List of therms» Graphic inks

Three different types of ink are used for lithographic hand printing. These are (1) roll-up ink, used in processing the image, (2) black or colored ink, used in proofing and printing of editions, and (3) transfer ink, used to transfer impressions from one surface to another. Each ink must have special working properties to function dependably, and the printer must understand the factors that govern the choice, behavior, and control of the inks he uses. Inks which are manufactured with the wrong properties or which are improperly altered by the printer may either destroy the printing stability of a work or produce impressions of poor quality.

The behavior of printing ink is usually related to its physical properties. The hand printer should become familiar with such terms as viscosity, thixotropy, tack, length, and body. Each term describes some physical aspect of ink.

Viscosity is the amount of resistance of the ink to flow. Comparatively, inks for hand lithography should be more viscous than inks for offset lithography. The vehicle (lithographic varnish) is the chief material governing the flow characteristics of ink. Since this is introduced during the manufacturing process, the viscosity of the ink is determined beforehand so that the ink meets certain basic requirements. Modifications of this viscosity by the printer are often necessary for special tasks. For example, inks of low viscosity flow too readily and cannot be easily controlled during inking. Inks with high viscosity may not transfer readily from the printing element to the paper. Roll-up ink requires greater viscosity than printing ink so as to prevent excessive build-up of ink during the critical stage of processing. Edition printing usually requires a less viscous ink which can penetrate the paper easily and which will not pIuck the fibers of the paper when the impression is removed from the printing surface.

Ink viscosity can be modified by adding varnishes with different flow characteristics or by using one of the modifying agents (magnesium carbonate). Temperature change can also affect ink flow. Warm temperatures tend to lower the viscosity and cold temperatures to increase it.

Lithographic ink may appear rather firm when first removed from the can. It becomes looser and smoother after being worked on the ink slab for a few minutes. The ink is like a plastic material; its internal forces break down when it is worked, and they gradually re-form when left standing. This phenomenon is called thixotropy. Because lithographic ink is thixotropic, the volume to be used for printing must be thoroughly agitated on the slab before it is distributed. During prolonged printing, the remainder of the ink pile must again be worked (for it will have re-formed) before the slab is replenished. Meanwhile the film of ink distributed on the slab will remain in its looser state, since it is under continual agitation from the rotation of the ink roller. Occasionally ink in the can will form a gel-like structure that cannot be broken down on the slab. Ink in this condition is no longer usable and is described as being "livered." Livering is most often caused by inferior materials and careless manufacturing.

Tack is the measure of the stickiness of ink. Tack can be roughly estimated by the pull on one's finger when it is rapidly lifted from the ink. When the finger taps the same ink on a sheet of paper, the pull will be even greater because some of the ink vehicle is absorbed into the paper.

Tack manifests itself in printing in the following way. Both the printing image and print paper are wet with ink during printing. When the paper is removed from the printing surface, the film of ink splits between the two surfaces, with the greater amount being affixed to the more absorbent.

A certain amount of tack is necessary in lithographic ink if the sharpness of the image is to be retained when the ink is rolled on. Excessive tack, however, prevents the penetration of ink into the paper and may even pluck fibers from the surface of soft papers. During printing there will be a gradual reduction of tack as the ink becomes emulsified with water from the printing surface.

The lenght of an ink is determined by the distance it can be pulled into a long string without breaking. A long ink may stretch 12 to 15 cm without hreaking: a short ink may break within 2,5 cm. Ink length is important because it affects the distrihuticon of ink on the paper. Shorter inks are preferable for hand printing because of the shorter distance of travel between the ink roller and the printing surface than in offset printing. Being more compact, they are esier to control on the inking roller and the printing surface. A longer ink has a tendency to overink each printing dot.
The length and the tack of an ink are different properties although often related. Each or both may be affected by changes of varnish or pigment ratios, or by additions of modifying agents (magnesium carbonate) . For example, adding a stiff varnish will make the ink tackier and shorter, whereas adding magnesium carbonate may only shorten the ink without affecting its tack. Like tack, length is reduced when the ink becomes emulsified with water during printing. It will be noticed that the printing ink on the slab is considerably shorter at the end of a job than it was at the beginning.
Ink body and consistency are generic terms usually referring to the over-all physical properties of the ink. Inks for hand printing should be heavier in body than those used for offset printing. This means that they should be compoundcd with greater loadings of pigments, varnishes, and modifiers, and that their consistency should be short and tacky.