The Discovery of a Watermark on the St Cuthbert Gospel ... · The Discovery of a Watermark on the St Cuthbert Gospel using Colour Space Analysis eBLJ 2014, Article 2 2. Colour spaces

1 eBLJ 2014, Article 2

The Discovery of a Watermark on the St Cuthbert Gospel using Colour Space Analysis Christina Duffy

1. Introduction

Watermarks since their introduction in the thirteenth century have been used as a method of establishing the provenance and origin of paper, identifying mill trademarks and locations, and determining the sizes and intended functions of papers. Watermarks are designs such as a name, initials, or a decorative motif impressed on paper in a similar way to chain and laid lines.1 A watermark design was incorporated by manipulating wire into a recognizable shape and affixing it to the mould. The thickness of the paper is reduced over regions of wire which results in the familiar appearance of watermarks, chain lines, and laid lines when paper is held up to the light. Chain and laid lines are important even if they obscure the watermark design as their relative positions can aid in determining the orientation of the paper when the pages were laid out for printing.

Imaging of watermarks has been problematic for curators and scholars with partial marks often found in non-adjacent gutters owing to the ordering and cutting of the folios. Techniques such as tracing, transmitted light and the Dylux method2 have been used in the past. More sophisticated methods such as beta radiation, IR imaging and thermography have improved results, but are still highly dependent on the material and only successful in certain cases.

Best results are usually found when the watermarked folio can be accessed from both sides to allow light to pass through from the back, and the image to be photographically captured from the front. Indeed the majority of watermarks are initially observed accidentally by curators or researchers turning the folios and capturing the light which illuminates the watermark. This region can then be imaged later with better techniques to identify the mark more thoroughly, but initial observation is highly unlikely when the folio is adhered to a backing such as another folio or a binding cover. Therefore the probability of observing with the eye a watermark on the inner paste down of a binding is very low. The inner paste down is often a region where author’s notes, signatures or inscriptions are placed, and is an important aspect of the collection item. Even if the folios have been rebound and post-date the main content there is still a great deal of information to be gained from understanding the full history of a collection item.

In this paper we explain the basics of colour spaces and introduce software which allows users to convert standard RGB images into other colour spaces. The recent discovery of a watermark on the lower board of the St Cuthbert Gospel inner pastedown is used as a case study to illustrate how image processing can reveal and enhance information. A discussion of the watermark and the implications and potential uses of colour space analysis to curators and scholars are presented.

1 Chain and laid lines are the result of fine mesh wires lying parallel (laid lines) and thicker mesh wires running perpendicular (chain lines) to the long sides of a rectangular mould.

2 The Dylux method for making watermark prints was developed by Thomas L. Gravell. It was used extensively by Gravell in his publications A Catalogue of American Watermarks, 1690-1835 (New York, 1979) and A Catalogue of Foreign Watermarks Found on Paper Used in America, 1700-1835 (New York, 1983) from documents in the Library of Congress. The method placed the watermarked paper on top of a sheet of commercially produced Dylux 503A photosensitive paper which was then exposed to blue daylight fluorescent light. The watermarked paper was removed and the Dylux exposed to UV light to reveal the image of the watermark. This image was fixed by further exposure to blue daylight fluorescent light.

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The Discovery of a Watermark on the St Cuthbert Gospel using Colour Space Analysis

eBLJ 2014, Article 2

2. Colour spaces

Colour is an attribute of visual perception: it is not a physical property. Reflection occurs when light or any other wave bounces off a surface. The light reflectance of a surface is its ability to reflect light. Light reflection makes it possible to see objects that do not produce their own light (light emission). The perceived colour of an object is completely determined by its reflectance spectrum, although objects with different reflectance spectra can have the same perceived colour. Colour can be specified by three parameters in a colour space and there are mathematical relationships that enable the parameters of one colour space to be transformed into another. Alternative ways of describing colour numerically are useful for making certain calculations easier and making colour identification more intuitive such as by describing colours by their hue, saturation and luminance. The colour of an image is most often described in terms of the percentage of red, green and blue components combined. Images such as these exist in RGB colour space, but there are other ways to describe the colour of a pixel using different colour spaces.

A single colour space which provides satisfying results for the enhancement of a watermark does not exist. Therefore it is important to convert an RGB image into as many colour spaces as possible to determine where most new information is found. Vandenbroucke3 proposed to classify the various colour spaces into four categories based on their definitions and properties (fig. 1).

