Fountain pen ink
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 pre–dates fountain pens by centuries is Iron gall ink. This blue–black ink is made from iron salts and tannic acids from vegetable sources. It 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. 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 and 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 × 10−3 N/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 was too high, then it would not flow through the pen; if it was too low, then the ink would run out of the pen with less control.
Fountain pens traditionally carry ink within the barrel, either inserted at one end in bulk with a syringe or eyedropper pipette, or through a mechanical filling system built-in to the pen (such as a piston or vacuum-pump mechanism). For such fountain pens, ink is available in bottles which will typically refill an individual pen many tens of times. Simpler (and therefore often less expensive) fountain pens tend to rely upon pre-filled ink cartridges, although in many cases the cartridge can be replaced with a converter which replicates the mechanical filling action of more expensive pens. The cost per millilitre of ink tends to be lower for ink bought in bottles, as opposed to in pre-sealed cartridges.
Most fountain pen manufacturers also provide a brand of ink; for example Parker sell 'Quink', and Sheaffer sell 'Skrip'. The recent resurgence of fountain pen use has also created a market for companies that specialize in ink, such as the British company Diamine and the American company, Noodler's Ink. These manufacture ink in dozens of different colours.
Durability and security 
In the late 20th century, particular attention has been paid by ink manufacturers to the durability of their products against the effects of time, moisture, and efforts at forgery or falsification. Such inks use dyes that chemically bond with the paper's Cellulose, and which therefore resist removal by solvents. A disadvantage of them is that, if spilled, they will form irremovable stains on clothing made from cotton as that also contains cellulose.
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