5.1 Evaluation of tomographic data
The information that could be learned from the bookbinding by direct observation was significantly broadened due to the tomographic research. Bookbinding is a combination of different materials, from those with low Xray attenuation such as paper, textiles and leather, through those that are more attenuating, such as wood, to those that are highly attenuating, such as metals. In the case of a dense book block with the presence of metal clips, it was necessary to use a relatively high voltage with sufficient Xray power.
Binding structure, materials, accessories and decoration
The gilding on the spine, including the text and lines between the raised bands (Fig. 2), was visible in the surface layers. The font appears in the images of the book block in the form of light, even white vertical lines. The most prominent is the red font, which contains mercury. In the next image, the visualization was set to highlight the structure of the spinal bindings (Fig. 3b). The bookbinding consists of (two) lengths of twisted cord.
Images of the layers on the book’s boards did not reveal the use of stickers or secondary materials under the leather covering. The next Fig. 3a shows the wooden board with the ends of the cords clearly visible. This concerns a layer close to the book block, whereby the taped ends of the frayed cord are clearly visible on the end paper. The initial visual research is very important for the subsequent good orientation in the layers of the image. Navigating individual layers from different angles can be confusing for the observer and lead to misidentification of the location of the element being studied. Figure 3a shows the annual rings in the wooden boards.
A very useful tool for viewing the internal structures of the book is the 3D visualization of the tomographic model. The elements are plastic and from the previews it is possible to determine the method of stitching, bookbinding fixation and layering (Fig. 4). On the far right are the layers of the box in which the book was placed. In close proximity to the material of the box is the compact layer of the leather covering, which is no longer glued to the entire length of spine, but which is noticeable in the space between the bookbinding of the cavity. The leather fits tightly against the protruding right bookbinding. We can see a section of the individual bookbinding, which are formed by (two) lengths of twisted cords wrapped around a thread. The passage of the thread through the component is also noticeable. The tomographic model enables to investigate the condition of the thread and its integrity throughout the book block to be thoroughly checked. There are no visible layers on the spine that indicate the presence of fractions – fragments of recycled materials described. The red font is also clearly visible. The text from the book is clearly visible due to the high elemental differentiation of the historical red colour, with its high iron and mercury content, from the book block [6]. The overall Fig. 3a shows tomographic 3D visualization with reduced visibility of lighter elements, highlighting details of the red lettering, bookbinding and clasps. If the angle of the section is selected correctly, the text can be read in parts. However, this is made more difficult by the considerable rippling of the pages.
Other images (Fig. 5) document the structure of the metal clasps. A 3D visualization of the entire book, with the settings for the metal elements presenting the opportunity to document different materials separately.
5.2 Testing the ability to read a closed book based on tomographic data
In addition to studying the construction of the book, an interesting option is to read the text in a closed book on the tomographic reconstruction basis. In the case presented here this is not necessary. However, in the collections there are a number of books (or scrolls and other documents) that are difficult or impossible to be opened (unpacked) due to their current state [7]. Not many papers have been published on this topic, with those that have mostly presented simpler cases in which the pages do not overwhelm each other [8], or, in the case of denser book blocks, only small fragments of text have been read, see the review article [9].
In the case of reading closed books by means of tomographic reconstruction, we are faced with the problem that, especially in old prints, the pages are wavy, while it is most convenient to examine the tomographic volume in its planar sections. However, if the pages are wavy, we will see text coming from different pages in the given section, see Fig. 10. On the Fig. 6 is a sample of a section of a book block, perpendicular to the pages. The page waviness exceeds 6 mm, which in this case includes 100 pages of text, due to which an unclear mismatch of text can be seen in one planar section of the initial CT reconstruction, that is very difficult to navigate. Since the voxel size is larger than the page thickness in the reconstruction discussed above, this reconstruction was resampled 4x, perpendicular to the page plane.
