Iron chemistry is a constant feature of my workshop, and for an upcoming project I will need to revisit the reaction between iron and tannins, as seen before in my stain and blackening method for wood. Before doing this, however, I wanted to explore iron gall ink, which is in many ways the most fundamental version of this reaction. This ink relies on the fact that ferrous iron is soluble in tannin solutions, while ferric iron produces an insoluble black resin; the change between the two being caused by exposure to air. The ink is extremely stable and can last for hundreds of years in books and manuscripts, and in the form of wood stain I found it capable of withstanding over a year of outdoor exposure without fading at all.
This ink has one major fault, however; it is well-known to be slightly corrosive, and over time it weakens the paper it is written on. This is because the vast majority of recipes for this ink use ferrous sulfate as the source of iron, which leaves behind a residue of sulfuric acid as the ink dries. Furthermore it tends to react incompletely, leaving soluble ferrous ions which act catalytically with air to decompose the paper. Having worked extensively with iron acetates in the past, I decided to improve this type of ink by using steel wool and vinegar to produce the necessary soluble iron.
I began by dissolving one part of oak gall powder (from a local natural dye supplier) into three parts of vinegar, by weight. I then added a small piece of steel wool to the solution, and heated the mixture to a simmer on my hot plate. The steel wool began to bubble, and over the course of an hour the solution first darkened, then thickened and formed a layer of foam before the bubbling eventually slowed. At this point the solution was saturated with iron and the ink was complete, so I filtered it into a small bottle.
When used for writing or painting this ink is initially somewhat transparent, but in a few seconds it becomes black and opaque, and within a minute it is dry to the touch. The dry ink is smooth and glossy, but is also flexible enough for the page to bend without it flaking off. After drying for an hour, I then soaked a section of inked paper in a small amount of water, and the ink remained in place without dissolving or running. I then tested the water with pH paper, which gave a reading of 7; the dry ink is totally neutral. Finally, I added a small amount of potassium ferricyanide solution to the water and there was no change in color, indicating that there were no soluble ferrous ions. If this remains the case as the ink ages, there should be no mechanism by which it could decompose the paper.
This ink is quite versatile, and can likely be made using any source of tannins; a batch made with quebracho extract performed very similarly, although the color was slightly different and it took longer to dissolve the steel wool. Even fairly dilute sources of tannins could possibly be used, such as extracts from black tea or acorns, if they were sufficiently concentrated by evaporation. Overall I am highly pleased with the outcome of this project; a very short period of experimentation yielded exactly the results I was looking for, and has possibly solved a long-standing problem in the field of permanent ink.
Although this preparation is simple, it is fairly messy; the purpose of ink is to permanently stain a surface, and it does this readily to equipment and glassware. Fortunately, I found that oxalic acid dissolves this ink (any many other iron-based stains) rapidly and completely, even in low concentrations (one small spoonful per liter of water). This should not be used on the hands, however, but thankfully ink stains tend to leave the skin within a day or two.