Printing inks are categorized according to the process for which they are intended, and, within each category, they may be further divided according to color and categories of quality. Letterpress inks, for example, include colored inks and black inks (halftone ink, jobbing ink, ornamental printing ink, rotary printing ink and special ink). There are also newspaper inks, lithographic printing inks (collotype ink, litho ink and offset ink), photogravure inks (either actual photogravure ink or copperplate ink) and die stamping ink, aniline printing ink, and special inks of various kinds, e.g., carbonizing ink.
Printing inks are also categorized according to the manner in which they dry, because rate of drying is considered to be one of the most important properties of an ink. Lithographic and ordinary letterpress inks dry by oxidation, and partly by penetration and evaporation; newspaper ink dries by absorption; and aniline and photogravure inks dry by evaporation.
Modern inks which dry on contact with the paper have been perfected, and are used principally on high-speed rotary presses. A recent development in inks is the monomeric ink which dries instantaneously when exposed to certain radiations, such as ultraviolet light or gamma radiations. This type of ink is designed to be used on ultra-high-speed presses. There are also heat-set inks which dry when the paper is passed through a heating chamber at a temperature of about 300° C. (infrared radiation is also used for heating). Cold-set inks, which are solid at room temperature, and which must be heated for printing, dry when the paper is conveyed, following printing, over a cooling cylinder. Steam-set inks, which consist of artificial resins dissolved in a hygroscopic solvent, such as ethylene glycol, dry when the paper web is passed through a steam chamber, where a small quantity of water is absorbed by the layer of ink causing the artificial resin to be deposited in solid form. Another type of quick drying ink is one which is combined with a vehicle which ordinarily solidifies at room temperature but is prevented from doing so by an admixture of substances. In printing, the admixtures are absorbed by the paper and the impression then dries quickly.
Consistency of a printing ink is also of considerable importance. Ink is said to be thin when it is easily set in motion, and stiff when it offers comparatively strong resistance to changes in form. A so-called long ink is viscous and can be drawn out into threads, while the opposite is a "short ink." In printing varnishes, a distinction is made between weak, medium, and strong (rigid) varnish. Oils are also used as vehicles, and at one time linseed oil was used most often because of its good drying properties, although tung oil was also used extensively. Tung oil dries very rapidly but loses its gloss in the process. Both have been largely replaced by tar and mineral oils. Inks for newspaper printing are compounded in mineral oils, and are very thin. See also:ANILINE INK ;CARBONACEOUS INK ;CARBON INK ;CHINESE INK ;DOUBLE-TONE INK ;HEAT-SET INK ;IRON-GALL INK ;METALLIC INKS ;OFFSET INK ;SEPIA INK . (21 , 140 ,143 , 195 )