Documenting Parchment Degradation Via Multispectral Imaging

Cultural Heritage Destruction: Documenting Parchment Degradation via Multispectral Imaging

Alejandro Giacometti Alberto Campagnolo Lindsay MacDonald Dept. of Medical Physics Ligatus Research Centre Photogrammetry, 3D Imaging & Centre for Digital Humanities University of the Arts, London Metrology Research Centre University College London [email protected] University College London [email protected] [email protected]

Simon Mahony Melissa Terras Stuart Robson Centre for Digital Humanities Centre for Digital Humanities Photogrammetry, 3D Imaging & University College London University College London Metrology Research Centre [email protected] [email protected] University College London [email protected]

Tim Weyrich Adam Gibson Dept. of Computer Science Dept. of Medical Physics University College London University College London [email protected] [email protected]

This paper describes the methodology and presents preliminary results of a project using multispectral imaging to document the deterioration of parchment. A series of treatments has been applied to degrade samples from a deaccessioned manuscript using both physical and chemical agents. Each sample is photographed before and after the treatment by a multispectral imaging system to record the effect of the treatments on both the writing and the parchment. Here we present the initial imaging of the samples, and we show how some of the agents affect the writing and parchment.

Multispectral imaging. Manuscript. Parchment degradation.

1. INTRODUCTION rendered document to its supposed original state, before the damage occurred. In this project, we aim Digital surrogates of documents have become an to investigate the processes of controlled document indispensable tool for scholars and archives. They degradation using multispectral imaging. We have provide an easily accessible, non-degrading obtained a manuscript on parchment, which we are resource for research and study (Terras & subjecting to a variety of degradation procedures. Robertson, 2006). Numerous imaging and image We are documenting the process via multispectral processing technologies have been employed to imaging. In this paper, we present the programme, examine the characteristics of the originals, such give an outline of the methodology for degrading as X-ray imaging, imaging in the infrared spectrum, the manuscript, and describe the approach to and multi-spectral imaging. These offer additional imaging. Finally, we show some samples from the information about the physical characteristics and initial imaging trials, and preliminary results from condition of the document. The results have been our tests with the various degradation methods. better images, more possibilities for analysis, and additional information recovered from ancient We obtained an 18th-century manuscript de- documents. (Chabries, Booras, & Bearman, 2003; accessioned from the London Metropolitan Salerno, Tonazzini, & Bedini, 2007; Tanner & Archives. Although it was in good physical Bearman, 2009). condition, this manuscript was deemed to hold no historical value, and it was kindly provided to us for Digitisation projects generally concentrate on experimentation. It comprised two large sheets of documents in their existing state. Typically the prepared animal skin, i.e. parchment, written in iron focus of image processing is to restore the digitally gall ink and highlighted in red ink. Each sheet

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Cultural Heritage Destruction: Documenting Parchment Degradation via Multispectral Imaging Alejandro Giacometti, Alberto Campagnolo, Lindsay MacDonald, Simon Mahony, Melissa Terras, Stuart Robson, Tim Weyrich and Adam Gibson measured approximately 70×70 cm (Figs. 1 and 2), satellites were also designed to capture multi- and the two had been tightly folded together for spectral reflective images from Earth, the most storage. It was dated 11th August 1753, in the reign famous being Landsat, first launched in 1972, with of George II, and appeared to be an indenture eight spectral bands. The original purpose was to (contract) between a Mr John Sherman and Mr identify the kinds of resources that could be found Christ Gardiner. in a specific location without the need for high spatial resolution. The idea was to analyse spectral variations in a pixel rather than the spatial variations in higher resolution images (Landgrebe & Chen, 1999).

Multispectral imaging (MSI) has been successfully applied to the imaging of fine art paintings (Hardeberg, 2001). It has also been utilised to image ancient documents (Attas, 2004; Salerno et al., 2007). Bearman and Spiro (1996) were the first to use digital MSI on the Dead Sea Scrolls, with a liquid crystal tunable filter in front of a monochrome CCD camera. They captured digital images from 400 to 1050 nm, and noted the great improvement in legibility at 970 nm, compared with the image at 700 nm.