The next step involved virtual page straightening. First it was necessary to find a surface describing the shape of the selected page of the book. This was implemented as follows: In the selected section, the approximate position of the letter (appear as a white line in the section) was indicated at the edge of the reference page. Its position was refined automatically by software searching for a maximum density voxel in a small, predefined area. Likewise, the next character is searched for horizontally within a defined area, at a distance roughly corresponding to the letter pitch of the text. Once the positions of at least three letters are known, additional characters are searched on the basis of the extrapolation curve passing through the previously found characters. Finding all the characters on the edge of the page over the entire width of the book block yields a curve describing the shape of the page in the given section, see the top Fig. 6 - green line in the book block. In the part outside the described area, this curve is extrapolated from the last character found. The found curve then serves as the initial condition for finding the page shape in adjacent sections. Using the procedure described, the area of the entire page is finally found, see Fig. 6. It shows that it has a wave range of 6.7 mm.
In the last step, the book block was virtually straightened according to this found area. Basically, this involves the transformation of the coordinate system, see [10, 11], which is defined according to the page shape found. The result is significantly improved readability of the text. Figure 7 shows a tomographic section before straightening on the left, a page after straightening in the middle (this is the page of book 239), and a photo of the same page on the right for comparison. Note that the page has not been perfectly straightened, so we still see text from different pages, although at a range about 10 times smaller than without straightening. The main reason for the imperfect straightening is the fact that some areas without tomographically visible text exist on the entire reference page, so that during the search for the page area there are “jumps” to adjacent pages. Even so, the result is considerably clearer than in the initial state. Moreover, it is quite easy for researchers to walk through the resulting virtual model and find the necessary contexts.
Similar results from the virtual straightening can, of course, be presented for other pages of the book. Since the waving of the book pages is not the same through the book block, the presented virtual straightening is only valid for a limited number of pages. However, it has been shown that it is possible to apply the algorithm to highly waved pages that would otherwise be very difficult to read directly from the non-processed tomographic model.
5.3 Dendrochronological analysis of book covering
Dendrochronological analysis was performed from tomography of the wood rings on the book boards in order to date them. The cross-sections of the left and right wooden boards in the tomographic model (Fig. 8a and Fig. 8b) were used. The images provided important information about the width of the tree rings and their number. It was proven that they were of oak. The determination of wood was performed using common xylotomy methods [12]. The dendrochronological processing method was divided into several steps and followed the standard chronology of tree ring widths [13, 14]. First, the width of the tree rings on Xray tomography images was measured using OSM32. Subsequently, relative synchronisation of the obtained tree-ring series was performed. The ring curves were then compared and relatively synchronised. The sets of mutually synchronous curves were averaged into sum curves. The last step was to attempt absolute dating using the PAST program. This program includes data verification and synchronization of tree-ring series with standard chronology. When using this program, the degree of similarity of the compared series or chronology is assessed using the correlation coefficient and the concurrency coefficient (Gleichlaufigkeit) after standardisation using Hollstein high-frequency filters and the Baillie & Pilcher method [15]. Available standard oak chronologies for Central Europe, France, Poland and the Baltics were used to date the oak. The attempt at relative synchronization of the ring curves of the two book boards was not successful. The book boards were therefore dated separately. Absolute dating of tree ring curves with standard oak chronologies resulted in the discovery of a synchronous position in only one row. This curve was reliably datable to the 2021BLT1 and 2021BLT3 chronologies, which represent oak material originating in the Baltics [16]. These particular chronologies probably represent oak trees originating in the area of present-day Lithuania [16]. A tree ring dating back to the year of felling of the used trees has not been preserved on either of the book boards. In these cases, therefore, it was only possible to equivocally identify the earliest year after which the used tree was cut, namely after 1608, which corresponds to the preserved historical information about the book and confirms the originality of the preserved book boards. [17, 18]
5.4 Investigation of chemical composition of inks based on Xray fluorescence
Xray fluorescence (XRF) is an important non-destructive method for investigating the chemical composition of materials. The object under investigation is irradiated by Xrays (or gamma rays), which excites electrons in atomic shells into higher layers. Deexcitation occurs, i.e. the return of electrons to the inner layer. This releases energy in the form of electromagnetic radiation, whose wavelength or spectrum is characteristic for every chemical element. The energy of the emitted photons is relatively low, so there is strong attenuation in the material being studied. This fact represents a major limitation of this method because secondary photon attenuation makes it impossible to perform analysis at greater depths of the material. However, this fact does not pose a significant obstacle to measuring the composition of the inks in the directly visible pages of the historical book.
