Gray wash techniques originated in prisons in the 19th century. It was difficult for inmates to get colored tattoo ink, so they used variations of black ink, from dark black to light shades of gray, to enhance tattoo designs. It carried over into mainstream tattooing and became a popular style for many tattoo artists in the 1970s. Artists typically learn gray wash techniques through apprenticeships.
Gray wash techniques are typically used by tattoo artists in portrait tattoos or other realistic images. Skulls, animals and flowers are also common gray wash designs. It's also commonly used in the case of biomechanical tattoos, which is a style of tattoo that depicts living robotics. Gray wash tattoos don't feature any other colors, just different shades of black.
Tattoo artists can either purchase pre-formulated gray wash inks from tattoo ink suppliers or make their own shades of gray. Those who choose to make their own inks will need to mix glycol, vodka, glycerin and black pigment to create their desired shade. Purchasing it from companies is often the easier, cheaper option. Gray wash colors can typically be purchased in at least three different shades of gray.
According to How-to-Tattoo.com, the most effective way of shading a tattoo is with a tattoo machine. These machines use a group of needles, arranged on a bar, to rapidly push needles in and out of the subject's skin. One of the major benefits to using a tattoo machine is that the depth of the needle punctures can be controlled. This is important because it can help the artist control the shade coloring.