I mix a tiny bit of my blood in the paint ...
I always tend to cut myself when I do art and craft projects, and when I do, I drip a bit of blood on images of the passion of Christ. Just a little. It dries brown. One painting even has some hairs from my beard embedded. Once, doing a wood cut and printing it on linen and silk, I cut myself really bad and most of the pigment is my blood. I still have the block and perhaps on a very hot day, I can use my sweat to do a print - just to see if aging in the sun would cause it to discolor and leave an imprint?
So anyway. Must get busy painting the icon I'm working on.