It wasn't too long ago that tattoos were found exclusively on a very specific group of people, convicts, and researchers from the Department of Forensic Medicine at Jagiellonian University collected a series of Preserved Prison Tattoos to study. Taken from dead inmates, these tattoos were contained in formaldehyde to ensure that no rotting would take place. Very morbid, Beautiful Decay points out that "photography might have been the simpler (and more obvious) solution."
Yet the Preserved Prison Tattoos series is interesting in and of itself. Encased in glass, they don't look like tattoos at all, but another form of hieroglyphs. Considering that crude tools like razor blades, broken glass or paper clips were used with pencil refills, charcoal or crayons mixed with water, fat or urine as ink, that should come as no surprise.