Why does wool shrink and felt, and can it be undone?

People treat a shrunk jumper as one problem with one fix, and it is not. Cotton that has shrunk has relaxed, and relaxation can be pulled back out. Wool that has shrunk has usually felted, which is a structural change to the fabric, and the soak-in-conditioner method that circulates online only helps the small number of cases where the wool has merely tightened rather than matted. Knowing which you have takes ten seconds and saves an hour.

Step by step

1. Look at a wool fibre the way a microscope does.The shaft is wrapped in overlapping scales, called cuticle cells, arranged like roof tiles from root to tip. Run a fibre between your fingers one way and it slides; the other way it catches. That directional friction is the whole story.
2. Add warm water and detergent.Water and alkali make the scales swell and stand out from the shaft, so they engage with neighbouring fibres far more aggressively than they do when dry.
3. Add agitation.Every push and pull moves a fibre relative to its neighbours. Because it can only travel tip-first, each cycle moves it further in and never back out. This is a ratchet, and it is why the process is one-way.
4. Watch what that does to the fabric.Fibres migrate into each other until the yarns lock together and the spaces between the knitted loops close. The fabric becomes thicker, denser, smaller and stiffer, and the stitch definition disappears.
5. Tell felting from ordinary shrinkage.Felted wool has lost its stitch pattern and feels like a solid sheet rather than a knit. Shrunk-but-not-felted wool still shows its stitches clearly and simply feels tight. Only the second one is worth attempting to rescue.
6. Understand why the reversal attempts fail.Soaking in conditioner or baby shampoo lubricates fibres and relaxes the yarn a little, which recovers tension shrinkage. It cannot un-tangle fibres that have physically migrated into one another. There is no product that does.
7. Take the prevention from the mechanism.Cool water, minimal movement, no rubbing, no wringing, and never a rinse at a different temperature from the wash — a sudden temperature change on its own can shock wool into felting.

Tips

• This is the same process, deliberately applied, that makes felt hats, tennis-ball covering and industrial felt. It is a manufacturing technique, not a fault in the wool.
• Superwash wool has had the scales stripped or coated so they cannot ratchet, which is why it can go in a machine. If a garment says machine washable wool, that is what it means.
• Silk is also a protein fibre but has no scales, so it does not felt. It is damaged by heat, alkali and enzymes instead.
• A felted jumper is not wasted. It cuts without fraying, which makes it good material for slippers, cushion covers, pot holders and patch material.

Common mistakes

• Rinsing wool in cold water after washing it in warm — The temperature shock alone can trigger felting even without much agitation. Wash and rinse at the same temperature.
• Wringing a wool garment out — Wringing is concentrated agitation applied to soaking wet warm wool, which is the exact recipe. Press the water out between towels instead.
• Trusting the wool cycle on an old machine — A true wool cycle barely moves the drum. Some older machines simply run a short cool cycle with normal agitation, which will felt a jumper.

Questions people ask