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Browsing through sealed historical manuscripts by using 3-D computed tomography with low-brilliance X-ray sources - PubMed

  • ️Mon Jan 01 2018

Browsing through sealed historical manuscripts by using 3-D computed tomography with low-brilliance X-ray sources

Daniel Stromer et al. Sci Rep. 2018.

Abstract

Severely damaged historical documents are extremely fragile. In many cases, their secrets remain concealed beneath their cover. Recently, non-invasive digitization approaches based on 3-D scanning have demonstrated the ability to recover single pages or letters without the need to open the manuscripts. This can even be achieved using conventional micro-CTs without the need for synchrotron hardware. However, not all manuscripts may be suited for such techniques due to their material and X-ray properties. In order to recommend which manuscripts and which inks are best suited for such a process, we investigate six inks that were commonly used in ancient times: malachite, three types of iron gall, Tyrian purple, and buckthorn. Image contrast is explored over the complete pipeline, from the X-ray CT scan and page extraction to the virtual flattening of the page image. We demonstrate, that all inks containing metallic particles are visible in the output, a decrease of the X-ray energy enhances the readability, and that the visibility highly depends on the X-ray attenuation of the ink's metallic ingredients and their concentration. Based on these observations, we give recommendations on how to select the appropriate imaging parameters.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1

(a) The book consisting of 56 handmade paper pages covered in buffalo leather. (b) Schematic of the book placement within the scanner with the cover orthogonal to the rotation axis z. While the turntable rotates, a set of projection images are acquired, from which a 3-D volume is calculated. (c) X-ray attenuation plot for the different inks and paper–malachite ink has the highest attenuation, followed by iron gall and Tyrian purple. As buckthorn ink and paper are both made of cellulose their absorption is the same.

Figure 2
Figure 2

Energy-dispersive X-ray spectroscopy of the inks: malachite ink consists of Cu. For all three iron gall inks, the elements Fe and S were detected (one exemplary ink is shown). Tyrian purple ink consists of Al, S, Cl and Sn. Buckthorn ink does not consist of metallic particles, but only C and O. All spectra show C and O which is caused by the cellulose of the paper.

Figure 3
Figure 3

(a) Exemplary X-ray volume xy-slice of the scanned book: The high resolution and the orientation of the pages in the volume make it impossible to read each page separately with the naked eye. Multiple pages are present in one slice such that a virtual flattening is necessary. (b) yz-slice of the volume–the pages are separated by air gaps and look like small vessels. (c) The same yz-slice after applying the volume binarization step–the pages are binarized but still contains overlaps and gaps. (d) The final output after page segmentation for the yz-slice of (b)–overlapping pages are separated and gaps are closed.

Figure 4
Figure 4

Volume processing pipeline for automatic page extraction. Within pre-processing (blue boxes), the volume is denoised, binarized and overlapping pages are separated. Next, the pages are smoothed and holes are filled iteratively (green boxes). A texturing step stores the maximum value along the height of a page in a 2-D image resulting in the final 2-D mapped page (orange boxes).

Figure 5
Figure 5

3-D X-ray CT reconstructed volume xz-slice with different book placement where the orange boxes denote the areas of interest. (a) Vertical book placement–the penetration beam length varies due to the placement resulting in metal artifacts that penetrate the pages. (b) Horizontal book placement with cover orthogonal to the rotation axis–no metal artifacts are visible.

Figure 6
Figure 6

Photographs of the original pages are shown in the top row–(a) malachite ink, (b–d) iron gall ink 1–3. (e) Tyrian purple ink. The center row shows the reconstructed and 2-D mapped pages of the closed book for the 50 kV scan, the bottom row the 30 kV scan output. Tyrian purple ink has a small attenuation and the writings can be seen only slightly compared to iron gall and malachite ink.

Figure 7
Figure 7

Zoom on letter ‘C’ from the 30 kV scan for three inks inks. (a) Malachite ink, (b) iron gall ink 1, (c) iron gall ink 3. A plot along the orange line shows, that the intensities at the border regions are higher than in the central region. This can be explained by the chromatographic effect when the spread of the metallic particles is stopped within the paper in border regions resulting in a higher concentration of these particles and thus higher X-ray attenuation.

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