Figure 1: Outer sheet of the complete manuscript, Multispectral images can be used to identify, and showing writing in Iron Gall ink on parchment hence to separate, different types of inks in a single document, allowing a researcher to determine whether the document has been edited, or if it was written by several different hands. Additionally, the reflectance profiles of known materials can be stored in a database and used to classify unknown materials. Recognising the type of ink can help determine the age or location of a specific writing.

The type of ink used has considerable impact on the imaging approach. The earlier carbon black ink consisted of graphite or soot particles suspended in an organic binder and applied with a stylus. These inks do not penetrate or stain the parchment but rest on top, with particles adhering to the micro- structure of the surface. Iron gall ink, also known as gallo-tannin ink, was introduced around the third century AD, prepared with organic material that Figure 2: Inner sheet of the complete manuscript penetrates the substrate more deeply and reacts to dye it black. Chabries et al (2003) noted that the parchment reflectance increases at longer 2. MULTISPECTRAL IMAGING wavelengths, resulting in greater contrast of text in the infrared images. A multispectral image is a set of images acquired through narrow band filters for consecutive wavebands. Each image shows the intensity of 3. METHODS radiation from the scene in the corresponding waveband (Vane & Goetz, 1988). Images are From the original manuscript, 23 flat square acquired at visible wave-lengths (400–700 nm), sections of 8×8 cm were cut, with each section and may also include regions of the non-visible containing written text. Two sections have text spectrum: ultraviolet (<400 nm) and infrared (>700 written on both sides. Each sample will be exposed nm). Images that include more channels over a to an external deteriorating agent in order to force a broader range of wavelengths are referred to as rapid and controlled form of degradation. The hyper-spectral images (Landgrebe & Chen, 1999). agents include mechanical damage, heat, humidity, Multispectral imaging was developed for remote abrasion and a variety of substances with different sensing to determine the composition of objects in chemical properties (Table 1). Each will affect the space by measuring their emission spectra. NASA appearance and condition of the samples in

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Cultural Heritage Destruction: Documenting Parchment Degradation via Multispectral Imaging Alejandro Giacometti, Alberto Campagnolo, Lindsay MacDonald, Simon Mahony, Melissa Terras, Stuart Robson, Tim Weyrich and Adam Gibson different ways, typical of the actual damage violet ultraviolet light. The energy in suffered by parchment in real archives. In the light photons can cause inter- following sections we describe the methods for molecular and extra-molecular degrading the parchment and for imaging. cross-linking in the collagen fibres. When this occurs, at a 3.1 Degradation treatments microscopic level the collagen fibres will shorten, fray, and flatten; at a microscopic level Large collections of parchment documents exist in prolonged exposure to light will public and private libraries, archives, and museums eventually lead to discoloration, in varying degrees of preservation. Parchment has fading, and fragility. The process a predominantly organic composition, mainly based is slow, but cumulative. on protein collagen in association with small 5. Mould Keep parchment in an enclosed quantities of elastin, globular proteins, and fats. high-humidity (>90%RH) Parchment is prepared from a pelt, i.e. an animal environment for an extended skin that has been wetted, immersed in lime water period of time until mould growth (a saturated solution of Calcium Hydroxide), is detected. The parchment is dehaired, and scraped. The pelt is then left to dry subsequently desiccated. Mould under tension on a wooden frame. The stretching grows on documents in humid (>75%RH) and warm (>22-25°C) of the soaked pelt has the effect of reorganizing the conditions. As microorganisms collagen fibre network into a laminate structure, grow and feed on the parchment, and, as the pelt dries, the fibres are locked into the multiple effects can be detected. stretched condition. The resulting material is a fairly At the microscopic level the stiff sheet which, without any further treatment, is collagen fibres appear split and durable and can last for centuries, provided it is shortened; at the microscopic kept cool and dry (Reed, 1972; Clarkson, 1992; level the appearance of the Larsen, 2007). medium is altered, with changes in colour and mass – e.g. thinning. (Součková, 2007). Parchment is an extremely hygroscopic material that reacts readily to changes of humidity with 6. Scraping Forcibly remove the surface layer changes in thickness and physical appearance, and the visible writing off the parchment through the use of such as cockling and curling. It is also endangered pumice stone. Scraping was a by biological, thermochemical, and mechanical common way of deleting, and degradation. correcting text on parchment, as We have selected some methods of artificial well as a way to reuse the degradation for our parchment samples to simulate medium for a completely new text the kinds of damage that can affect parchment – e.g. palimpsest documents. documents found in cultural heritage collections. 7. Mech- Alter the surface of the These degradation methods fall within the main anical parchment via double folding and categories of mechanical, thermochemical and damage rolling. The surface of parchment humidity, biological, and physical damage. These document can be affected by act directly upon the writing medium – i.e. the wear and tear. This is usually parchment – changing its physical characteristics at most notable in the first and last membranes of a codex, both microscopic and macroscopic levels, as well especially if the binding has as damage caused by external substances that act partially or totally failed. The text directly to obscure the text. can become difficult to read, and the surface of the medium Table 1: Methods for degrading the parchment becomes softer and velvety as the grain and fibres are broken by No. Treatment Description the repeated mechanical action. 1. Control Control parchment from inner 8. Scrunching Alter the surface of parchment by sheet sheet, no treatment. scrunching. Badly stored 2. Control Control parchment from outer documents, especially single-leaf sheet sheet, no treatment. ones not in codex or scroll form, 3. Fold Control parchment containing a or pages that have become control fold – no treatment. Fibres at the detached, can become crumpled, fold are more degraded than altering the physical appearance fibres from other parts of the of the medium. The writing inks, document due to their exposure especially on a degraded to physical and mechanical parchment, can flake away under damage and dirt. these conditions; miniatures and other coloured portions of text are 4. Ultra- Expose the parchment to particularly susceptible to this

