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.

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Will cotton wool shrink up at all when I wash it?

Cotton yarn expands when washed. It lacks the little scales that wool has, which can interlock causing shrinkage. So that fitted cotton sweater you bought will most likely become a baggy sweater after the first wash.

A blend of cotton and wool will, on balance, retain its shape – the cotton expanding and the wool shrinking should cancel each other out, although a lot depends on the relative composition.

Cotton wool pads – the kind of thing you take your makeup off with – will appear to shrink, but that’s only because they’re floofed up with air in the package. As soon as they get damp – with water or makeup remover – the air gets removed, and you get a flat pad of cotton.

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Why do clothes made from cotton feel softer than those made from wool?

Because you’re weak and soft.

Wool comes in hundreds of different qualities, some suitable for next to the skin, others better suited to outerwear. For thousands of years, people wore wool – either as fabric or fleece – from swaddling to shroud, with none of this crybabying about thquatching their thoft dewicate thkin. You got used to it, or you scratched.

Nowadays, people haven’t the skill or knowledge to select the right quality of wool for the purpose, and are too precious to give themselves time to get used to wool. They pronounce themselves ‘allergic’ (only a very tiny proportion are allergic to wool; ETA: Claire Jordan reminds me that more people are allergic to lanolin, the oil in sheep’s wool – that one is nasty), and never wear wool again.

Here’s an experiment. Grab a cotton wool ball or a face flannel, and scrub it, dry, over your skin. Or actually look when you’re towelling off after a bath. They all scrape your skin. In the case of the towel, you might well see what looks like dandruff flaking off your body as you dry. You’ll probably need to slap on a load of moisturiser, because the cotton strips the oils off your skin as well, adding to the flakiness.

The only reason cotton “feels” soft, is because the specific cotton fabric in your clothes has been chemically and mechanically treated to feel soft. Untreated cotton sandpapers the top layer of your skin off.

Quora linky.

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