Fabric guide

Shrinking, creasing and ironing are the same thing happening three times

Cotton and linen are both cellulose, and almost everything that goes wrong with them comes down to one mechanism: water breaks the bonds holding the fibre in shape, and they reform wherever the fabric happens to be lying.

Last updated 26 July 2026

The mechanism, once

Cellulose is a long chain of glucose units, and each unit forms hydrogen bonds with its neighbours on the chains alongside it. Individually those bonds are very weak. Collectively they are what makes a cotton shirt hold its shape rather than behaving like a net.

Water is a plasticizer. Water molecules push in between the cellulose chains, break those hydrogen bonds, and let the chains slide past one another. When the fabric dries, new bonds form — but wherever the chains have ended up, not where they started.

That is the whole of it, and it explains three things people normally treat as separate problems:

• Creases — the fabric was crumpled while wet, the chains slid, and the bonds reset in the crumpled position
• Shrinkage — tension built into the yarn at the factory finally gets to release
• Ironing — heat and moisture break the bonds again so the chains can be pressed flat and re-bonded straight

Everything below is that one idea applied.

Cotton does not shrink. The yarn relaxes.

Nothing about a cotton fibre gets smaller in the wash. What happens is that tension put into the yarn during spinning, weaving and knitting — held there for months, all the way through the shop — is finally released.

On first wetting the fibre swells, its diameter increasing by as much as 25%. That swelling lubricates the polymer chains, the built-in mechanical stress lets go, and the fabric contracts to where it would have been if nobody had stretched it.

Which is why the first wash does most of the damage and the fifth barely does any. There is a fixed amount of stored tension and you only get to release it once.

Typical figures from trade sources put untreated cotton at around 3–5% in length and 1–3% in width on a first wash and dry, and untreated fabric can reach about 10%. Treat those as typical rather than precise — they come from industry testing rather than one published standard — but the shape of the number is reliable.

What “pre-shrunk” actually is

It sounds like marketing. It is in fact a specific mechanical process, patented by Sanford Lockwood Cluett in 1930 and still in use.

In sanforizing, the cloth is moistened to around 15% moisture content and fed between a heated cylinder and a thick rubber belt. The belt is squeezed so it compresses and spreads sideways, then allowed to relax — and the cloth, pressed against it, is forced to compress along with it. The warp yarns are physically pushed closer together, and drying locks them there.

The fabric is made to do its shrinking at the factory, mechanically, before anyone cuts a pattern from it.

The dryer is where shrinkage actually happens

Washing wets the fabric. Drying is what sets it.

In a tumble dryer, evaporation is fast and the garment is being tumbled while its fibres are relaxed, so the new and shorter bond network forms quickly and completely. On a line, water leaves slowly and nothing is agitating the fabric, so fibres have time to recover part of the way back toward the geometry they were held in.

Trade lab comparisons put the difference at roughly one to two percentage points of total shrinkage between high-heat tumble drying and line drying after the same wash. Modest, but it compounds over a garment's life.

This is the third time this site has arrived at the same conclusion from a different direction. The energy arithmetic reached it through mechanical wear, the wool article through felting, and now shrinkage. The dryer does more damage than the wash, every time.

Linen creases because it does not stretch

The usual explanation is that linen is more crystalline than cotton. That explanation is wrong, and it is worth saying so plainly because it is repeated everywhere: cotton is also roughly 65–70% crystalline. Crystallinity does not separate the two fibres.

What separates them is elongation at break — how far a fibre stretches before it fails — and the elastic recovery that goes with it.

A flax fibre has almost no give. Bend it past its very short elastic limit and it does not spring back — it sets into the new geometry and stays there. Cotton, with several times the stretch, has room to recover. That is the entire difference, and it is why linen creases if you look at it and cotton merely rumples.

Linen is stronger wet — the opposite of viscose

Flax is commonly cited as around 20% stronger wet than dry. Water swells the fibre and improves how load is shared between the cellulose bundles inside it.

Compare that with viscose, which loses roughly half its strength the moment it is soaked. Both are cellulose. One is at its toughest wet and the other is at its most fragile, which is a useful reminder that “natural fibre” tells you almost nothing about how something will behave in water.

Practically it means you can be firm with wet linen. Wringing it will crease it badly, but it will not tear.

So iron it damp

Now the mechanism pays off.

Ironing works because water breaks the hydrogen bonds so the chains can be repositioned, and heat and pressure hold them flat while new bonds form as the fabric cools and dries. Water does the chemistry. The iron only supplies heat and pressure.

Ironing bone-dry linen therefore does almost nothing. There is no plasticizer present, so the bonds do not release, and you are pressing a crease that is structurally locked in. People conclude their iron is not hot enough and turn it up, which scorches the surface without touching the problem.

Iron linen while it is still damp from the wash, or spray it thoroughly and give the water a minute to soak in before starting. A steam iron on dry fabric is a weaker version of the same trick — surface steam does not penetrate nearly as well as water already inside the fibre. The dots on the iron symbol tell you how hot you may go; for linen it is almost always three.

The short version

• One mechanism. Water breaks hydrogen bonds in cellulose, and they reform wherever the fabric is sitting.
• Cotton shrinks because the yarn relaxes, not because fibres contract. The first wash does most of it.
• Pre-shrunk means mechanically pre-compressed, to under 1% for the trademark. Not zero.
• The dryer sets the shrinkage. Line drying costs a point or two less.
• Linen creases because it barely stretches — not because of crystallinity, which is a myth.
• Linen is stronger wet. Viscose is the opposite.
• Iron linen damp. Dry linen has nothing for the iron to work with.

Cotton and linen are the forgiving end of the wardrobe. Nothing here is irreversible the way felting is — a shrunken t-shirt is still a t-shirt, and a creased linen shirt is one damp press away from being fine. It is worth knowing why, because the same understanding tells you when to stop worrying.