Jan 11, 2020

Oak Gall Ink Explained

Alex Margarit

In my study of the Ripley Scrolls and trying to understand how medieval documents were made,
I created a large batch of iron gall ink. Iron gall ink is made from the tannins found in oak galls and iron salts. The recipe for iron gall ink differs depending on who writes it, but the general idea remains that it is a simple ink made from a vegetable dye and a mordant. One thing I discovered about oak galls, aside from the way they grow on trees, is that they differ based on the region they come from. In the image below are Canadian oak galls (top) and Eurasian oak galls (below):

Fig. 1: Canadian oak galls (above) and Eurasian oak galls (below). All images (c) Alex Margarit.

What can immediately be seen is that Canadian oak galls are smaller and smoother than their Eurasian relatives. Not only do these oak galls differ in appearance, they also differ in weight and the tannin levels they possess. The Canadian oak galls are practically hollow and very brittle whereas the Eurasian oak galls are very dense, almost wood-like in density, and are much harder to break open. The species of oak or the gall wasp are unknown in both cases, but the characteristics of the galls are likely due to the conditions the trees grow in, as is seen by the size difference between galls from Canadian oaks and European oaks.

After finding a good source of oak galls, the next step in making ink is to crush them in a mortar with a pestle. The galls do not need to be pulverized, but broken small enough to facilitate the next step. After crushing, the galls need to be soaked for at least a couple of days to release the tannic acid into solution. Medieval people likely used rainwater for this step; I substituted distilled water. The solution will become a murky, light brown colour and some bubbles will be seen rising. I am not sure if oak galls contain any sugars, so the bubbles could be a result of air escaping from the porous galls, or from some sort of fermentation. The next step after soaking the galls is to strain out the solids, which medieval people likely used a fine sieve or cheesecloth to do. This is another reason why the galls should not be pulverized, since straining gall dust is significantly more difficult than straining out gall chunks.

Fig. 2: Oak galls soaking in water.

After the solution of tannic acid is separated from the solid matter, an iron salt solution needs to be added to finish the process. It is not known definitively what the iron source was in medieval times, since anything from nails to modern chemicals would work for this. Adding nails to the solution would eventually produce ink, but this would require much more time. To speed things along, I made a solution of ferrous sulphate by simply adding iron (II) sulphate to distilled water.

Iron (II) sulphate was known to medieval people, since two common early names for this salt are green vitriol, or copperas. Neither name has a clear etymology, but it is likely that both names were given to the salt on assumption that the green salt was actually copper-based. Vitriols are copper salts, and most of them are either blue or green, so the confusion can be easily understood in a time without modern chemistry. The reaction between ferrous sulphate and tannic acid is immediate, and once the iron is added to the solution, the ink turns a very dark blue-black colour and begins to thicken significantly. The cause of the thickening is likely due to the high concentration of my iron salt, which I countered by simply adding more distilled water to the solution.

Fig. 3: Green vitriol.

The ink needs to be thinned out at some point in order to make writing with it possible, which I discovered through making a batch of very thick, concentrated ink. The pH level of this ink is low, so it would not be an ideal candidate in creating something archival. Some medieval documents show significant damage from ink from the ink literally dissolving the parchment or paper beneath. It is worth noting that the pH level of the ink is low, but not so low that it is guaranteed to dissolve the surface it is put on; certainly, many old documents still survive in relatively good condition that were written with either oak gall ink or walnut ink.

ink was not difficult to make, but it was difficult to make it right
My biggest takeaway from conducting this experiment is that ink was not difficult to make, but it was difficult to make it right. Oak gall ink also has unique qualities when it is being written with, the main one being that when it goes on paper it is initially very transparent and bleak-looking. In comparison to most modern inks, which go on dark but dry light, oak gall ink is initially transparent (almost blueish) but quickly turns black. The other feature of oak gall ink is that it does not need to be stirred since the mixture is homogenous. Lamp black, or any other ink for that matter that is made by adding a pigment to a binder, needs to be constantly stirred to maintain consistency. Furthermore, pigmented solutions tend to dry darker, while pigmented binder inks tend to dry lighter.

Further Reading:

"Make Ink." The Iron Gall Ink Website. Accessed November 28, 2019.

"Making & Testing Iron Gall Ink." School of Conservation Blog, West Dean College. Accessed November 28, 2019.


Alex Margarit has a BFA in Studio Art and is currently working on a History BA at the University of Saskatchewan. This post was written for his CMRS 433 course. All images in this post are (c) Alex Margarit.

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