The short answer
Rinse it under cold running water from the back of the fabric, then soak in cold salted water. Never use warm water, at any stage.
- Rinse from the back, pushing the stain back out the way it came in rather than through the weave.
- Soak in cold water with a tablespoon of salt for half an hour for anything set.
- Then wash cold with ordinary detergent.
- For a mattress or something that cannot be rinsed, make a paste of cornflour and cold water, spread it over the mark, let it dry completely, and brush it off.
What will ruin it
Warm water cooks it in. Blood is a protein, and protein coagulates with heat. Warm water at any stage will set the stain into the fibre and no amount of scrubbing afterwards will lift it.
Why it works
The same reason an egg white goes from clear to white in a pan. Proteins unfold and bond irreversibly when heated, and once that has happened in a fibre the stain is part of the fabric rather than sitting on it.
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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