From The Giant: The Definitive Obey Giant Site
Simply, Rubylith is a separable two-layer acetate film of red or amber emulsion on a clear base. It has extensive uses in graphics, most often for color separations by hand in the composition or stripping departments. Red masking film is also often referred to as Rubylith.
The Ulano Corporation invented and owns the trademark on the name Rubylith (although it has become somewhat of a genericized term). Their promotional material states the following:
The Ulano Corporation gained worldwide recognition within the graphic arts industry with the invention and promotion of the first masking film: Rubylith®. Rubylith® is used in every segment of the graphic arts and has gained such prominence that the Rubylith® brand has become synonymous with masking film. Ulano continues to produce and distribute Rubylith® and Amberlith® products to a broad graphic arts customer base, which includes the screen printing industry. Rubylith® is a red masking film, which is suited for use with orthochromatic films. Rubylith® is safe for use with camera speed darkroom films as well as indirect gelatin stencil films and diazo, diazo acrylic, or SBQ sensitized stencil films or emulsions. Rubylith® is primarily used in the camera and plate making operations for offset lithography, flexography, gravure, and screen processes where orthochromatic films and plates are used.
Rubylith® masking film is a light-safe, knife-cut film coated onto a polyester backing sheet. This film can be cut manually or on a plotter to produce masks for film contacting and plate making and film positives for photographic stencil production. Rubylith has an emulsion thickness range of 1.1 - 1.2 mils.
Shepard Fairey began producing framed fine-art Rubyliths around the time of the "Supply and Demand" show at the Merry Karnowski Gallery, which ran from October 9, 2004 to November 6, 2004. The following Rubylith pieces were shown at "Supply and Demand":
Rubyliths were also featured at Shepard's 2005 show "Manufacturing Dissent" at the Black Floor Gallery. Philadelphia Weekly commented on this show, stating, "A series of 16 'Rubylith' images offer a sense of Fairey's work method. These red acetate film cutouts are a byproduct of the screenprint process-they're not prints themselves-and the framed, high-contrast Warholian images have no words or message." The following picture shows the "Manufacturing Dissent" Rubyliths. Although distant, Rubyliths of the graphic used in both Angela Davis and Angela Davis Small, the graphic used in both Slick Rick Red and Slick Rick Blue, the graphic used in LL Cool J Red and LL Cool J Blue, the graphic used in both Joey Ramone Pink and Joey Ramone Red, and the graphic used in Unknown Black Panther can be seen, amongst others.