Fig. 1. Colour Space Families from Busin, Vandenbroucke and Macaire and Postaire, fig. 1. The proposed categories are the primary spaces, the luminance-chrominance spaces, the perceptual spaces and the statistical independent component spaces.

2.1 Primary colour spaces (RGB, RGBW, XYZ)

Primary colour spaces are based on the trichromatic theory which assumes that it is possible to generate any colour using a combination of the three primary colours in varying proportions. The default colour space for most available image formats is RGB which corresponds to the three primary colour components: red (RRGB), green (GRGB)

3 C L. Busin, N. Vandenbroucke, L. Macaire and J.-G. Postaire, ‘Colour Space Selection for Unsupervised Colour Image Segmentation by Analysis of Connectedness Properties’, International Journal of Robotics and Automation, vol. xx, no. 2 (2005), pp. 70-7.

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and blue (BRGB), respectively, called tristimulus values. These components match the absorption spectra of the three visual pigments in the eye. The visual colour perceived is the combined effect of one or several of these normalized components.

RGBW has four colour components red (RRGBW), green (GRGBW), blue (BRGBW) and white (WRG-

BW).4 The RGBW colour space is a method for more efficiently displaying colours by using an amount of white light to substitute for the part of each of those three channels that are the same. XYZ is the first international colour space and was developed by the CIE5 in 1931. Col-our is described in this 3D space as the luminance component Y (YXYZ) and two additional components X (XXYZ) and Z (ZXYZ). The components are based on results from psycho-physical experiments and are weighted relative to their importance in the human visual sys-tem. A human’s visual response is what allows hidden information to be observed making XYZ a useful colour space for locating hidden information. Other colour space categories are obtained from a linear or non-linear transformation from the primary colour spaces.

2.2 Luminance-chrominance colour spaces (AC1C2, Lab, Luv, YIQ , YUV, YQ1Q2)

Luminance-chrominance colour spaces consist of one component representing ‘lightness’ and two components representing ‘colour intensity’. RGB information is converted into luminance and chrominance information. The separation of luminance and chrominance components of a colour space is advantageous over basic colour spaces in compression applications.

AC1C2 is an opponent (antagonist) colour space containing one luminance achromatic channel (A) and two opponent chrominance channels (C1, C2). It was derived by an examination of the human visual system and creating axes which pass through the most populated colour region.6

In the Lab colour space light distribution can be optimized and operations in this space are used on digital photographs to enhance particular regions by separating layers. Lab is based on the XYZ space and is non-linear. L describes lightness, and a and b describe the red/green and yellow/blue axes. This space is used as part of a shade-matching tool in dentistry where the lightness, chroma and hue of teeth are matched.7

Orthogonal colour spaces or device-dependent colour spaces are the TV transmission colour spaces. Two colours deemed to have the same CIE colourimetry will only match if they are viewed under the same conditions. The International Colour Consortium (ICC) defined a Colour Management System (CMS) allowing colour information to be compatible between input, output and display devices. Specific devices are assigned colour information profiles that can be mathematically translated to function accurately on any device. YIQ was defined by the National Television Systems Committee (NTSC) and is used in televisions in the United States. In the YIQ space greyscale information is separated from colour information so that the same signal can be used for both colour and black and white TV monitors. Y represents luminance, I hue and Q saturation.

4 WRGBW is the common base shared by R, G, and B i.e. WRGBW = min(RRGBW, GRGBW, BRGBW).5 The International Commission on Illumination is known as the ‘CIE’ from its French title ‘Commision

Internationale de l’Eclairage’. The CIE is an independent, non-profit organization founded in 1913 and is devoted to international cooperation and collaboration in regards to the science and art of light and lighting, colour and vision, photobiology and image technology.

6 Garrett M. Johnson and Mark D. Fairchild. ‘Full-Spectral Color Calculations in Realistic Image Synthesis’, IEEE Computer Graphics and Applications, vol. xix, no. 4 (1999), pp. 47-53.

7 Nicoleta Corcodel et al., ‘Evaluation of Two Different Approaches to Learning Shade Matching in Dentistry’, Acta Odontologica Scandinavica, lxx (2012), pp. 83-88, http://informahealthcare.com/doi/pdf/10.3109/00016357.2011.600705.

