To dilute black tattoo ink to make shades of gray, try mixing vodka, propylene glycol, glycerin, water and black pigment in various combinations to reach the desired shade. Tattoo ink manufacturers do not have to disclose the contents of the ink, so the dilution solution for each bottle may be different.
If you want to use water to make gray, do so drop by drop; otherwise, by the time you've hit the desired color, the ink may be so thin that the gray will look washed out instead of solid. Iron oxide, carbon, logwood, magnetite crystals, powdered jet, wustite, bone black and soot are typical ingredients in natural black ink. However, the tattoo industry is not regulated by the U.S. Food and Drug Administration, and manufacturers claim their ink mixtures are proprietary. Tattoo artists can therefore never be sure exactly what is in the ink. The oldest tattoo inks were made from ground-up minerals and carbon. Most inks are now made from metal salts, but they can also contain minerals, modern industrial organic pigments and vegetable-based material. Some contain plastic. The inks that glow in the dark or show up in ultraviolet light are particularly risky, because they may contain potentially harmful chemicals.