Invisible Inks

Recipes fluctuate in time and space, between cultures and languages, surfacing in different textual genres and contexts of application. In the course of their transmission, some elements of these ‘erratic blocks’ undergo alterations and the texts describing similar procedures see the introduction of different variants.

The preparation of invisible inks with gallnuts and a metallic component has been textualized in various forms, and partially overlapping recipes have been preserved in Greek, Syriac and Arabic. If only two recipes remain to us to bear witness of invisible inks in the Greek and Syriac traditions, the Arabo-Islamic tradition yields a  considerable  number of  examples.  These recipes are  attested  not only in handbooks and treatises  devoted to  ink making and the arts of books, but also in technical texts focusing on amusing, playful or even deceptive applications of natural properties. Thanks to the richness of materials offered by the Arabic sources, we can observe even better the presence of many variants in the described procedure, concerning indications about the most suitable writing surface, the modes of applications, and the consistence of the ingredients (in dry form or in solution with water).

Greek recipe

Philo of Byzantium’s Compendium of Mechanics (3rd cent. BCE) ranges among the earliest sources on this procedure. During wars and sieges, an invisible ink was prepared to exchange secret messages that could be delivered in unusual writing surfaces, like a hat or the messenger’s skin :

Syriac recipe

A strikingly similar procedure reappears in the Syriac alchemical tradition of the Graeco-Egyptian alchemist usually referred to as Pseudo-Democritus (1st cent. CE) :

Replicating the procedure Complete replication   

The preparation of a gallnut infusion consists in a simple procedure: gallnuts (fig. 1) are ground in a mortar (fig. 2) and left to macerate in water for a week (fig. 3). The obtained mixture is filtered (fig. 4), in order to separate the infusion from the residues.

Video: Filtering soaked gallnuts

Chemical compounds identified in the gallnut infusion includes tannic acid and gallic acid . These molecules, in presence of metal cations, form stable complexes characterised by dark colour. This reaction was exploited in the past for the production of purple-black ink. Since the ink is obtained by the combination of the infusion of gallnuts, which is dark yellowish, with the almost colourless metal cations, this reaction is particularly suitable for the production of invisible inks. The darkness of the produced ink depends on the concentrations of tannic/gallic acid, which can vary due to the kind of gallnut that is used and its state.

The gallnut infusion was used to write ‘invisible letters’, which become visible after being treated with metallic ores ground in water. ‘Flower or copper’ and misy in the recipes refer to copper-iron ores, which can be identified with different kinds of vitriol, i.e., the source of metal cations:

(1) blue vitriol, i.e. copper sulfate — referred to as copper (ii) in the videos and pictures below

(2) green vitriol, i.e. iron sulfate – referred to as iron (ii) in the videos and pictures below

(3) whitish vitriol, i.e. iron (iii) sulfate – referred to as iron (iii) in the videos and pictures below

(4) a combination of the blue and green vitriols – referred to as copper (ii) + iron (ii) in the videos and pictures below

In replicating the recipes, we used a thin brush dipped in the gallnut infusion to write ‘invisible letters’ on paper (fig. 6). Once the writing has dried, a solution of vitriol is applied to it (fig. 7-8). We used the four vitriols mentioned above diluted in water. Immediately the writing appears in a black/brownish colour, due to the formation of the above-mentioned complexes between tannic/gallic acid in the gallnut infusion and the metal cations in the vitriol solution.

Video: Writing ‘invisible letters’ on paper with gallnut infusion Video: Treatment with solutions of vitriols

By following the instructions given in Philo’s recipe (see above, Greek source), we also used the gallnut infusion to write ‘invisible letters’ on the skin (fig. 9). The gallnut infusion gets dry in about 5-7 minutes and the letters become invisible (fig. 10). Then, we soaked a cotton ball in a solution of green and blue vitriols. After applying this solution to the letters, they became visible again (fig. 11-12).

Video: Writing on the skin Video: Treatment with vitriols on the skin

Arabic sources

The 9th century physician and alchemist Muḥammad ibn Zakariyyāʾ al-Rāzī writes :

The Arabic term al-zāǧ, ‘vitriol’, can refer to the same range of vitriols already discussed in the previous page: copper (ii) sulfate; iron (ii) sulfate; iron (iii) sulfate; a combination of copper (ii) sulfate and iron (ii) sulfate (see above). In replicating the Arabic recipes, we decided to use a mixture of copper and iron sulfates.

Al-Rāzī’s recipe also refers to a white vitriol, whose identification raises some questions. In another alchemical work , he claims that ‘white vitriol’ corresponds to alum (šabb) of a pure white colour, perhaps to be identified with potassium alum.

We added potassium alum to the gallnut infusion and wrote with this liquid on paper (fig. 13). Then, we treated the paper with a solution of copper (ii) sulfate and iron (ii) sulfate (fig. 14-15), i.e. the source of metal cations reacting with the tannic/gallic acid of the gallnut infusion:

Moreover, in his recipe Al-Rāzī adds an interesting remark on the different writing supports that could be used to write with gallnuts and vitriols. We tried with papyrus, also mentioned by Zosimus of Panopolis in the Syriac recipe. Invisible letters were written on papyrus with the gallnut infusion (fig. 16); after being treated with a solution of blue and green vitriols, the letters became visible (fig. 17).

Video: Writing on papyrus with the gallnut infusion Video: Treatment with the solution of vitriols

In his treatise on inks and dyes, al-Qalalūsī (d. 1307) offers just a brief sketch of the procedure, where the order in which gallnut infusion and solution of vitriols are applied on the writing support is inverted :

Invisible letters were written on paper with a solution of blue and green vitriols (fig 18.). Then, the letters were treated with the gallnut infusion (fig. 19). As showed in the replication below, the final result is less satisfactory, since the ink tends to run and spread on the paper.

Video: Writing on paper with a solution of vitriols Video: Treatment with the gallnut infusion

In one of the works of the 13th century alchemist al-ʿIrāqī, it is possible to find both versions of the procedure. They are included in two different chapters of the ʿUyūn al-ḥaqāʾiq (‘The best of true facts’), namely ch. 23 on metallic inks and ch. 18 on dyes and wondrous ways of writing :

In both versions, the leaf with the invisible writing (either written with the gallnut infusion or with the solution of vitriols) is soaked in either ‘vitriol water’ or ‘gallnut water’ in order to make the writing visible.
We replicated the first option, by writing with the gallnut infusion on paper (fig. 20) and then soaking the paper in ‘vitriol water’ (i.e. a solution of vitriols; fig. 21):

Video: Writing on paper with the gallnut infusion Video: Soaking the paper in ‘vitriol water’

A variety of methods for applying vitriols on the invisible writing is recorded by al-Iskandarī (d. 1243) in his Al-ḥiyal al-bābiliyya (‘Babylonian stratagems’). He also urges to expose the invisible writing to the vapours of vitriols, along with soaking the leaf into ‘vitriol water’ :