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2.3 Perceptual spaces (LCH, HSI, HSV, HSL)

Some of the features of colour such as hue, saturation, and lightness are indescribable by the basic RGB colour space. This is due to their definitions being in terms of human sensations. Hue, saturation and lightness can be described by reference to the reflectance spectrum. Hue is related to the wavelength of maximum reflectance; saturation is related to the difference between the minimum and maximum values of the reflectance; and the lightness/intensity is related to the minimum reflectance. Perceptual colour spaces such as HSV are often used in colour selection, e.g. pigments or inks from a colour wheel as the space approximates more closely how colour is experienced by humans. HSI is a perceptual colour space with components of hue (HHSI), saturation (SHSI) and intensity (IHSI). Other perceptual colour spaces include HSL (hue, saturation, lightness) and HSV (hue, saturation, colour value).

2.4 Statistical independent component spaces (I1I2I3)

I1I2I3 is a colour space proposed by Ohta, Kanade and Sakai8 for image segmentation applications. It results from different statistical methods generating components with little correlation.

Colour space conversions have been used in many fields and specific spaces have been identified for analysis operations such as edge detection (differentiating between shadow edges, reflectance edges and actual object boundaries9) and object segmentation.10

These types of analyses are useful in food quality studies11 and skin detection systems.12 In skin classification systems it has been observed that skin colours differ more in intensity than in chrominance. Therefore the omission of the luminance component of an image’s colour space allows for higher accuracy in skin classification.13

3. Colour space processing tools

Optimum results from colour space processing are found when high resolution image files are used. Images should be cropped to the region of interest and saved as lossless formats such as TIFs. Lossy compression file formats such as JPG lose detail on consecutive high order saves and the cumulative effects are irrecoverable.

Converting an RGB image into another colour space can be achieved with most commercial image-processing software packages including Adobe Photoshop, JASC Paintshop and MATLAB. However there are many free image processing packages available to users such as GIMP and ImageJ, the latter, which produced the results in this paper.

ImageJ is a powerful Java-based open source image processing and analysis software

8 Y. I. Ohta, T. Kanade, and T. Sakai, ‘Colour Information for Region Segmentation’, Computer Graphics and Image Processing, xiii (1980), pp. 222-41.

9 Erum A. Khan and Erik Reinhard, ‘Evaluation of Colour Spaces for Edge Classification in Outdoor Scenes’, IEEE International Conference on Image Processing, Genova, Italy, September 11-14, vol. iii (2005), pp. 952-5.

10 C. Vertan and N. Boujemaa, ‘Color Texture Classification by Normalized Color Space Recognition’, Proceedings of the 15th International Conference on Pattern Recognition, Barcelona, Spain, vol. iii (2000), pp. 580-3.

11 Karen L. Bett-Garber, Elaine T. Champagne, Jessica L. Thomson and Jeanne Lea, ‘Relating Raw Rice Colour and Composition to Cooked Rice Colour’, Journal of the Science of Food and Agriculture, vol. xcii, issue 2 (2011), pp. 283-91.

12 Wei Ren Tan, Chee Seng Chan, Pratheepan Yogarajah and Joan Condell, ‘A Fusion Approach for Efficient Human Skin Detection’, IEEE Transactions on Industrial Informatics, vol. viii, no.1 (February 2012), pp. 138-47.

13 P. Kakumanu, S. Makrogiannis and N. Bourbakis, ‘A Survey of Skin-Colour Modeling and Detection Methods’, Pattern Recognition, vol. xl, issue 3 (March 2007), pp. 1106-22.

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package available for download. It was originally designed by the National Institutes for Health for medical imaging but has since found applications in many fields. ImageJ offers features similar to commercially available image-processing software packages such as brightness/contrast adjustment, frequency domain filtering, binarization and pixel values. A ‘plugin’ architecture allows for extensible programming. The nature of open source software allows for the constant update and availability of new plugins designed for specific tasks. Two such plugins allow for the conversion of a standard RGB image into another colour space called ‘Colour Transformer’14 and ‘Colour Space Converter’.15

These ImageJ colour space conversion plugins have been incorporated into a website called retroReveal designed by Hal Erickson, specifically for the purpose of finding hidden information in cultural heritage collection items. Users can freely upload high resolution images and observe their conversion into fifty-two alternative colour channels almost instantly.