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kind of damage. are thus not usually removable. 9. Heat Submit the parchment to Moreover, blood looks similar to temperatures above 200 °C. Dry iron gall ink as haemoglobin is heat removes most of the water rich in iron. from the collagen structure and 16. Aniline Stain the parchment with aniline the molecules are thus closer dye dye. Although fairly modern – it together and cross-linking occurs was first isolated in 1826 and more frequently and freely under used in the dye industry since the these circumstances. Collagen 1850s – this chemical compound fibres will shrink, flatten, and split, is used to dye leather. while at the microscopic level the Colouration of skin by aniline document will undergo quite dyes is permanent, making drastic changes in dimensions. reading the text very difficult if not (Vest et al., 2007). impossible to the naked eye. 10. Smoking Apply smoke to the parchment. Chemically, aniline dyes are very Smoke temperatures are kept low different to iron gall ink. in order to separate the effect of 17. Oil Stain the parchment with an oily smoking and heat. Smoke alters or greasy substance. Oily stains the colour of the medium, while are common in historical soot obscures writing further. collections. Oil replaces the air 11. Iron Stain the parchment with modern trapped inside the skin and gall iron gall ink. Sometimes a text makes the parchment was corrected, or altered, at a translucent. If a document later stage, by obscuring letters contains text on both recto and or words deemed wrong with ink. verso, both writings will be visible metallogallic based ink recipes from both sides making reading varied greatly from person to difficult. person, and period to period. 18. Desiccant Enclose the parchment in an 12. India Stain the parchment with India airtight container with desiccant ink ink. India ink is composed of for a period of time, until the lampblack, combined with water humidity drops below 30% RH. and a binding agent like gelatine, When parchment is stored in a shellac, or gum Arabic. Carbon- very dry environment, irreversible based inks were amongst the first cross-linking of the collagen kind of writing inks used. Their structure can take place as the chemical composition and the collagen molecules are in close physical characteristics of their contact in the absence of dried form is very different from intermolecular water content. those of metallogallic inks. (Vest et al., 2007). 13. Black Stain the parchment with black 19. Sulphuric Apply Sulphuric acid (H2SO4) to tea tea. Black tea is rich in tannins. acid the parchment. The concentration Tannins have marked astringent of acid is kept low in order to properties and draw collagen observe the changes, while fibres closer together, reducing maintaining its structural integrity. the water content of the skin. Beginning in the early 19th Vegetable tannins have also a century, sulphur compounds colouration effect, mainly shades were used to speed up the of reds and yellows. The effect of process of preparing the pelt. this colouration would not wash These sulphur compounds away once set. (Reed, 1972). remain in skin even after rinsing and eventually convert into

