The process of making printing ink consists of grinding a pigment, black, white, or colored, into a suitable varnish. The pigment is that constituent which makes the impression visible, while the varnish is the vehicle which carries the pigment during the operation of grinding and during its distribution on the press to the type, from the type to the paper, and ultimately binds it to the paper.
A complete factory for the production of printing ink consequently consists of three distinct plants,--one for the production of the varnishes, one for the manufacture of the pigments, and one for the grinding of the pigments into the varnishes.
Roughly speaking, the varnishes are divided into three classes, the first and second of which are the varnishes proper, _i.e._ the resin and the linseed varnishes, while the third class consists of dryers, etc., whose purpose is to influence the drying and consistency of the inks.
Taking up first the proper varnishes, we find that these are produced by the destructive distillation of resin in huge cast-iron stills. By this process, the solid resin of colophony is split up into water, various resinic acids or naphthas, and resin oils of various specific gravities and consistencies, all of which are separated from each other into separate containers which are ready to receive them. As one distillation is not sufficient to purify the resin oils from the water and acid, which would not only give the resulting ink an obnoxious odor but be detrimental to type, plates, etc., the distillation is repeated a number of times until the oils become perfectly pure. The grades of varnishes made from these resin oils are used for the cheaper classes of printing inks, not only on account of their lower cost, but because they are more suitable for the class of work for which such inks are used.
The linseed varnishes are made by boiling refined linseed oils at a very high temperature. The linseed oil loses its acrid elements by volatilization, and gradually becomes thick and viscous, the various "numbers" or consistencies of these varnishes being dependent upon the length of time during which the oil is subjected to the process, and to the temperature applied.