P 13500, dated to the fourth century BCE, closely followed by P 13501 and the group of Greek texts (Delange et al. 1990), dated to the period from the third to the first century BCE, all containing copper, might indicate the beginning of the transition from carbon to iron-gall inks. We suggest associating this ink with a recipe for sympathetic (invisible) ink reported by Philo of Byzantium. This recipe seems particularly relevant, since it is not only roughly contemporary with our documents but also presents the oldest extant record of the reaction between a substance based on copper and a tannic solution. Even though Philo’s ink has been well known to scholars for quite some time, most of the scientists analysing inks became aware of it only after 1983, when Zerdoun first drew their attention to it (Zerdoun Bat-Yehuda 1983, pp. 91–92). The ink produced according to Philo’s recipe has not yet been found experimentally. Although the exact date when Philo lived is unknown, specialists think he lived around the end of the third and the beginning of the second century BCE. However, the recipe was transmitted to us only as a part of Byzantine collections of ancient military texts, compiled in the tenth century CE, probably during the reign of Emperor Constantine VII Porphyrogenitus (de Rochas d’Aiglun 1881, pp. 19–21).Footnote 7
The Greek text of the recipe (chapter 77[102]) reads as follows:
Γράφονται δ’αἱ ἐπιστολαὶ εἰς καυσίαν καινὴν <ἢ > εἰς τὸν χρῶτα, κικῖδος θλασθείσης καὶ ὕδατι βραχείσης∙ ξηρανθέντα δὲ τὰ γράμματα ἄδηλα γίνεται, χαλκοῦ δὲ ἄνθους τριφθέντος ὥσπερ ἐν ὕδατι τὸ μέλαν καὶ ἐν τούτῳ σπόγγου βραχέντος, ὅταν ἀποσπογγισθῇ τούτῳ, φανερὰ γίνεται.
We suggest the following literal translation:
The letters are written on a new kausia,Footnote 8or on (human) skin, with crushed gallnuts soaked in water; when the characters dry out, they become invisible; when one grinds flower of copper, like black (ink) in water, and moistens a sponge with it, they become visible whenever one wipes them off with it as with a sponge.
Here, Philo describes how to create a sympathetic ink, made of a solution of gallnuts, that becomes visible only after exposure to “flower of copper”. It is the oldest attested recipe for ink using a tannic solution. The juice of gallnut is a very common ingredient for later iron-gall inks, but also for plant inks. It has a very light brown colour and is indeed likely to appear invisible when written on a brownish surface such as a felt hat or skin. The term “flower of copper” (“χαλκοῦ ἄνθος”), the main constituent of the revealing solution, is usually translated as “copper sulphate” in the modern editions of the text (Garlan 1974, 324; de Rochas d’Aiglun 1881). The exact same term (in two separate words) appears in several medical texts in Antiquity, in particular in the Hippocratic Corpus attributed to Hippocrates of Kos (fifth–fourth century BCE).Footnote 9 Of more relevance to us are mentions by Dioscorides and Pliny the Elder, both authors from the first century CE, who distinguish “χαλκοῦ ἂνθος” (Dioscorides De Materia Medica V 77) or “aeris flos” (Pliny Naturalis Historia XXXIV 24.107) from “χαλκανθὲς” (Dioscorides De Materia Medica V 98) or “chalcanthum” (Pliny Naturalis Historia XXXIV 32.123). The first material is defined as particles flying off melted copper when it is suddenly cooled with water (Dioscorides De Materia Medica V 77) or hammered (Pliny Naturalis Historia XXXIV 24.107). It may consist of small particles of metallic copper or of cuprite (copper oxide). The second material is described by Pliny as a blue substance resembling glass, which, when diluted, appears dark and is used to dye leather black (Pliny Naturalis Historia XXXIV 32.123). Dioscorides describes it as a solidified liquid (similar to Pliny’s description of a material that looks like glass) existing in three different varieties, before listing its medicinal properties (Dioscorides De Materia Medica V 98).
