Category: Crafty Science

How does wool keep you warm when you are wet?

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).

From: Wool fibre structure and properties

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.

Quora linky.

If I dye cotton fabric with food coloring, when washed does the dye fade out evenly, or would it be splotchy?

You can’t dye cotton with food colouring.

I say dye in bold, because dye means something quite specific. It means applying a colourant, possibly with other chemicals, such that the fibre may develop a hue as intense as the colourant, and that the hue should not wash out, fade, or otherwise greatly change in intensity over the course of its use.

It is, on the other hand, entirely possible to stain cotton with food colouring – either the E-numbers type or the vegetable-extracts type, or indeed ketchup, red wine, blood, grass, and all those other annoying stains that Stain Devils were developed to counteract. But stains wash out, fade, and greatly diminish in intensity. They may not disappear entirely – I’m sure you have plenty of cotton dishcloths which have resisted detergents, bleach and boil-washes – but they do fade to indeterminately-coloured marking:

You can dye animal fibres and some synthetic fibres with a mixture of food-colouring (only the E-numbers type, though) and an acid such as vinegar or citric acid, and heat. For more information, see here: Thorn Maiden Dyeing.

The reasons for this lie in chemistry. Animal fibres are mainly proteins, which react well with acids and form a variety of bonds with the dye molecules in food colouring. On the other hand, plant fibres such as cotton are cellulose, with a waxy coat.

Diagrams and close-up photo showing the physical structure of plant fibres, notably the waxy outer cellulose which resists dyeing.

The waxes keep the dye acid out, and, while it is possible to get cellulose to react with (some, strong) acids, the result is glucose (it is a polysaccharide, after all) – and its reaction with vinegar (acetic acid) is very, very slow, and very, very weak.

Your gut should tell you this: your stomach contains hydrochloric acid, which tears through a steak, but passes wheat fibre undigested.

Quora linky.

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