Fountain pen inks

Fountain pen ink is a water-based ink intended for use with fountain pens.

Because fountain pens operate on the principle of capillary action, ink for them is almost exclusively dye-based. Pigment-based inks (which contain solid pigment particles in a liquid suspension) tend to clog the narrow passages of the pens. Some pigmented inks do exist for fountain pens, but these are uncommon. India ink, a carbon pigment-based ink, also contains a binder (such as shellac), which can quickly clog such pens. The ideal fountain pen ink is free-flowing, free of sediment, and non-corrosive. These qualities may be compromised in the interests of permanence, manufacturability, and in order to use some widely available dyes.

A form of ink that predates fountain pens by centuries is Iron gall ink. This blue-black ink is made from iron salts and tannic acids from vegetable sources. Prior to the ready availability of manufactured ink, iron gall ink was often home made. Stark's ink was one ink recipe, named after the Scottish chemist who devised it after spending several decades experimenting with hundreds of formulations.

Iron gall ink was used in fountain pens when they were invented, but has the disadvantage of causing corrosion to metal parts. Modern formulations of Iron gall ink are somewhat less corrosive and are still occasionally used in applications that require permanence.

Red inks usually contain the dye Eosin. Blue inks often contain Triarylmethane dye. Black inks are mixtures. In addition to water, the non-dye components (collectively referred to as the vehicle) might include polymeric resins, humectants to retard premature drying, pH modifiers, anti-foaming agents, biocides to prevent fungal and bacterial growth, and wetting agents (surfactants). Surfactants reduce the surface tension of the ink; distilled water has a surface tension of 72 dyn/cm (72 × 10⁻³N/m), but the desirable value for ink is between 38 and 45 dyn/cm (38 to 45 × 10⁻³ N/m). If the ink's surface tension were too high, then it would not flow through the pen; if it were too low, then the ink would run out of the pen with less control.

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