Iron Gall Ink: Recipe using Knopper galls

Iron gall ink is literally the stuff of history; it was used to write many medieval documents, including the Magna Carta.
The ink It is made from oak galls (deformities of the leaf, acorn or other part of the tree, caused by the larvae of a parasitic wasp). If you search the Internet, you will find many recipes for it, including several impressively venerable ones. If you dig a little deeper, you will find that the chemistry involved is only partially understood - but I found enough information from modern science that I was able to adapt the recipes to make my own version, based, it has to be said, on a series of assumptions and guesses.

Each type of gall contains different amounts of tannins. My recipe is intended specifically for use with knopper galls.

The recipe

Ingredients

Quantities depend on the clean weight of the galls and are indicated in the method.

• Knopper galls
  These fall with the acorns in the autumn and can be collected from the ground beneath an affected oak tree. Some of mine had been kicked around a lot (the tree is on the edge of our village school's playground) and were rather muddy.
• Water
  I used tap water. It is probably better to use rain water or filtered water.
• Ferrous sulphate
  Many recipes suggest the use elemental iron (either as a cooking vessel or in the form of rusty nails). As I understood the process, ferrous sulphate (iron (II) sulphate) is a safer and surer reagent. It is inexpensive and can be bought, in crystalline form, from garden centres as a moss killer. 
• Gum arabic
  This is a natural binding and thickening agent. It is sap from the acacia tree, and is sold in the form of dried lumps of sap or as a solution. I used lumps.
• Cloves
  The spice is a natural preservative.

Method

1. Clean the galls with water and separate them from the acorns as necessary.
2. "Break" or "crush" the galls.
   I took this to mean that the galls did not need to be ground into tiny pieces. As knopper galls are very hard, I folded them into a cotton bag and hit them with a mallet. A hammer would also work.
3. Sort through the broken up galls and remove any larvae or foreign objects. Weigh the galls and put them in a jar.
4. For every 10g of galls, measure out 100ml of water. Add the water to the jar. It should cover the broken galls.
5. Put the jar in a warm place and leave it there for 10 to 14 days.
   
   I used the airing cupboard, where the mixture slowly and gently fermented as the tannins in the galls leached out and reacted with the water to make gallic acid (also a tannin). There was a little effervesence (bubbles) and the liquid turned dark brown. It did not smell or erupt.


6. Strain and filter the liquid into a clean jar. Discard the solids.
7. For every 10g of the original weight of galls, add about 5g of ferrous sulphate.
   
   Gallic acid and ferrous sulphate react to form ferric pyrogallate (iron (III) pyrogallate), an insoluble black solid. This is the ink pigment. Athough it is insoluble in water, it will form a suspension in water, and this suspension will be enhanced by the binding agent, gum arabic.
   
   
8. Add the same amount of gum arabic to the mixture
   
   Gum arabic dissolves very slowly in water. Heating and whisking may help it dissolve, but the most effective method seems to be waiting. My ink was ready to use the following day.
9. Decant, using a funnel, into your storage bottle(s).
10. Add a few cloves to help preserve the ink.

The rationale

What follows is my attempts at trying to make sense of a complicated and much debated situation. Chemists, conservationists and assorted others (like artists!) have all contributed. The starting parameters are uncertain (how much of what is in this gall?), the reactions are complex, intertwined and poorly understood. Time, measured in centuries, plays a key part, and that makes judgement of results difficult. Definitive numbers are elusive.

or

Warning: science bit.

Iron gall ink can sometimes cause corrosion. There appears to be some confusion about how and why it does this, but from my reading, it seems likely that the corrosive nature has very little to do with the tannic acids (which are mild acids, no more dangerous than vinegar).

Edit: This article suggests that the acids do affect the ink's stability.

The corrosion causes old manuscripts to disintegrate when they are handled and seems to be caused by the oxidisation of a soluble ferrous salt that is present in the ink (it oxidises to ferric oxide, commonly known as rust). This salt is the product of an incomplete reaction between gallic acid and ferrous sulphate, which I interpreted to be the result of an excess of iron during the reaction. (Iron is dual valence; it can form compounds at +2 or +3 valencies, but the +2 compounds tend to be less stable than the +3 compounds.)

So, to make ink that does not cause the corrosion, care must be taken not to add too much iron.

I tried to calculate the ideal amount of ferrous sulphate. I found figures that suggested that British knopper galls typically contain around 40% tannins (a high value; marble galls - the other type of oak gall that I had found locally - have around 16% and the bark of the tree has around 9%). The molecular weight of gallic acid is 170.12g/mol, and that of ferrous sulphate heptahydrate is 278.01g/mol. I found the abstract of a scientific paper about the reaction, from which I inferred two things: firstly, that it was a complicated reaction, and secondly, that the reaction ratio might be 1:1.

I was using the kitchen scales to weigh everything (they are not very accurate), so it seemed reasonable to approximate the ratio by weight of gallic acid to ferrous sulphate crystals to 1:1.5 or 2:3.

Remember the 40% tannin content? If I assumed that all of the tannins were converted to gallic acid by the fermentation process, then the relative weights of galls to ferrous sulphate crystals should be 5:3. Given that I wanted to err on the side of caution regarding the amount of iron involved, I adjusted this to 2:1. This made it easier on the kitchen scales, too.

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Note: According to the mostly authorative Iron Gall Ink Website,

"Recent research indicates that a 3:1 ratio of gallotannic acid to iron sulfate produces the most stable inks."

(1998) https://irongallink.org/igi_indexd7ce.html

I discovered this nugget after I had made my ink. Was I wrong? (It's entirely possible; there were an awful lot of guesses and approximations.) However, they are talking about the ratio of gallotannic acid to "iron sulfate" (which is the same thing as ferrous sulphate), not gallic acid. But is this a molar ratio or does it refer to the weights used? It's impossible to tell.

Gallotannic acid is the main tannin found in oak galls. It has a molar mass of 1701.19g/mol, ten times that of gallic acid, to which it hydrolyses during fermentation. It is the gallic acid that actually reacts with the ferrous sulphate (iron sulfate).


Before I got too lost in my calculations, I discovered a more recent scholarly article ("A few thoughts on iron gall ink reconstruction) which stated that,

" a 3:1 molar ratio of FeSO₄·7H₂O to gallic acid [...] has been known to have the most chemical stability"

I used a molar ratio of 1:1, with the result that I had only a third of the iron that I should have had.

I stopped trying to make the numbers work then. The ink is nice and black and it works now.

Call me in 300 years if it oxidises and makes the picture fall off the paper. In the meantime, I shall treat it as "non-archival" and I shall enjoy drawing oak trees using it.