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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 ink can be of a paste form, and this kind of ink is used most extensively in letterpress and lithographic printing.

Types of ink

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 (known as sepia ). 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

Pigmented inks contain other agents that ensure adhesion of the pigment to the surface and prevent it from 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 are advantageous when printing on paper because the pigment stays on the surface of the paper. This is desirable because more ink on the surface of the paper means less ink needs to be used to create the same intensity of color.

Dyes in inks

Dyes, however, are generally much stronger and can produce more color 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 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 to resolve this include harder paper sizing and more specialized paper coatings. The latter is particularly suited to inks that are used in non-industrial settings (and thus must conform to tighter toxicity and emission controls), such as inkjet 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 radiation as in sunlight.
Pigments are the main components of ink, containing the different colours. The size of the pigment is very important for the ability of diffuse in the solution inks. Qualities such as hue, saturation, and brightness are inherent in the ink, varying dependant on the source and type of pigment.

History of ink

Approximately 5000 years ago, the Chinese developed ink for blackening the raised surfaces of pictures and texts carved in stone. This early ink was a mixture of soot from pine smoke, lamp oil, and gelatin from animal skins and musk. Other early cultures also developed inks (of many colors) from available berries, plants and minerals.
In an article for the Christian Science Monitor, Sharon J. Huntington describes these other historical inks:
Scribes in medieval Europe (about AD 800 to 1500) wrote on sheepskin parchment. One 12 century ink recipe called for hawthorn branches to be cut in the spring and left to dry. Then the bark was pounded from the branches and soaked in water for eight days. The water was boiled until it thickened and turned black. Wine was added during boiling. The ink was poured into special bags and hung in the sun. Once dried, the mixture was mixed with wine and iron salt over a fire to make the final ink.

In the 14 century, a new type of ink had to be developed in Europe for the printing press. Two types of ink were prevalent at the time; The Greek and Roman writing ink (soot, glue, and water) and the 12 century variety composed of ferrous sulfate, nutgall, gum, and water. Neither of these handwriting inks could adhere to printing surfaces without creating blurs. Eventually an oily, varnish-like ink made of soot, turpentine, and walnut oil was created specifically for the printing press.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Ink".