Fountain pens use liquid ink to mark paper. Usually, it is some pigment or dye suspended in water. In 1941, detergents and other chemicals were added to adjust the properties--how it flowed or the intensity of the color.

Ink is traditionally sold in bottles, though more recently (since the 1960s), cartridges have become very common. In general, so long as it is kept in an airtight container, not exposed to light or allowed to freeze, and no sediment forms in the container, the ink should be usable. Some collectors will only put vintage ink in their pens.

Ink may dry out in the pen. This should not be a problem--run some water through the feed, and it will come out. Evaporation is typically the biggest threat. On the other hand, ink tablets have been available for quite some time. Water would be added to the tablet in a container, then the pen filled. This was convenient for soldiers in the field.

Do note that not all liquid inks are compatible with fountain pens. Metallic, scented, or invisible ink contains elements that will clog the feed, at best requiring a more invasive cleaning. Likewise, drafting ink, india ink, and other substances are not appropriate for use in a pen.

Sources:

da Book
http://www.pendemonium.com/

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