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The ink was prepared by adding some green vitriol ( ferrous sulfate, Fe2 S O4) to a solution of tannin ( gallo-tannic acid ). The later was usually obtained from gall nuts (from oaks or other trees), hence the name.
A binder such as gum arabic was usually added to thicken the ink and make it flow properly from the pen. To extract the gallo-tannic acid, the gall nuts could be powdered and allowed stand in cold water for varying amounts of time, or boiled for several hours, or allowed to ferment for several weeks. The last two methods would hydrolyze the gallo-tannic acid into gallic acidGallic acid is an organic acid, also known as 3,4,5-trihydroxy benzoic acid, found in gallnuts, sumac, witch hazel, tea leaves, oak bark, and other plants. The chemical formula is C H( OH)COH. Gallic acid is found both free and as part of tannins. Externa, which yielded a deeper black color.
After filtering, the resulting pale-gray solution was used to write on paperPaper is a thin, flat material produced by the compression of fibres. The fibers used are usually natural and based upon cellulose. The most common material is wood pulp from pulpwood (largely softwood) trees such as pines, but other vegetable fiber mater, vellumVellum ( Latin for the animals' wool hair) has two meanings: A sort of parchment, a material for the pages of a book or codex, usually made from calf skin. See also: vellum parchment, Golden Fleece. A manuscript or book written on such material. (specially prepared calfCalves are young animals. The term is mainly used for cattle, although whales and elephants also have calves. A cattle calf is a child of a cow and a bull. The plural is calves. Calf meat is called veal. Fine calf skin used for pages in early codexes is c hideTo hide is to make someone or something less visible; see camouflage, information hiding. Hide is the skin of animals; see leather. A hide is a shelter or other structure used for observing wildlife in natural habitats, particularly birds; see ethology, b) or parchement ( lambA lamb is a young sheep. When eaten as meat, lamb is taken from an animal between one month and one year old, with a carcass weight of between 5. 5 and 25 kg. The meat of an older sheep is known as mutton. Milk-fed lamb is from an unweaned lamb, typically hide). A well-prepared ink would gradually darken to an intense purplish black. The resulting marks would adhere firmly to the vellum or parchement, and (unlike india ink or other formulas) could not be erased by rubbing or washing — only by actually scraping off a thin layer of the writing surface.
The gradual darkening of the ink was due to the oxidation of the iron ions from ferrous (Fe++) to ferric (Fe+++) state by atmospheric oxygen. (For that reason, the liquid ink had to be stored in a well-stoppered bottle, and would become unusable after a time.) The ferric ions then reacted with the tannin or some derived compound (possibly gallic acid or pyrogallol ) to form a polymeric organometallic compound, which would make a strong chemical bond with the proteins of the hide.
While a very effective ink on vellum, the formula was less than ideal for paper, since the iron-tannin pigment would not make chemical bonds with the cellulose fibers. The ink still stuck firmly to the paper, but largely by mechanical bonding — namely the dried ink would penetrate the spaces between the fibers and, after drying would become entangled in them. Besides causing unsightly "gost writing" on the obverse face of the paper, any excess of ferrous ions remaining by the ink, over years or decades, would create a rusty halo around the marks, and would ultimately cause the paper to disintegrate.