iron-gall ink
An INK produced by
the reaction of tannic acid with an iron salt,
such as ferrous sulfate (FeSO). The reaction
produces no immediate change in the color of the
solution, but, when the ink is applied to paper
and is thus exposed to air, it darkens by
oxidation, forming ferric tannate. The difficulty
of writing with a colorless fluid was partially
overcome by the addition of gum arabic to the
solution, as well as some pre-exposure to air, so
as to form of the ferric tannate before use. The
gum arabic served to prevent the ferric tannate
from settling out of solution. There were
disadvantages to this procedure, such as the
tendency of the ferric tannate to settle out
despite the gum arabic, and the tendency of the
ink to remain on the surface of the writing
material instead of impregnating the fibers. This
was due to the fact that the ink had formed before
meeting the paper, and the result was that it
could be removed rather easily by washing once the
water had loosened the gum. In an effort to
overcome these problems, various coloring matters
were added to the original solution, including
extracts of logwood, and, at a later date, indigo.
Other than being colored at the outset, iron-gall
inks treated in this manner had several
advantages: the oxidation and consequent
deposition of solid matter was restrained and the
ink therefore penetrated the fibers more readily;
the penetration was assisted by the absence of gum
arabic because the ink could then flow more
freely; oxidation occurred only on the paper, and
the ink then changed from blue to black; and the
addition of the indigo also increased the
resistance of the ink to fading and bleaching.
Iron-gall ink does have one serious disadvantage.
Free acid is often present, which not only
corrodes steel pens badly, but, far worse, attacks
the paper, as well as certain of the dyes used to
color them.
There is also the so-called Japan ink, which
consists of almost completely oxidized iron
tannate with large proportions of gum, which gives
a black glossy effect immediately; and an ink made
from logwood and iron, without the addition of
tannic acid, which has a greenish shade which
eventually changes to black.
Iron-gall inks came into use in the 9th century
and by the 11th century had largely replaced
carbon inks as a writing medium. (143 )
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