Sample characteristics

The Herculaneum scrolls were burnt by a blast of furnace-like (c. 320 °C) gas from the volcano into a vastly homogenous sample matrix. In Antiquity, the most common writing material used for books and other documents was papyrus. The stalks of papyrus plants were cut into thin strips, then two strips were laid atop and perpendicular to one another; these strips were then combined by pressing them until the plant’s natural starch that acted as a paste. The resulting sheets of papyrus were then smoothed and bleached. A papyrus roll consisted of some number of such sheets pasted together end on end. The resulting sheet, which could reach 15 m or more in length, was then rolled up from right to left, so that it could be read from left to right. Papyrus writing material, therefore, consisted of two layers, whose fibres ran perpendicular to one another; the scribe would usually write the text in vertical columns on the side of the papyrus sheet where the horizontal fibres were uppermost. The carbon-based ink he used did not penetrate into the papyrus fibres, but sat atop them. As we shall see, this fact proved to be crucial for our experiments.

In general, the letters stand out in slight relief from the papyrus surface, a phenomenon that is familiar to papyrologists reading unrolled Herculaneum texts with the aid of a microscope. It is precisely this morphological feature that allows the highlighting of letters by XPCT, although the difference between ink composition and carbonized papyrus is not sufficient to enhance any differential absorption using conventional XCT. The relief of the letters, which rise at least one hundred microns above the surface of the papyrus fibres, is by itself sufficient to produce appreciable phase contrast.

Writing detection in a fragment of papyrus scroll PHerc.Paris. 1

To prove the effectiveness of the proposed technique, a first experiment was performed on a very small part of fragment 101 from PHerc.Paris. 1, removed from the roll thanks to an ‘unrolling’ operation conducted by the Oslo method in Naples in 1986/7, and showing several superposed layers of papyrus. On one of the hidden layers, we have succeeded in reading two words in successive lines. In Fig. 1a the sequence of Greek capital letters ΠIΠTOIE(pi-iota-pi-tau-omicron-iota-epsilon), could be read either as ππτOιε[ν or as ππτOι followed by a word beginning with an epsilon, for example, [ν (or ε[ισ), and meaning ‘would fall’. In the following line (Fig. 1b), it is possible to read another letter sequence, almost certainly EIΠOI (epsilon-iota-pi -omicron-iota), meaning ‘would say’. The spatial resolution of the detector (see Methods) is 100 microns so that the letters observed in this papyrus, which are about 2–3 mm, are fully disclosed by the imaging process.

Figure 1: Two words read on a hidden layer of the fragment PHerc.Paris. 1 fr. 101. (a) The sequence of Greek capital letters ΠIΠTOIE (pi-iota-pi-tau-omicron-iota-epsilon); (b) The letter sequence of the next line, EIΠOI (epsilon-iota-pi-omicron-iota). Full size image

Writing detection in papyrus scroll PHerc.Paris. 4

The main object of our investigation has been a nearly complete scroll that is typical of rolled-up Herculaneum papyri. This specimen is relatively well preserved and is inventoried as PHerc.Paris. 4 (Fig. 2a). In Fig. 2b three orthogonal slices of the reconstructed volume (Fig. 2c) give a picture of the morphology of this scroll. The tremendous pressure of the pyroclastic material compressed the scroll and deformed its internal spiral structure, the layers of which are folded in a nearly chaotic and badly entangled fashion. Moreover, in many places the whorls have adhered to one another. Therefore, it should be emphasized that the letters (which were capital letters in all the literary papyri) may be more or less distorted, since they lie on highly non-planar surfaces that are cut by selected planar sections for rendering purposes. All this makes data analysis a challenge, since it is necessary to ascertain the phase contrast associated with the text on a surface, the shape and topology of which exhibit an extremely high degree of complexity. Nevertheless, we were able to discern evidence of this contrast in the reconstruction.

Figure 2: The Herculaneum papyrus scroll PHerc.Paris. 4. (a) A picture of the PHerc.Paris. 4 lying in its transport holder, fabricated by means of a three-dimensional scan of the external papyrus shape profile (~16 cm length). The carbonized papyrus is extremely fragile. Its deformation was caused mechanically by the eruptive material; (b) three orthogonal slices, and (c) a volume rendition of the reconstructed papyrus, which highlights the huge complexity inside the scroll, where the papyrus convolutions were exposed to tremendous stresses. Full size image

In Fig. 3a, it is possible to make out three letters in linear sequence. The first is undoubtedly an A (alpha) and the third an N (nu). The middle letter, however, requires a more careful interpretation: the loop, which we believe is its topmost part, evokes a P (rho), that is, a letter equivalent to English R. We would then have the sequence ‘APN’, which can either come from a single word like αρν-εĩσθαι ‘to deny’ or be divided into the two-letter sequence ‘AΡ’ and the single letter ‘N’, giving, for example, the particle [γ]αρ, ‘for’ (which is always in second position in a clause or sentence) and preceding a word starting with N. In Fig. 3b, the sequence of letters HEY (êta-epsilon-upsilon), equivalent in English to a long E followed by the syllable EU, is clearly identified. This sequence falls naturally into the single letter H and the two-letter sequence EU, the former being presumably the feminine definite article ‘the’ and the latter the first syllable of a nominal (cf. for example, in English euphonia, euphonic). In Fig. 3c, we can identify the sequence KI, κι (kappa-iota), found, for example, in the words of the verb family κινεν ‘to move’. Unfortunately, the risk of confusion is always possible when dealing with horizontal or vertical traces, since they can correspond either to actual letters or to fibres in the papyrus material. The only clearly distinct marks in such a document are rounded or oblique traces, shapes that are not easily confused with papyrus fibres, which run vertically and horizontally, and are therefore letters or parts of letters. Hence, the following letters are relatively easy to distinguish: A, B, Δ, E, Z, Θ, K, Λ, M, N, O, P, C, Υ, Φ, X, Ψ and Ω; while Γ, H, I, Ξ, Π and T are more easily confused. The script here is noticeably different from that of PHerc.Paris. 1, shown in Fig. 1.

Figure 3: The letters discovered inside PHerc.Paris. 4. (a) The sequence of Greek capital letters APN (alpha-rho-nu); (b) the letters sequence HEY (eta-epsilon-upsilon) and (c) the sequence KI (kappa-iota). Full size image

The alphabet of this papyrus that we present in Fig. 4 comes from different images of the scroll acquired during the experimental scans. It is to be read as follows: the 24 letters of the Greek alphabets are arranged in three superimposed lines. Lines 1 and 2 consist of letters taken from several internal regions of the sample (for several letters, line 2 shows a second, slightly different, example we were able to obtain). One should not forget that the letters may be heavily distorted, due to the highly non-planar surfaces on which they lie. Line 3 reproduces the corresponding letters extracted from infrared images of a Herculaneum papyrus (PHerc. 1471) opened previously and preserved in the National Library in Naples, Italy; it contains a work by the Epicurean philosopher Philodemus, ‘On Frank Criticism’, which has been studied in depth by one of the authors of the present article. The letters from PHerc. 1471 are meant to help the reader in recognizing the layout of the letters appearing in PHerc.Paris. 4.