ink used to form monochromatic or multicolored images on paper, cardboard, fabric, sheet iron, and other materials in publishing and for packaging and marking. They are colored, homogeneous viscoplastic pastes or liquids consisting of a fine stabilized dispersion of a pigment (sometimes a dye solution) in a binder.

Printing inks are applied by printing presses of various designs at a rate of up to 40,000 impressions per hour. The ink is transferred to the elements of the printing plate, and under pressure of the printing cylinder it is transferred to the printed material. The thickness of the layer of ink on an impression depends on the printing method; it is usually 0.8–60.0 microns. Depending on the conditions of color application and fixing, the nature of the printing plate, the speed of printing, and the properties of the printed material’s surface, printing ink should satisfy certain requirements. These include structural and mechanical requirements (viscosity, elasticity, adhesion to the rollers and plate, degree of transference to the paper, and ability to thicken at rest and liquefy upon mixing), optical requirements (hue, brightness, luster of the impression, light-resistance, and transparency, which affects the production of mixed tones in multicolor printing), and requirements as to the degree of dispersion of the pigment in the binder, and the ability to adhere to the impression (because of selective absorption in the pores of the paper, oxidizing polymerization, or other chemical transformations of the binder, and also evaporation of a highly volatile solvent).

The selection of pigments for printing inks is determined by the variety of conditions of the printing process and by the demands made on the finished product. Carbon, channel, or furnace black is used for black printing ink, with an added undercolor of dark blue and violet pigments or oil-soluble dyes to strengthen the blackness and improve the hue of the impression; a wide assortment of organic and inorganic pigments is used for white and colored inks. Substances used as binders include natural, semisynthetic, and composition drying oils; oil varnishes; and volatile solvents.

Printing inks are classified according to the printing method for which they are intended. They are subdivided into groups according to the construction of the printing press, the nature of the work (lustrous, two-tone, cartographic, and so on) and the grade of paper. Each ink has its own commodity number depending on its purpose; the number also indicates the color of the ink and the possibility of its use for multicolor printing.