Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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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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