What should I use on a greasy mark on a wool jumper?

Why the usual answer works

Washing soda is a genuinely effective degreaser. It attacks fat by saponification, converting it into a soap that dissolves and rinses away, which is why it is sold for heavy greasy soil and why it appears in laundry boosters. On a greasy mark it does exactly what it is bought to do.

Usually recommended: Sodium carbonate.

Why it must not be used here

Wool is a protein fibre, and the mechanism that converts fat to soap also hydrolyses protein. A strong alkali therefore attacks the garment on the same chemistry it uses on the stain, and the damage is to the fibre rather than to its appearance — it is not a mark that can be cleaned off afterwards. The wool body approaches the same problem from the other end, warning that methods primarily aimed at garments made from cotton or synthetic fibres are not all suitable for wool, and specifying a dilute wool detergent in TEPID water rather than anything stronger or hotter.

This damage cannot be undone by cleaning. There is nothing left on the surface to remove.

More about wool and grease.

What to do instead

Change the mechanism from saponification to dissolution, and respect the order. The wool body's instruction for oily or greasy stains is that they are best treated with SOLVENTS FIRST, and that the solvent MUST BE ALLOWED TO EVAPORATE before any water-based treatment is subsequently carried out. That waiting step is the part most likely to be skipped and it is stated explicitly. The water-based stage that follows is a dilute solution of an approved wool detergent in tepid water — dilute rather than strong, tepid rather than hot. Technique carries the rest: scoop or blot the excess quickly with white tissue or an absorbent towel, work from the edge of the mark towards the centre, and do not rub. Where the mark survives that, the honest next step is professional cleaning rather than a stronger alkali.

• Solvent wiping — Dissolving an oily soil and letting the solvent evaporate. The answer where water would leave a mark or cause harm.
• Wool detergent — A mild, near-neutral detergent without enzymes or bleach. What the wool industry body recommends, and what it recommends against is the more useful half.
• Blotting — Lifting a spill straight up into an absorbent cloth rather than working at it. The opposite of what almost everyone does, and the trade bodies are explicit about why.
• Absorbent powder treatment — Lifting an oily mark out of a porous surface with a powder rather than dissolving it with a liquid. The gentlest route on anything water would ruin.

This is a case where a specialist is a reasonable answer. Restoring a surface is a different trade from cleaning it, and continuing to try cleaning products on damage generally makes the eventual restoration harder.

The chemistry behind both halves

• pH in cleaning — Why the acid or alkali choice decides both what a cleaner removes and what it damages. The two are the same property seen from opposite sides.
• Material compatibility — The reason a useful answer is about the surface rather than about the product. The same agent is the right answer and the wrong answer depending on what it lands on.
• Classifying a stain by its origin — The stone trade puts identification before treatment, because the origin decides the mechanism — and three origins that look similar need three different answers.