in a hot wash is not shrinking, and calling it that hides the reason it cannot be undone. The real mechanism is mechanical, one-way, and it explains every rule about washing knitwear.
Last updated 26 July 2026
Scales, like shingles on a roof
A wool fibre is keratin — the same protein as hair and fingernails — and its surface is covered in overlapping cuticle scales arranged like tiles on a roof. The scales are directional: a rough protruding edge facing one way, a smooth overlapping edge the other.
Put two wool fibres together in warm water and move them, and the scale edge of one locks into the gap between scales on the other. Textile researchers describe it as a ratchet: the scales work as one-way barbs, letting fibres slide past each other in one direction while blocking the return.
Agitate a garment full of fibres doing this and they migrate steadily inward, tangle, and cannot get back out. The fabric gets denser and smaller. That is felting, and the reason it is permanent is that there is no force you can apply to unhook several million microscopic barbs at once.
This is worth separating from what happens to cotton. Cotton shrinks because tension in the yarn relaxes — a one-off release of stress built in at the factory, which is why it happens on the first wash and then stops. Wool has no such limit. Every wash can felt it further, because the barbs are always there.
A review of shrink-resistance methods puts the scale of it plainly: untreated wool garments can shrink to an average of 55% of their original dimensions after the first laundering. Half the jumper, in one wash.
The counterintuitive part: wet wool felts harder than damp wool. Water swells and softens the scales so they lie flatter against the fibre, which lets more of them make full face-to-face contact. Soaking does not lubricate the problem. It arms it.
Three ingredients, and the good news hidden in them
Felting needs moisture, heat and mechanical agitation together, with alkalinity speeding it along. Remove any single one and the process does not start.
That is worth sitting with, because it means water on its own is not the enemy. Cool water with no agitation is safe. A wool jumper can be submerged, gently pressed, and lifted out with nothing happening to it at all. What destroys knitwear is the drum — and the tumble dryer, which supplies heat and agitation simultaneously and is named in the research as a felting condition in its own right.
If you take one thing from this page: the danger was never the water.
The detergent nobody warns you about
Biological detergents contain protease — an enzyme whose entire job is breaking down protein. It is in there to digest egg, blood, milk and sweat, and it is very good at it.
Wool is protein. So is silk. The enzyme has no way to tell a protein stain from a protein fibre, and it does not try.
What makes this genuinely nasty is the timing. Nothing appears after the first wash. Protease weakens the fibre gradually across repeated washes, and what eventually shows up is a scatter of small, random holes — weeks later, long after anyone would think to blame the detergent. Peer-reviewed work on protease and protein fibres describes exactly this pattern of severe cumulative damage.
Use a wool wash, a non-bio detergent, or plain soap. Anything advertising stain-digesting power is the wrong bottle — which is also why the enzyme pre-treatment that works on a protein stain must never go near a wool or silk garment.
A note on our own record here: the wash temperature article retracted a claim that detergent enzymes stop working above 40 °C. That was wrong and it stays retracted. Enzymes really are a wool problem — just an entirely different one, and this is it.
Why some wool goes in the machine anyway
Superwash wool exists because the ratchet can be disabled. The standard treatment, developed in the 1960s and still the common one, works in two stages.
First, chlorination at very low pH strips the fibre's natural lipid layer and etches the scale edges down. Then a polymer resin is applied at a couple of percent by weight, which swells and fills the gaps between the remaining scales. The barbs are blunted and the sockets are filled, so there is nothing left to catch.
That is a real trade-off rather than a scandal. It is why superwash feels slicker, takes dye differently, and why some knitters avoid it — the fibre has been chemically altered to behave like something else. If your label says superwash or machine washable wool, believe it; the treatment works. If it does not say so, assume the scales are intact.
Cashmere: finer, shorter, and it will pill
Cashmere is not better wool. It is finer and shorter wool, and both cut in two directions at once.
| Cashmere | Fine merino | |
|---|---|---|
| Fibre diameter | ~13–19 µm | ~17–19.5 µm |
| Staple length | ~28–42 mm | ~50–120 mm |
Fineness is what makes cashmere feel the way it does. Shortness is what makes it pill. A shorter staple means more fibre ends per centimetre of yarn, and every loose end is a pill waiting to happen. Cashmere also has less crimp than merino, which produces the drape people pay for and simultaneously makes the fibre less able to resist friction.
So pilling is geometry, not a sign you were sold something bad. Staple length is the honest quality signal — cashmere spun from fibres under about 30 mm pills aggressively — but some pilling is inherent to the material. Comb the pills off and the garment settles down as the shortest ends work their way out.
The best thing you can do is wash it less
Wool is hygroscopic: it absorbs water vapour into the fibre rather than letting it sit on the surface, holding a substantial fraction of its own weight in moisture. Drier skin means less for odour-producing bacteria to work with, and odour molecules that do form are absorbed into the fibre and held there until washing.
This is the exact opposite of what happens in polyester, which repels water but soaks up the skin oil that bacteria actually feed on — and it is why a merino base layer and a synthetic one behave so differently on the same body.
Woolmark — the wool industry's own promotional body, which is worth knowing when weighing the number — publishes New Zealand research using thirteen trained assessors that found wool retained 66% less body odour intensity than polyester and 28% less than cotton. Industry-funded, but consistent with the underlying physics.
The practical version needs no study. Air a jumper overnight on a hanger and it will be fine. Most knitwear needs washing a handful of times a season rather than after every wear, and every wash you skip is a wash that cannot felt it.
When you do wash it
The label will usually show a wash tub with two bars under it, or a hand in the tub — both mean drastically reduced mechanical action. In practice that comes down to the following, and the hand-washing method has its own guide.
- Cool water, by hand, in a basin. Or the wool programme — which works by soaking rather than agitating, and holds far less than the cottons cycle for exactly this reason.
- Wool wash or non-bio. Never biological.
- No temperature shock. Wash and rinse water at the same temperature; a sudden change is its own stress.
- Press, never wring or rub. Rubbing is the ratchet in action.
- Roll in a towel to take up the water, then dry flat. A wet jumper hung up will stretch permanently under its own weight.
- Never tumble dry unless the label explicitly permits it.
The same method covers most things labelled hand-wash — including a fair amount of what says dry clean only, because an unstructured knit has nothing inside it to come apart.
The short version
- It felts, it does not shrink. Microscopic barbs interlock and cannot release.
- Moisture, heat and agitation together. Remove one and nothing happens.
- Water alone is safe. The drum and the dryer are the risk.
- Biological detergent digests wool, slowly, and the holes show up weeks later.
- Superwash has had its scales etched and filled. That is why it survives the machine.
- Cashmere pills because its fibres are short, not because it is cheap.
- Air it instead of washing it. The most effective thing on this list.
Felting is one of the few laundry mistakes with no remedy at all. Stains come out, creases fall out, dye can sometimes be rescued. A felted jumper is a smaller, denser object that used to be a jumper, and the only real defence is understanding what it takes to start the process — and then simply not doing it.