An ink is a liquid containing various pigments and/or dyes used for colouring a surface to render an image or text. Common perceptions consider ink for use in drawing or writing with a pen or brush. However, inks are used most extensively in printing.
Early varieties of ink include Indian ink, various natural dyes made from metals, the husk or outer covering of nuts or seeds, and sea creatures like the squid. India ink is black and originated in Asia. Walnut ink and iron-gall nut ink were made and used by many of the early masters to obtain the golden brown ink used for drawing.
Pigmented inks contain other agents to ensure adhesion of the pigment to the surface and prevent its being removed by mechanical abrasion. These materials are typically referred to as resins (in solvent-based inks) or binding agents (in water-based inks).
Pigmented inks have the advantage when printing on paper that the pigment stays on the surface of the paper. This is desirable, because when more ink stays on the surface of the paper, less ink needs to be used to create the same intensity of colour.
Dyes, however, are generally much stronger and can produce more colour of a given density per unit of mass. However, because dyes are dissolved in the liquid phase, they have a tendency to soak into paper, thus making the ink less efficient and also potentially allowing for the ink to bleed at the edges, producing unsightly and poor-quality printing.
To circumvent this problem, dye-based inks are made with solvents that dry rapidly or are used with quick-drying methods of printing, such as blowing hot air on the fresh print. Other methods, particularly suited to inks that are used in non-industrial settings (and thus must conform to tighter toxicity and emission controls), such as ink-jet printer inks, include coating the paper with a charged coating. If the dye has the opposite charge, then it is attracted to and retained by this coating, while the solvent soaks into the paper. Cellulose, the material that paper is made of, is also naturally charged, and so a compound that complexes with both the dye and the paper surface aids retention at the surface. Such a compound in common use in ink-jet printing inks is polyvinyl pyrrolidone.
An additional advantage of dye-based ink systems is that the dye molecules interact chemically with other ink ingredients. This means that they can benefit more than pigmented ink from optical brighteners and colour-enhancing agents designed to increase the intensity and appearance of dyes. Because dyes get their colour from the interaction of electrons in their molecules, the way in which the electrons can move is determined by the charge and extent of electron delocalisation in the other ink ingredients. The colour emerges as a function of the light energy that falls on the dye. Thus, if an optical brightener or colour enhancer absorbs light energy and emits it through or with the dye, the appearance changes, as the spectrum of light re-emitted to the observer changes.
A disadvantage of dye-based inks is that they can be more susceptible to fading, especially when exposed to ultraviolet light, as in sunlight.