When investigating iron-gall inks, the fingerprint model has been suggested in the past as a possible approach to perform qualitative and quantitative studies of ink traces on paper writing surfaces (Hahn et al. 2004). We often successfully apply a simplified version of this model that considers only an approximation of the contribution from the support and yet gives an indication of the different writing phases involved in the production of a manuscript. The model involves at first the subtraction of the net peak intensity of all the elements in the support from the values of the corresponding elements in the inked areas. This is a necessary step given that the X-rays are penetrating and measuring the contribution from the support and ink together. The values thus obtained for each element are then normalised to iron, under the assumption that a coherent writing phase is characterised by the same ratio of the satellite elements to iron, the main component of the vitriol used to make iron-gall inks in medieval recipes. However, iron is contained in different proportions in the most common writing supports as well. Therefore, for this method to provide reproducible results, the content of iron in the support must show a certain homogeneity and its median value should be considerably lower than the median value of iron intensity detected in the inked areas. This is generally the case of medieval manuscripts written on parchment or European paper. For instance, the simplified version of the fingerprint model could be successfully used in advanced codicological studies on Hamburg, Staats- und Universitätsbibliothek, Cod. germ. 6 (Rabin et al. 2014; Geissbühler et al. 2018). Unfortunately, in the case of ancient papyri, the writing support is hardly ever homogeneous. To begin with, the structure of the plant itself is rather heterogeneous, made of contiguous capillary fibres of different thickness. Despite the pressure applied during the manufacturing process of the papyrus leaves, the fibrous structure is still recognizable even to the bare eye. In addition, processes of deterioration acting over centuries generally increase the heterogeneity of the leaves, physically affecting each fibre differently. Finally, archaeological material is usually contaminated by different elements depending on the diagenetic surrounding. Unfortunately, iron belongs to the primary pollutant agents, especially in sandy environments. All these processes result in a writing support that is characterised by a widely heterogeneous iron content. Figure 1 shows the distribution of the values for iron measured in the papyrus and in the ink on two different leaves of Apostolic, Vatican Library, Pap. copt. 9. First of all, we see that the iron intensity is far from constant with a larger spread corresponding to the ink. This is not surprising since the thickness of the ink is not necessarily constant throughout the inscribed portions subjected to analysis. Deterioration processes such as flaking or abrasion certainly also contribute to the heterogeneity of the ink thickness. Figure 1 illustrates that exceptional heterogeneity of iron in both papyrus and inks makes quantitative and semi-quantitative evaluation rather difficult. Note that the gap between the maximum intensity value in the support and the minimum one in the ink is very small in the case of leaf 90, and non-existent on leaf 92, where the two intervals largely overlap. Under these circumstances, even when it is possible to conduct the XRF measurements for the ink and the support in great proximity, we use the fingerprint model rather cautiously.
However, even in cases in which the semi-quantification of the elements contained in the ink is not possible, we found that the qualitative information collected using our analytical protocol was often enough to discriminate among different clusters of manuscripts when performing a study on a large geographical area and a broad time span. In the following paragraphs, the differences in the elemental composition of the inks examined will be highlighted and discussed.
The literary manuscripts: clusters 1 and 2
Figure 2 shows the results on Turin, Museo Egizio, Codex IX, classified as cluster 1. Observing the visible and near-infrared micrograph, we notice a significative change in opacity, indicating that the manuscript investigated had been written with iron-gall ink. Looking at the XRF spectra, we observe that the net peak intensity of the Kα line of iron at 6.4 keV is substantially higher in the inked area as compared with that of the support, confirming this result. The content in potassium may be attributed to the binder and the tannins, while it is surprising that satellite elements like copper and zinc, usually found in vitriol (and largely attested in Medieval European manuscripts and recipes) are absent (Aceto et al. 2017; Geissbühler et al. 2018). During the sixteenth century, the addition of metallic iron to a solution of vitriol became a common procedure to cause the copper to precipitate, leaving a solution rich in iron sulphate (Karpenko and Norris 2002). It seems unlikely that such a technology could have been applied eight centuries before. We believe this might be an indication that common iron filings were used instead of vitriol to prepare this type of iron-gall ink. We have record of the use of this ingredient in Arabic recipes from the Middle Ages onwards.Footnote 5 For this reason, we designate the inks containing only iron as ‘non-vitriolic iron-gall inks’. The collection from the library of the Cathedral of Thi(ni)s showed a great homogeneity in the ink composition, given that a similar result was obtained on all the other leaves from cluster 1, after collecting 30 measurement spots on inks from 6 leaves belonging to 4 different codicological units.
