What temperature does to a fibre's shape

Short answer: Heat makes the molecules in a fibre move more, and above a certain limit they can shift places permanently. The fibre then takes on a new shape that cannot be undone. That is why temperature in washing and drying matters more than many people think.

Below the limit nothing lasts

At thirty to forty degrees, the molecules move a little more but settle back into place as the fabric cools. Clothes may feel slightly different while warm and wet, but the shape returns. That is why a cold or lukewarm wash is a safe choice for everything in bamboo sweatwear.

Above the limit the new shape sets

At high heat, the long chains in the fibre can slide past each other and find new positions. When the fabric cools, they stay there, and the garment has shrunk or stretched for good. That is why the dryer costs most on clothes that need to keep their fit, including clothing for women.

Every fibre has its own limit

Fibres react differently to heat. Elastane gives way first, then viscose, while cotton tolerates the most. Blends follow the most sensitive fibre, so a fabric with a little elastane must be treated by the elastane limit. Always read the care label, including on your favourites among the most popular styles for women.

Moisture makes fibres more sensitive

A wet fibre is softer and more mobile than a dry one. Water gets in between the molecules and makes it easier for them to move. So the combination of heat and moisture is the riskiest, as in a hot wash or a dryer full of wet clothes. Dry heat from an iron works in a different way.

Pull and heat at the same time

When wet clothes hang or tumble, weight or movement pulls on the fibres. If they are warm at the same time, they can stretch and set in a longer shape. That is one reason clothes can become longer and narrower after washing. Lay heavy wet items flat to dry so they keep their original shape.

Ironing is the highest heat

An iron is hotter than any wash, and it presses directly on the fibres. Use the lowest setting that works, and iron from the inside or through a thin cloth. Keep the iron moving so the heat does not build up in one spot. Many soft everyday fabrics smooth out on their own if hung neatly after washing.

Signs the limit was crossed

Shrinking is the best known sign, but a changed surface can also show the heat was too high. The fabric may turn stiff or shiny, or lose some of its softness. Waistbands and elastic can go slack and stop pulling back. If you notice this, lower the temperature next time.

A simple low heat routine

Wash at a low temperature, use a gentle programme and let clothes air dry. Give them a light shake when you take them out and gently pull the seams into shape. Avoid radiators and direct sun for drying. That way clothes keep their fit and softness much longer.

Frequently asked questions

What temperature is the limit

It depends on the fibre. Elastane is the most sensitive, viscose tolerates a bit more, and the care label gives the safe limit for that garment.

Can a shrunk item be stretched back

Partly, with moisture and flat drying while you gently ease the fabric into shape, but rarely all the way back to its original size.

Does this apply to ironing too

Yes, and there the heat is higher than in any wash. Use low heat and iron from the inside to protect the fibres.