COMPOSITION OF OFFSET PRINTING INK
Ink is made of three main ingredients: Pigment, which is the coloring material in the ink; Vehicle, which is the liquid that holds the particles of pigment; and Modifiers, which control the drying of the ink as well as other factors such as smell, scuff resistance, and fading.
· PIGMENT: There are two basic types of pigment used in offset printing inks.
Organic pigment, which is made from carbon, is used for making black ink. Inorganic pigments, which are made by mixing various chemicals together, are used for colored inks. For example, sulfur, silica, or china clay can be combined with either soda ash or sulfate salts to make ultramarine blue ink.
· VEHICLE: Vehicle is the liquid that holds the particles of pigment and carries them to the paper. There are two kinds of vehicles used in offset inks: oils such as soya oil or linseed oil (which is a yellowish oil made from flax); and synthetic vehicles, which are liquids resulting from the mixture of chemicals. For example, phenol and formaldehyde mixed together make phenolic resins, sometimes used in printing inks as a vehicle.
· MODIFIERS: Modifiers are ingredients added to the ink to control drying and other qualities such as smell and resistance to fading.
Visual properties of inks are a function of the colorant or pigment, in relation to the vehicle system used. They include color, transparency or opacity, and gloss. By far, the most widely used ink color is black. Then come cyan, magenta and yellow which are used in process printing to create the millions of colors so familiar to us in printed matter. While the physics of color is a highly sophisticated science, in simplest terms color comes from reflected light. White light contains the entire rainbow of colors. When that light passes through a filter or is separated by a prism or raindrop we see the individual colors in the light spectrum. An ink film acts as a filter on the light reflected from the printed surface, e.g., a red ink film allows the red segment of the reflected spectrum to pass through while blocking the rest of the colors. Because printed surfaces vary in color and in reflectance, they, too, will affect the reflected color. Thus, various ink colors printed individually or “trapped” one on top of the other create different filter effects resulting in different visible colors. Similarly, these same ink colors printed on different substrates will result in visible colors that are different yet.