A different type of ink was identified on the leaves of the literary fragments from cluster 2, originally produced around the fourth century CE, presumably in the area of Panopolis. Figure 3 shows the net intensity profile of the elements iron, potassium, copper and manganese (often associated with iron) extracted from the XRF measurements on Montserrat Abbey, Roca Puig collection, Inv. 145, along the line connecting the papyrus and an inked area, a so-called line scan. We observe that these elements show a similar profile, increasing in intensity when moving from the support to the inked area, attesting that they are all present in the ink. Iron, copper and manganese could be attributed to the metallic salt(s) used to make an iron-gall ink, while the potassium could be present as a sulphate salt or attributed to the binder or the tannins. However, the interpretation of near-infrared reflectography challenges the possibility that iron-gall ink was indeed used in this case, posing some interesting questions. The ink appears quite pale under near-infrared light, but the comparison between visible and near-infrared micrographs shows only a slight change in opacity, which is in contrast with the one observed on the inks of cluster 1, or with those we generally observe on medieval iron-gall inks. This makes the typological characterisation of this ink rather uncertain, casting doubts on its nature. Are we looking, instead, at a mixed ink containing both iron-gall ink and carbon? As far as we know, this type of ink was not necessarily produced intentionally. We have records of some Arabic recipes in which the ingredients were roasted, (e.g. Grohmann 1967, p. 129). If this procedure was ever applied to iron-gall inks, carbon would have formed, entering the blend as a by-product. Alternatively, it is plausible that at the very beginning of the existence of iron-gall inks, the scarce knowledge regarding their chemical structure resulted in darkish inks rather than black ones, thus carbon may have been used to blacken the mixture. A recent study seems to corroborate this idea, demonstrating that, in contrary to what Robert Fuchs asserts in his definition of ‘imperfect inks’ (Fuchs 2003), not only gallic acid but also other polyphenols commonly contained in tannins can form darkish complexes with Fe2+, but the complex iron gallate remains the blackest (Díaz Hidalgo et al. 2018). This raises another question: could it be that the ink on Montserrat, Roca Puig, Inv. 145, rather than being a mixed ink, is simply evidence of an undocumented type of ink, prepared using metallic salt(s) and a vegetal ingredient containing mainly other polyphenols than gallic acid? May this be a rough attempt at producing iron-gall ink, obtaining a sort of precursor? In this case, the metal-polyphenol complex(es) formed may be characterised by its own behaviour at 940 nm.
The results obtained on Montserrat, Roca Puig Inv. 145 are coherent within all the 5 leaves examined from the same manuscript and similar to those found on the other codicological units from cluster 2, namely Apostolic Vatican Library, Pap. copt. 9 (Fig. 4) and Montserrat, Roca Puig, Inv. 14. Clearly, the portable equipment we normally employ is not sufficient to fully elucidate the nature of these inks. To unequivocally establish the presence of carbon, the method of choice seems to be IR reflectography performed at longer wavelengths, at which only carbon remains visible. Unfortunately, the dimensions of the equipment partially limit its portability. Another possibility is the application of Raman spectroscopy. However, given the limitation of portable Raman instruments, this analysis often requires bench equipment or the collection of samples. Even in such a case, poor conservation state of the ink often renders the examination fruitless.
Even if possible, the identification of carbon would not provide a complete understanding of the nature of these inks. As discussed, recent experiments proved that the complexity of the inks containing vegetal matter and metallic salts can hardly be described using the term iron-gall ink in a ‘traditional’ way, i.e. considering only the complex iron-gallic acid as responsible for the ink’s properties (Díaz Hidalgo et al. 2018). In addition, the recipes collected in the works of ColiniFootnote 6 and Zerdoun show the variety of organic ingredients employed in the manufacturing of the inks (Zerdoun Bat-Yehuda 1983). Against this background, the characterisation of the organic compounds becomes crucial to enable an accurate classification of the typologies of ink. We believe that mass spectrometry performed in atmospheric solid analysis probe (ASAP) mode could well serve this purpose. Because of its high sensitivity, this technique may lead to satisfactory and reliable results even on degraded materials and, compared with current mass spectrometry, it has the advantage of being quasi-non-invasive. Including this method in our standard protocol would lead to new insights and a greater level of accuracy in the characterisation of many inks from Late Antiquity that are so far still poorly understood. Some tests on mock samples performed with this technique are presented at the end of this article.
