Is it possible to dye leather/cloth with blood without it fading, perhaps by using a sealant of some-kind?
Note: This would be in an apocalyptic world where advanced chemical compounds would have long expired.
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Fresh blood has a bright red color, so if you would dip a cloth in fresh blood, it would get the same bright red color.
But then, as the enzymes and proteins present in the blood would start interacting with oxygen, it would turn to a brownish tint.
Anyway, this is what can be found online:
What you want to do is basically the opposite of what one is supposed to do for blood stains that you want to wash out. You should cook the blood into the fabric, and then let it age for some time.
Heat will denature protein. The denatured protein loses its shape and tangles around the other molecules of denatured protein. A good way to apply the heat would be to spread the fabric out, as much as possible, and bake it. Perhaps you could air-dry the blood on the fabric, then wrap the fabric around layers of crumpled unprinted newsprint paper, to allow air between the layers, and bake it in the oven, on a very low heat, for several hours.
I would advise you to not wash the fabric any more than you have to. Blood cannot be used as a true dye, but it can stain the fabric very effectively. Repeated washing will cause it to fade and gradually wash out, however. Rinse no more than you need to to get the artwork to look the way you want it to.
It is traditional in Japan to use freshly made soymilk as the binder for hand-painted earth oxides on fabric. The soymilk, of course, like blood, contains mostly protein. (Blood also contains some iron.) Treating your blood like the soymilk in the iron oxide painting recipes would probably be a good idea. Do NOT wash the fabric for several months, if possible. The longer the soymilk ages, the more permanent and washable the fabric designs become, though it should never be machine-washed or used for clothing that must be washed frequently; the same might be true of blood, as well.