Wool, Silk and the Fibres That Punish a Hot Wash

The points below about washing delicate fibres are ordered by how much difference they make, not by how often they get repeated.

Why wool shrinks and never comes back

Wool fibres have overlapping scales along their surface, rather like roof tiles, which lie in one direction. Heat and moisture open those scales, and agitation causes them to ratchet past one another and lock together permanently.

That is felting, and it is a mechanical interlocking rather than a chemical change, which is why nothing reverses it. The three ingredients are heat, water and movement, and removing any one of them largely prevents the problem. Superwash treated wool has had the scales chemically modified or coated, which is why some woollens are genuinely machine washable.

Washing wool without ruining it

Use cool water, a detergent formulated for wool, and as little movement as possible, which means no rubbing or wringing. Press water out gently between towels rather than twisting, since twisting supplies exactly the agitation that felts the fibre.

Dry flat and reshape while damp, because a wet wool garment hung on a hanger stretches under its own weight and stays stretched. Where a machine has a genuine wool programme, it uses a very low agitation drum action rather than merely a lower temperature. Wool needs washing far less often than cotton, since it resists odour well and airing overnight restores most garments.

Silk, viscose and the plant regenerates

Silk is a protein fibre that dislikes alkaline detergents, high heat and direct sunlight, all of which weaken and dull it. Hand washing cool with a mild detergent works for many silks, though the dye is often the limiting factor rather than the fibre. Viscose and its relatives lose a large share of their strength when wet, which is why they distort and tear more easily in a wash.

On a Sunday evening, those fibres also shrink readily, and garments made from them are frequently labelled for professional cleaning for exactly that reason. Test any of these on a hidden seam allowance first, and accept that dry clean only labels on structured garments usually mean it.

Elastane and the fit that disappears

Elastane, the stretch fibre blended into jeans, sportswear and underwear, degrades with heat far faster than the fibres around it. Hot washing and tumble drying are the two main causes, which is why stretch jeans go baggy at the knee after a year. Chlorine, high heat and body oils all accelerate the breakdown, and none of the damage is visible until the garment stops recovering.

Wash stretch garments cool, spin them gently and air dry them, and they hold their shape for considerably longer. Sportswear also traps body oils that ordinary washing removes poorly, which is why technical fabrics develop persistent smells.

The fibres that tolerate almost anything

Cotton and linen are strong when wet, tolerate heat well and are the reason towels and bedding can be washed hot. Their weakness is shrinkage on first wash and creasing, neither of which is a durability problem. Polyester and nylon are extremely robust mechanically and are damaged mainly by heat, which melts rather than shrinks them.

By the second month, blends behave according to their weakest component, so a cotton garment with five per cent elastane follows the elastane rules. Reading the fibre composition on the label is more informative than the care symbols, because it tells you why the symbols say what they do.

Buy the cheap version first; a month of use tells you whether it deserved better.

Rescuing a mistake

A felted wool garment cannot be restored, though gentle stretching while damp with a wool conditioner sometimes recovers a little size. A stretched knit can often be improved by washing gently and drying flat in the correct shape.

In a small flat, a dye-transferred item should be rewashed immediately without drying, since heat sets transferred dye permanently. A garment shrunk from cotton drying is usually recoverable in length by damp stretching, but only slightly. The honest summary is that most of these are prevention problems rather than cure problems, which is why the label is worth ten seconds.

Everything above, in order of what to do first

1. Why wool shrinks and never comes back. Wool fibres have overlapping scales along their surface, rather like roof tiles, which lie in one direction.
2. Washing wool without ruining it. Use cool water, a detergent formulated for wool, and as little movement as possible, which means no rubbing or wringing.
3. Silk, viscose and the plant regenerates. Silk is a protein fibre that dislikes alkaline detergents, high heat and direct sunlight, all of which weaken and dull it.
4. Elastane and the fit that disappears. Elastane, the stretch fibre blended into jeans, sportswear and underwear, degrades with heat far faster than the fibres around it.
5. The fibres that tolerate almost anything. Cotton and linen are strong when wet, tolerate heat well and are the reason towels and bedding can be washed hot.
6. Rescuing a mistake. A felted wool garment cannot be restored, though gentle stretching while damp with a wool conditioner sometimes recovers a little size.

The takeaway

Heat plus water plus movement is what kills wool, and heat alone is what kills stretch.

Do the ten-minute version tonight rather than the proper version never.