A liquid containing various pigments and/or dyes used for colouring a surface in order 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 for printing.
Early varieties of ink are 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 in colour 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 will contain other agents in order to ensure adhesion of the pigment to the surface, and prevent it being removed by mechanical abrasion. These materials are typically referred to as resins (in chemical solvent based inks) or binding agents (in water-based inks).
Pigmented inks have the advantage when printing on paper that the pigment will stay on the very surface of the paper. This is desirable, as the more ink that stays on the visible surface of the paper, the less ink that can 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, as dyes are dissolved in the liquid phase in the case of inks, 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 use solvents that will dry more rapidly, or quick-drying methods of printing, such as blowing hot air on the fresh print. Other methods, particularly suited to inks that have to be used in non-industrial settings (and thus must conform to tighter toxicity and emission controls), such as ink-jet printing inks include coating the paper with a charged coating. If the dye has the opposite charge, then it will be attracted to, and retained by, this coating, whilst the solvent soaks into the paper. Cellulose, the material that paper is made of is also naturally charged, and so a compound that will complex with both the dye and the paper surface will aid retention at the surface. Such a compound in common use in ink-jet printing inks is PVP - Poly Vinyl Pyrrolidone.
A further advantage of dye based ink systems is that the dye molecules can 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. As 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 present 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 can absorb more light energy, but emit 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 in conditions of UV light, such as sunlight.