sulphuric acid. Skin treated in this 14. Wine Stain the parchment with red fashion is weaker and greyish in wine. Wine is also rich in tannins, appearance.(Reed, 1972) but the colouring effect is less 20. Hydro- Apply Hydrochloric acid (HCl) to conspicuous and the tanning chloric the parchment. The concentration effect is less prominent, which acid of acid is kept low in order to will influence spectral response. observe the changes, while 15. Blood Stain the parchment with animal maintaining its structural integrity. blood. Blood stains can occur in Non-oxidising acids like hydro- historical documents. These chloric were used in production of appear as brownish stains that iron gall inks. Although parch- can sometimes obscure the text. ment is more resilient to acidic Unlike paper documents, (and alkaline) substances than parchment cannot be washed paper, an excess of acidity in ink during conservation and stains

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can perforate the parchment At the end of this project, there will be a total of (Banik, 1997). 3900 images, captured in 2 sessions, in 4 21. Calcium Apply calcium hydroxide modalities (Table 2). There are 23 square hydroxide (Ca(OH)2) to parchment. parchment samples in total, but two samples Calcium hydroxide is a contain written text on two sides. These samples strongly alkaline substance, are imaged once on each side. used in the production of parchment as a dehairing Table 2: Imaging details and defattening agent. (Reed, 1972) Modality Squares Filters Session RGB Reflective 25 17 425 3.2 Multispectral imaging RGB Transmissive 25 17 425 Each sample is imaged before and after the Monochrome Reflective 25 22 550 treatment, using two different cameras, in Mono. Transmissive 25 22 550 combination with two lighting systems for both Session Total 1950 reflective and transmissive imaging, under a range Total (2 Sessions) 3900 of bandpass filters. The cameras are: (1) Nikon

D200 with 105mm lens, an RGB digital SLR, which We punched four 1mm round holes in each sample captures images in the visible spectrum; (2) Kodak at about a third of the distance from the borders. Megaplus 1.6i scientific monochrome camera, These holes are apparent in each image and will which captures images in both the visible and near- be a persistent feature after the samples are infrared spectrum over the range 400–1000 nm. degraded, both as reference points for registration With each camera, a set of bandpass optical filters of the multispectral image channels and for centred at regular intervals is used. comparison of image sets of the samples before

and after treatment. Each of the first 16 filters has a bandwidth of approximately 20 nm, at wavelength intervals of 20 0.8 nm throughout the visible spectrum from 400 to 700 0.7 nm inclusive (Fig. 3). The last 5 filters have a 0.6 bandwidth of approximately 50 nm, at intervals of 50 nm in the near-infrared spectrum from 750 to 0.5 950 nm inclusive. 0.4 0.3 90 Reflectance factor Reflectance 0.2 80 0.1 70 0.0 60 380 430 480 530 580 630 680 730 Wavelength (nm) 50 40 Figure 4: Measured reflectance spectra of (top to 30 bottom) parchment, yellowed parchment, dirty parchment, red ink, blue seal, and iron gall ink

Transmittance factor (%) factorTransmittance 20 10 As a reference for reconstruction of the reflectance 0 spectra from the multispectral image sets, we 347 377 407 437 467 497 527 557 587 617 647 677 707 737 imaged a Mini-Macbeth colour checker chart using Wavelength (nm) an X-Rite i1Pro spectrophotometer. This instrument Figure 3: Transmittance factors of 16 optical bandpass measures the reflectance factor of each patch at filters in the visible spectrum 10nm intervals from 380 to 730 nm. Typical spectra from the manuscript are shown in Fig. 4, also We use two lighting systems. The sample is measured by the i1Pro. The parchment has a very illuminated at 45° angles by a set of four tungsten - smooth spectrum, lower at the blue end, giving its halogen lamps in a standard photographic document copystand. This setup allows the camera characteristic yellow hue. The loss of reflectance at to capture light reflected from the parchment short wavelengths is accentuated in the ‘yellowed’ regions. At a middle wavelength of 550 nm the without specular highlights. The second system reflectances of the ink and parchment are 0.1 and uses a lightbox with two fluorescent lamps and a 0.65 respectively, giving a contrast ratio of ~ 6.5:1. diffuser to illuminate the parchment from behind, thus capturing the transmitted light. 4. INITIAL IMAGING TRIALS

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We imaged the parchment under a glass sheet to hold it flat, using the Nikon RGB camera. Each sample was first imaged without a filter on the camera (Fig. 5). The four reference holes can be observed as round black spots.