The presence of both copper and iron in the inks attributed to cluster 2 may suggest that vitriol (a mixture of sulphates including iron and copper) was used in their preparation. However, sulphur is a light element; therefore, XRF analysis is sometimes not sensitive enough to detect its presence in the inks examined. Even when it is possible to detect it, it does not deliver an unequivocal proof that its presence must be related to sulphates. Though unlikely, we cannot exclude the possibility that iron and copper entered the ink preparation in a form other than vitriol. In addition, we must consider the possibility that more than one ingredient, each containing different metallic component(s), was employed in the preparation of these inks. The textual examination of the existing recipes does not cast light on this matter, given that the exact composition of many of the ingredients mentioned is still rather unclear, as discussed before. However, although just as a speculation, it is interesting to point out the possible parallel between the inks identified on the manuscripts from cluster 2 and the recipe written in the Papyrus V of Leiden, which dates at the third century CE and was found in a tomb in Thebes, not far from the area around Panopolis, where the papyri from this cluster may have been produced. The fair amount of copper contained in the inks seems to provide evidence of the use of chalcanthon (a copper-based substance) mentioned in the recipe. If we assume that this compound did not contain any other metallic element besides copper, we could suppose that some iron-based substance may have been added in an attempt to obtain a black complex more suitable for writing. Interestingly, the recipe mentions also misy, a metallic salt of unknown composition that could have provided the content of iron necessary to form the black iron-gallate complex.
The documentary texts: cluster 3
The results obtained on the documentary texts examined were very different from those obtained on clusters 1 and 2. Figures 5 and 6 show the near-infrared reflectography performed on inked areas of Montserrat, Roca Puig, Inv. 308 and Inv. 715, respectively. We observe no change in opacity between visible and near-infrared light, thus indicating that both fragments were written using a carbon-based ink. Also, the XRF analysis performed on both documents confirmed this result since it did not detect any consistent presence of metallic elements.
This is an interesting result considering that these manuscripts are respectively dated at the fifth century CE and eighth century CE. By this time, iron-gall ink (or at least an ink showing a very different behaviours than carbon under near-infrared light and containing a fair amount of iron and other metals, maybe a precursor?), was already in use, as demonstrated by the results obtained on the cluster 2. This poses an interesting question: is it possible that the type of ink used was correlated to the type of manuscript produced, when considering the literary and documentary genres? The results obtained on the full corpus of manuscripts examined so far highlighted the predominant presence of two types of ink that clearly show different features, although an accurate characterisation was not possible due to the limitation of our current analytical protocol. On the one hand, we have a group of inks showing a black colour in the near-infrared region (940 nm) and no change in opacity when comparing near-infrared and visible images, revealing the presence of a significative amount of carbon black. In some cases, XRF analysis on this group of inks revealed the presence of metals such as iron, copper and in some rare cases, lead. On the other hand, we have inks showing a pale colour in the near-infrared region (940 nm), at least a slight change in opacity when comparing visible and near-infrared micrographs and containing a significant and consistent amount of metals. In some cases, they are clearly identified as iron-gall inks, while in others, the characterisation is more problematic, but given the consistent presence of iron and other metals, they may be defined as ‘similar to iron-gall ink’. These inks occasionally presented signs of corrosion. It is very interesting to discuss the distribution of these two groups of inks along the corpus. Carbon-based ink has been found on 60 out of 70 documentary texts, representing more than 80% of cases in a time span between second and eighth centuries CE. For the sake of clarity, Table 2 shows the chronological distribution of the units analysed, along with the typology of ink found on their leaves. In contrast, iron-gall ink or similar has been found on 31 out of 34 literary texts, representing more than the 90% of cases in the same time span. The use of this type of ink in literary manuscripts continues between the ninth and eleventh centuries CE, when it was found on all the 30 units analysed. Similarly, it was found on all the 13 literary manuscripts from the Michaelides collectionFootnote 7 that were analysed at the Cambridge University Library. Although this collection has been poorly studied and a date for every single manuscript has still not been established, its leaves can be placed in a time span between the sixth and tenth centuries CE.
The results obtained suggest that the transition between carbon ink, used everywhere in antiquity, and iron-gall ink, very popular in Medieval Europe, did not happen as a result of the mere function of time. Other factors, like the environment of production of a manuscript (given that documentary and literary texts were most likely produced in different environments), influenced the type of ink used. No other correlation of this dimension was observed between type of ink and support, as it was previously suggested (Lucas 1922; Macarthur 1995), neither was it found between ink and language, as demonstrated in Table 3, where the results obtained are sorted according to the language of each unit examined.
