Faint letter marks can be made visible using

Tattoo marks Tattoo marks are designs made on the skin by multiple punctures with needles dipped in coloring agents. The designs are permanent because the coloring material goes under the epidermis and stain the dermis. Dyes commonly used are indigo, cobalt, carbon, Vermilion, Prussian blue, and Indian ink. The tattoo marks are permanent and persists for many years. After 8 to 10 years it may fade and a faded tattoo can be made visible by ultraviolet lamp or infrared photography. Ref: FORENSIC MEDICINE AND TOXICOLOGY Dr PC IGNATIUS THIRD EDITION PAGE NO 101