Figure 7: Transmissive RGB multispectral image set of parchment sample 602, recto side

5. DEGRADATION TESTS Figure 5: Full spectrum reflective RGB image of Preliminary tests on the degradation methods, parchment sample 203 using ‘spare’ pieces of parchment, illustrate various effects on both the appearance and physical We then captured each sample 16 times, through characteristics of the parchment. Figure 8 shows successive filters (Fig. 6). The first and last images the difference between the effects of scraping, are completely dark, due to the low sensitivity of scrunching and mechanical damage. The scraping the camera sensor at these wavelengths. The appears to have removed almost all traces of ink progression of hues, from blue through green to from the surface. Mechanical damage is also red, matches the perception of those wavelengths effective in deteriorating the writing. by the human eye. Note that the second image also has a hint of red, because the red channel of the camera is designed to be somewhat sensitive to these short wavelengths in order to be able to differentiate the violet hues of the spectrum.

Figure 8: Three test samples submitted to scraping, scrunching and mechanical damage, imaged under reflective lighting

Figure 6: Reflective RGB multispectral image set of Samples were stained with India ink, iron gall ink sample 509 at 16 wavelengths from 400 to 700 nm and aniline dye (Fig. 9). Under natural light, the appearance of iron gall ink and India ink is Each sample was also imaged using the indistinguishable. However, they differ in both tone transmissive lighting system (Fig. 7). Sample 602 and intensity from the original iron gall writing. The has text on both recto and verso sides. In the aniline dye completely obscures the writing. images under transmissive light, the two layers of text can be observed. The reference holes in these Oil, tea and red wine affect the parchment in quite images appear as bright spots, rather than dark different ways (Fig. 10). The oil makes the ones, because the light is coming from beneath the substrate more transparent, causing the writing on parchment. the two sides to blend together. Tea, on the other hand, seems to have reactivated the original iron gall ink and made it bleed. Red wine appears to have deeply stained the parchment, and hidden the writing.

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some contrast between the parchment and ink (Fig. 14).

Figure 9: Three test samples stained with India ink, iron gall and aniline dye, under reflective lighting

Figure 12: Four test samples submitted to desiccation, smoking, soaking in blood and burning, imaged under transmissive lighting

Figure 10: Three test samples submitted to soaking in oil, tea and red wine, in transmissive lighting

When the same samples are imaged under reflective lighting, however, the writing on the sample treated with wine is perfectly readable (Fig. 11) and the effect of oil is less noticeable.

Figure 13: Four test samples submitted to desiccation, smoking, soaking in blood and burning, imaged under reflective lighting

Figure 11: Three test samples submitted to soaking in oil, tea and wine, under reflective lighting

The colour and transmittance of parchment can vary significantly with the degradation methods (Fig. 12). The desiccated sample has become lighter, while the smoke has turned parchment deep yellow. The blood has made the parchment Figure 14: A burnt test sample, imaged through red like meat, and obscured the writing to imaging transmissive lighting using a very long exposure under both transmissive and reflective lighting (Fig. 13). Burning has made the parchment very dark 6. DISCUSION and opaque, and shrunk it to half the original size. At the completion of this project we will have a set The burnt sample, moreover, has become of multispectral images showing both the initial and extremely fragile, and cracked in the middle. degraded state of a manuscript. These images will Reflective lighting reveals the writing on the burnt be fully documented and released publicly as a sample, especially at infrared wavelengths. Under resource for the research community. They will a much longer exposure with backlighting the burnt provide insight into how a parchment with text sample does allow some light through, and delivers reacts visually and physically to various forms of degradation. We offer these as a guide for

