In my story my characters find an altar belonging to an ancient culture that did large animal sacrifices on important dates. The altar is on a platform in what used to be a town square and has not been overgrown with plants yet. The climate is hot and dry but is near a confluence of rivers.
I want the staining to be visible to the naked eye ~4,000 years after the last sacrifice was performed. Is this even possible? If ~4,000 years is too long, how long COULD the stains be visible?
I can pick the best kind of stone for it. I can add a roof if need be, but it should be open on the sides because the sacrifices were public events.
I don't know how to google this. I googled variations "ancient blood stains archaelogy" and "ancient blood stains maya" and got a lot of scientific results about testing for proteins and such, but nothing about whether, under ideal conditions, any staining could be visible to the naked eye.