The choice of certain materials for the manufacturing of manuscripts must have been the result of economic factors. Poorer environments of production probably used cheaper ingredients to prepare the inks. It is possible that less expensive ingredients were mixed together with more pricey ones to function as diluents, and this may have conducted to the production of mixed inks. Unfortunately, to date we have no records stating the cost of one or another type of ink, nor of its ingredients. It may seem logical to think that carbon ink has always been cheaper, given that charcoal or soot can be easily produced by combustion of a variety of common materials, such as pine logs as described by Pliny. However, Pliny mentions also the soot obtained from the combustion of ivory (precious and therefore more expensive) and the importation of carbon ink from India, suggesting that different qualities of carbon ink were circulating at that time, given that these last ones must have been finer and more expensive. At this point, it is difficult to say whether iron-gall ink was higher-priced than ivory soot or Indian carbon ink, making it impossible to draw any conclusion on this matter until further analytical and historical information is made available. On top of the economic factors, there is a possibility that the choice behind the type of ink used was linked to the physical properties of carbon and iron-gall ink. The former simply sits on top of the support, adhering to it thanks to the binder, but it can be easily scraped off, even accidentally, and it is therefore more suitable in the case of ephemeral manuscripts, as in the case of receipts or private messages. In this regard, we have record of an epigraph by Martial dating from the first century CE who describes the process of removal of an ink using a ‘Punic sponge’. This proves that the practice of erasing an ink from the support once the text has lost its relevance was in use at that time (Martial 2015, pp. 67–69). In contrast, iron-gall ink penetrates the support deeply and it can be erased only in critical acidic conditions that can be achieved, for instance, by intentionally treating the ink with weak acids. Its durability makes this type of ink the best choice when writing a manuscript with the specific intention to make it last in time, as it is the case with literary texts, especially of literary codices produced as part of the library of a religious institution (it must be stressed that these represent most cases existing in the corpus examined). The manuscripts from the library of the Cathedral of Thi(ni)is, presented in this work as the cluster of manuscripts number 1, may well serve as an example. Clearly, the scenario described so far is only a partial interpretation of a much more complex phenomenon that still requires further investigation, especially focusing on those documentary texts that may have been produced with the intention to last in time, such as testaments. As a final remark, it must be stressed that the data collected so far is representative mainly of the areas of Bawit, Oxyrhynchus, Thi(ni)s and the area around Panopolis. There is a big portion of Egyptian geography that could not so far be covered, despite the logistical efforts. In addition, until now, it was not possible to compare directly a significative number of literary and documentary texts coming from the same area and dating to the same time. Therefore, the results presented in this work provide far from a homogeneous representation of the whole Egyptian reality in the time span considered. We must acknowledge that the general situation may have been very different from the one represented by the analytical data collected.
Tests with ASAP Xevo G2-XS QTOF waters
Given the difficulties previously described in characterising the inks of some of the manuscript in our corpus, the goal of this experiment was to prove the effectiveness of the method in the identification of organic compounds, this time centring our attention on hydrolysable tannins. These can be found in high amounts in gallnuts. The latter have been already studied using mass spectrometry in conjunction with chromatography (Arpino et al. 1977; Mämmelä et al. 2000). The main purpose of those studies was the identification of different tannins or tannin mixtures rather than the mere verification of their presence. We propose to use gallic acid and ellagic acid, with molar masses of 170.12 and 302.2 respectively, that can be found in high amounts in gallnuts, as well as pyrogallol (molar mass 126.11) detected as a daughter ion of gallic acid in the mass spectra obtained with different ionisation techniques (Wyrepkowski et al. 2014), as indicators of the presence of tannins in the mixture. Previous studies on the tanned parchment of Dead Sea Scrolls proved that it was possible to identify at least gallic acid even on heavily deteriorated materials (Reed and Poole 1964). For the purpose of this study, four mock samples of different typologies of ink were prepared using gallnut extract and analysed both liquid and dry, to make sure that their physical state would not influence the final result. The microsampling was performed by either rubbing a glass wire onto the dry sample or immersing it into the liquid. In both cases, the amount of sample collected was invisible to the naked eye. No further sample preparation was needed; the glass holder was inserted directly in the machine for examination. Since the analysis was performed using negative-ion pattern, the precursor ions of pyrogallol, gallic and ellagic acid are found respectively at around 125, 169 and 301 m/z. Table 4 lists the description of the samples and the results obtained.
We find it encouraging that all the samples investigated showed prominent peaks corresponding to at least two out of three characteristic components of the family of hydrolysable tannins and consider it to be a demonstration of the effectiveness of the technique in the identification of these components. In this preliminary study, we neither studied the effects of the ionisation conditions on the resulting spectra nor used a collision cell for controlling the fragmentation processes. Despite this fact, gallic acid and its fragment pyrogallol seem to be easily detectable whereas a lower amount of ellagic acid might pose a problem. Figure 7 shows the sampling technique as well as the mass spectrum of sample B, on which the peaks corresponding to deprotonated pyrogallol, gallic acid and ellagic acid have been highlighted.