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Cultural Heritage Destruction: Documenting Parchment Degradation via Multispectral Imaging Alejandro Giacometti, Alberto Campagnolo, Lindsay MacDonald, Simon Mahony, Melissa Terras, Stuart Robson, Tim Weyrich and Adam Gibson conservation, a resource for quantitative evaluation Clarkson, C. (1992). Rediscovering Parchment: of image processing algorithms for information the nature of the beast. The Paper Conservator, recovery, and for other research activities in 16(1), 5-26. conservation, image processing, computer graphics, and archaeometry. Hardeberg, J.Y. (2001). Acquisition and Reproduction of Color Images: Colorimetric & We have established a methodology for a multispectral approaches, Dissertation.com comprehensive study of how the degradation of parchment affects its appearance, changes its Landgrebe, D., & Chen, C. (1999). Information physical characteristics, and influences our ability Extraction Principles and Methods for to discern written text. Additionally, we have Multispectral and Hyperspectral Image Data (pp. presented an overview of how this degradation will 3-38). River Edge, NJ: World Scientific be documented using a multispectral imaging Publishing Company. system. Larsen, R. (2007). Introduction to damage and We have shown examples of how the effects of damage assessment of parchment. In Improved degradation treatment to parchment can vary, and damage assessment of parchment (IDAP): how multispectral imaging analysis can be assessment, data collection and sharing of instrumental in studying these effects. knowledge. European Commission, Directorate- Analysis of multispectral images of scraped or General for Environment, 17-22. mechanically damaged samples will enable us to Reed, R. (1972). Ancient skins, parchments and identify even faint traces of ink that might not be leathers, London; New York: Seminar Press. visible. It will enable the identification of seemingly identical inks, and could allow us to recover writing Salerno, E., Tonazzini, A., & Bedini, L. (2007). from darkly stained parchment, identifying the Digital image analysis to enhance underwritten staining agent, and from charred and burned text in the Archimedes palimpsest. International fragments. Journal on Document Analysis and Recognition, The data from our experiment will be made 9(2), 79-87. available to the public. Our dataset of the Součková, M. (2007). Characterisation of parchment samples before treatment will provide a microbial parchment damage based on the ‘ground truth’ for analysis of the effectiveness of assessment of microbiologically accelerated image processing algorithms in restoring images of aged parchments. In Improved damage the degraded samples. assessment of parchment (IDAP): assessment, data collection and sharing of knowledge. 7. REFERENCES European Comm., Directorate-General for Environment, pp. 31-35. Attas, E. (2004). Enhancement of Document Tanner, S., & Bearman, G. (2009). Digitizing the Legibility Using Spectroscopic Imaging. Dead Sea Scrolls. Proceedings of the IS&T Archivaria, 57, 131-146. Archiving Conference (pp. 119-123). Banik, G. (1997). Decay caused by Iron-Gall Terras, M., & Robertson, P. (2006). Image to Inks. In Iron Gall Ink Corrosion. Iron Gall Ink Interpretation: An Intelligent System to Aid Corrosion Symposium. Rotterdam; Amsterdam: Historians in Reading the Vindolanda Texts (p. Museum Boijmans van Beuningen ; Netherlands 252). Oxford: Oxford University Press. Institute for Cultural Heritage, 21-27. Vane, G., & Goetz, A. F. H. (1988). Terrestrial Bearman, G.H. and Spiro, S.I. (1996). imaging spectroscopy. Remote Sensing of Archaeological Applications of Advanced Environment, 24(1), 1-29. Imaging Techniques, The Biblical Archaeologist, 59(1): 56-66. Vest, M., Jacobsen, J. & Larsen, R. (2007). Accelerated ageing: effect of heat and relative Chabries, D.M., Booras, S.W., & Bearman, G. humidity. In Improved damage assessment of (2003). Imaging the past: Recent applications of parchment (IDAP): assessment, data collection multispectral imaging technology to deciphering and sharing of knowledge. European manuscripts. Antiquity, 77(296), 359. Commission, Directorate-General for Research, Directorate I–Environment, 67-68.

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