The possible confusion between the two expressions “χαλκοῦ ἄνθος” and “χαλκανθὲς” or “χάλκανθος” was already noted by Zerdoun (Zerdoun Bat-Yehuda 1983, p. 92 note 49), who claims that “χαλκοῦ ἄνθος” in Philo’s recipe is correctly translated as “copper sulphate”. Philo’s flower of copper, reacting with the gallnut solution to reveal the ink, indeed appears more likely to be Pliny’s “chalcanthum” than “aeris flos”, which would not react with tannins. Nowadays, we know that it is not the copper but iron ions that form black complexes with tannins during the production of iron-gall inks. Although both Pliny, who describes different possible variations of colour with chalcanthum, and Dioscorides, who mentions three different varieties, know of the existence of various kinds of vitriolic substances, they do not seem to understand that they are dealing with different metals. Furthermore, it is likely that even blue vitriol (chalcanthite) collected by the ancients would contain, as an impurity, the iron essential to the reaction. It seems that until a first classification of the different vitriolic substances was made by Muhammad ibn Zakkaríja ar-Rází around 900 CE, the distinction between green vitriol (melanterite) and blue vitriol (chalcanthite) was not clear, possibly hindering the development of iron-gall ink (Karpenko and Norris 2002).
It is tempting to associate the inks from P 13500 and P 13501 with Philo’s recipes. Indeed, as stated in the results, the halo observed and the behaviour of the ink under UV light (cf. Fig. 3a) suggest the presence of tannins, present in Philo’s recipe in the form of crushed gallnuts dissolved in water, and the presence of copper as a major constituent could fit with the use of “χαλκοῦ ἄνθος” (or possibly more correctly “χάλκανθος”). However, there are several objections to linking the inks analysed in this paper with Philo’s. First, Philo mentions only felt or skin as a possible support. This objection can be easily discarded, since this choice of support arises from the military context of Philo’s recipe. His aim is to obtain a sympathetic ink to be used with a courier in the event of a besieged city. Writing a message on papyrus or parchment, even with sympathetic ink, could draw the attention of the enemy, while writing on the skin or inside the hat of the envoy would make it undetectable. Second, although the inks on papyri P 13500 and P 13501 do not appear plain black as a carbon ink does, they do contain carbon, as clearly demonstrated by the infrared images. Again, in the case of Philo’s recipe, the aim of having an invisible ink would militate against using carbon together with the gallnut solution. Likewise, in the revealing stage, if flower of copper is enough to reveal the text, carbon is not needed. In the case of P 13500 and P 13501, it could have been used to make the text darker and more visible. The association of “χάλκανθος” is also not unlikely, since it is reported in the later recipe by Dioscorides (Dioscorides De Materia Medica V 162). Finally, although the halo and UV images suggest the presence of tannins, further analysis is needed to unequivocally prove it. For this purpose, it seems promising to apply Atmospheric Solids Analysis Probe (ASAP®) mass spectrometry, as suggested by Ghigo and co-authors (Ghigo et al. 2020).
Published data, together with the present examples P 13500 and P 13501, connect inks containing copper with the Hellenistic period. Moreover, since most of the evidence so far comes from Greek texts, we might even tentatively suggest that such an ink is a Greek invention. We should not forget, however, that the particular climate of Egypt and the Judean Desert contributed greatly to the preservation of papyrus rolls, and we cannot determine whether Egypt was indeed the birthplace of such inks or if we simply lack evidence from other regions. Until further evidence surfaces, we cannot determine whether the Greeks already knew of such an ink and brought it with them to Egypt during Alexander’s conquests.
We can suggest two hypotheses why ancient producers of ink deemed it necessary to add chalcanthum to their ink. The first one is that it was used to stretch the ink, allowing the ink maker to use less soot. Soot might seem like a product that is easy to get. However, soot of good quality was apparently not so common. Among other sources for soot, Pliny mentions atramentum indicum (Pliny Naturalis Historia 35, 25), which had to be imported from far away and was certainly expensive, while the best varieties of chalcanthum were readily available in Hispania (Spain) and Cyprus (Pliny Naturalis Historia 34, 32). It is possible that the large Diadochi kingdoms resulting from Alexander’s conquests needed much ink, be it only for their administration, which could explain why we observe the first occurrences of copper presence in inks from Hellenistic times. The second hypothesis is the need to make inks more durable. Several Latin authors mention the common use of sponges to erase ink. According to Suetonius, ink could even be erased with the tongue, which seems to have been a form of punishment (Suetonius, Caligula 20). Indeed, unlike pure carbon inks, which stay on the surface of the writing support, tannins and metallic ions penetrate the substrate, making inks containing metals harder to erase.