As it happens, the other day I had one of those very common sink-tap accidents which resulted in me being sprayed down the front with water.
That kind of thing, except all over my woolly jumper.
If I’d been wearing almost any other top, I’d have had to change. However, with my woolly pulli, I just brushed off the drops sitting on the surface of the fabric and carried on. Some spray did go right through the fabric and landed on my chest, which was unpleasant, but that water swiftly evaporated with my body heat, right back through the fabric. So I had no excuse but to carry on washing those damn dishes.
This is the kind of thing woollen garments excel in. You can wear your Aran sweater out fishing on a drizzly day, and stay perfectly warm and dry on the inside. Your Melton-weave wool overcoat will see you through moderate to heavy rain, and only require a shake-out on the doorstep when you get home. However, if you fall off the boat into the sea, or are unfortunate enough to walk home in torrential rain, neither is going to help you much (of course, no other fibre will help either – you’re soaked: get changed).
The above shows the structure of a single wool ‘hair’. Off the left-most end is the root of the fibre, where it grows out of the skin. For our purposes, the important bits are the Cuticle, and the Cortex.
Cuticle: The outer surface of the fibre is made up of overlapping cuticle cells, or scales. These have a chemically-bonded waxy coating, which repels water. This allows tap-spray and light rainfall to simply run off the fabric.
Cortex: The cortex is made up of two types of cell with different chemical compositions. One type attracts moisture more than the other, and therefore expands more than the other. This difference in expansion causes the fibre to ‘crimp’ (curl). It is the crimp in wool that allows it to trap air pockets, providing insulation. So, a slightly-damp woolly jumper becomes ‘crimpier’ and warmer.
I’ll mention a third region for completeness: the Matrix. This is mostly hydrophilic sulphur-based proteins, which allow wool to absorb up to 30% of its weight in water. If you get thoroughly soaked, this layer fills up, your woolly garment weighs a ton, and it takes forever to dry out. In an emergency, you can try wringing it out and relying on the cortical crimp to – at least – keep you warm, but the waterproof outer cuticle has been subverted. You need to get somewhere warm, with hot food and dry clothes, stat.