5.4.1 Description of the XRF experiment
A Prospector 3 handheld spectrometer manufactured by Elvatech Ltd. was used for the XRF spectrometric measurements. This device is equipped with Xray tube with a rhodium anode, with a maximum current of 200 µA and an acceleration voltage of up to 50 kV. Prospector 3 has energy resolution of 140 eV at the Kα line of manganese.
The book was supported by soft wedges to avoid excessive stress on the bookbinding when it was opened. XRF data were collected from five pages, whereby mostly an area of only one colour was taken, in one case both colours on the same side. An ink-free area, i.e. blank paper, was always taken as a reference sample. A collimator with a diameter of 2 mm was used for all the measurements. The measured area was targeted using an image from an integrated camera. Data acquisition was always carried out in two steps. In the first step, data suitable for emission edge analysis at higher energies (up to 35 keV, so-called “heavy spectrum”) were collected. The data acquisition took 200 s, with an integrated 0.8 mm thick aluminium filter placed in front of the Xray source. In the following step, data suitable for more sensitive spectrum analysis in lower energy regions were collected from the same area. The accelerating voltage in this case was 12 kV (“light spectrum”) and no filter was used. The data collection took 300 s.
5.4.2 XRF data evaluation
The ElvaX software tool was used to evaluate the data. The obtained spectra are quite complex and their evaluation must be approached with great caution. The XRF spectrometer used has a rhodium anode, therefore it does not enable to analyse the presence of chlorine, whose K edges are in the same energy region as the L edges of rhodium. Sulphur detection is similarly problematic. During the evaluation, the spectra measured in the pigment-free paper region were compared with the pigment region. During pigment analysis, more elements were gradually added to the set of analysed elements, starting with heavy elements. In this way, the possibility is minimised of adding an element that is not actually present in the pigment, but the spectrum shows a peak in the region of some of its edges.
The book contains three volumes with visually different printing and paper. By evaluating the spectra in the inkless paper area, it was found that the paper material throughout the book is similar. Calcium, iron, manganese, copper and zinc were found in all places; other elements were found in lower concentrations. In addition, however, we also found traces of mercury and lead in the paper area, which originate from ink abrasion and mercury diffusion. Despite the aforementioned similarity in paper composition, minor differences were found. The paper of the title page contained less calcium and iron than the paper in the first and third volume. It also lacked traces of mercury. The paper of the third volume had the highest iron content, the paper of the first volume the highest calcium content.
Subsequently, the ink analyses were carried out. The red ink showed an abundant presence of mercury and lead. Sulphur is also detectable. This finding confirms that the red ink is a vermilion-based pigment (cinnabarite). More precisely, quantitative analysis of the ink indicated that the pigment is composed of 97% mercury sulphide and 3% lead, which is either an admixture in the ore or a deliberately added essence during pigment preparation, see [19]. The spectrum directly from the Prospector instrument is shown in the Fig. 9 for illustration.
Comparison of the red ink spectra used in the first, second and third volumes revealed that the ink from the third volume showed a lower content of mercury but a higher content of lead (89% Hg, 11% Pb). Hence, the ink is based on the same pigment, but it cannot be ruled out that it is an ink from, for example, another workshop or a different batch.
No extra element was detected in the black ink area compared to red ink. From the analysis of tomographic sections from Xray scanning and their comparison with photographs of specific pages of the book, it is immediately clear that only red ink causes significant attenuation due to the mercury and lead content. In Fig. 10 is a comparison of the front page with the inscription Euchologion. The red angel and part of the inscription are clearly visible on the tomographic section, while the black drawing is not. However, the representation of red ink in the book is high, so it is difficult to measure the spectrum of black ink by XRF so that pulses from red ink on other pages do not leak. Measurements in the area of several pages with only black ink revealed a slightly higher concentration of mercury in the black ink than in the surrounding paper. Although the diffusion of mercury vapours into the whole book block is evident, its higher content in the black ink could also be attributed to the contamination of the ink during the print. Since no other significant element compared to paper was found, the composition of the black pigment cannot be determined. What can be said is that it is not a metal oxide pigment that can be detected by spectrometric analysis, which implies that it is very probably a common carbon-based pigment.