Fountain pens use liquid
ink to mark paper. Usually, it is some
pigment or dye suspended
in water. In 1941,
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.