According to Bob Kenworthy of the Chemical Heritage Foundation, blood on clothing is best removed with salt water.

Blood contain hemoglobin which contains iron. The iron gives blood its red color. When salt water is poured on the bloody clothes, the salt displaces the iron in the hemoglobin which causes the red color to fade. Concurrently, the water keeps the blood wet and cold which prevents the hemoglobin from binding to the fabric. With iron, sodium and chloride in an aqueous solution, the blood in the clothes is washed away.

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