The short answer
Fabric conditioner. It works by coating the fibres in a waxy film, which is the same film that stops a towel taking up water. Wash them hot with a cup of white vinegar and no detergent to strip it off.
- Run the towels on a hot wash on their own, with about 250ml of white vinegar in the drum and no detergent.
- Run a second hot wash with a half cup of bicarbonate of soda and again no detergent.
- Tumble or line dry as usual.
- Then stop using fabric conditioner on towels. They will be stiffer and they will dry you properly.
What will ruin it
Do not put vinegar and bicarbonate of soda in the same wash. They neutralise each other and you get neither effect. Two separate cycles.
Why it works
Fabric conditioner is a cationic surfactant that deposits a thin water-repellent layer on the fibre, which is what makes fabric feel soft. On a towel, whose entire job is to absorb water, that is working directly against you.
Household chemicals can be dangerous in combination. Never mix them, keep the room ventilated, and test anything on a hidden patch first. Full disclaimer.
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