The site was originally targeted at archivists, curators, conservators and scholars, but has since proven useful to others including archaeologists. The user can scroll through the colour space versions to quickly scan and see if new information has become apparent in a particular colour space. Any individual image can be re-viewed at different saturations and downloaded.

4. Case study: The St Cuthbert Gospel

4.1 Introduction

The St Cuthbert Gospel is a late seventh-century parchment volume and is the oldest intact European book. This Anglo-Saxon pocket gospel belonged to St Cuthbert of Lindisfarne (c. 635–687) and was discovered in 1104 in his tomb. The tomb was opened to translate St Cuthbert’s relics to a new shrine behind the altar of Durham Cathedral. The manuscript was acquired by the British Library in 2012 from the British Province of the Society of Jesus. It had been on loan since 1979 where it was exhibited first in the British Museum and later at the St Pancras site when the British Library was constructed.

The British Province of the Society of Jesus held the volume for nearly 200 years in the library at the Jesuit College of Stonyhurst, Lancashire, during which time it became known as the Stonyhurst Gospel. It was received by the English Jesuits based in Liège, now Belgium, through a donation by the Reverend Thomas Philips, S.J. (1708-74), who had himself been given the volume by the third Earl of Lichfield, George Henry Lee (1718-72).

The donation from Philips to the Jesuits on 20 June 1769 is recorded on an eighteenth-century paper leaf pasted on the inner lower board (fig. 2 left).16 The lower pastedown was formerly numbered f. 91 and is available to view on the internet as part of the Digitised Manuscripts website.17

A high resolution digital TIF image (640 MB) of this inner pastedown was analysed with image processing software to search for hidden information (faint inscriptions, missing text or watermarks). The image was captured by Imaging Services at the British Library on 13

14 Maria E. Barilla of the Digital Systems & Vision Processing Group at The University of Birmingham, UK is the author of the ImageJ plugin ‘Color Transformer’. The first version was released in 2007 and was last updated in 2012. http://rsbweb.nih.gov/ij/plugins/color-transforms.html.

15 Duane Schwartzwald is the author of the ImageJ plugin ‘Color Space Converter’. The first version was released in 2006 and was last updated in 2007.

16 The Latin transcription reads: ‘Hunc Evangelii Codicem dono accepit ab Henrico Comite de Litchfield, et dono dedit Patribus Societatis Iesu, Collegii Anglicani, Leodii, Anno 1769; rectore eiusdem Collegii Ioanne Howard: Thomas Phillips Sac. Can. Ton.’ which translates as: ‘This Gospel Book was received as a gift from Henry, Earl of Litchfield, and given to the Fathers of the Society of Jesus, of the English College, Liège, in the year 1769, the rector of the college, John Howard, Thomas Phillips Canon of Tongres.’

17 188 images of Add. MS. 89000 (The St Cuthbert Gospel) are available to view on the Digitised Manuscripts website at http://www.bl.uk/manuscripts/FullDisplay.aspx?ref=Add_MS_89000.

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December 2011. The physical dimensions of the paste down were determined, the image was processed through colour space analysis, and the chain and laid lines were calculated. Owing to the large file size the image was cropped into a top half and a bottom half (fig. 2 right).

Fig. 2. Left: Physical dimensions of the lower board inner paste down (ff. back i; formerly numbered f. 91). Right: Cropped image to reduce file size during image processing.

4.2 Colour space analysis

Both cropped images were uploaded onto the website retroReveal and processed to generate fifty-two images in different colour channels. No new information was found on the upper half of the pastedown, but what appeared to be a watermark became visible in the right hand corner of the bottom half of the paste down. Forty of the colour channels of the bottom half of the paste down are shown in figure 3. Not all colour spaces reveal new information but several show the presence of a watermark in the lower right hand corner. Many of the colour spaces replicate the same information with most details observed in colour channels RGBW B, YUV V, YIQ I, Luv v, Lab b, AC1C2 C1, I1I2I3 I2 and HSL H.

Original Brightness/ RGBW W Contrast Adjustment

RGB R

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RGB G RGB B

RGBW R RGBW G RGBW B

XYZ X XYZ Y XYZ Z

YUV Y YUV U YUV V

YIQ Y YIQ I YIQ Q

Luv L Luv u Luv v

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Lab L Lab a Lab b

AC1C2 A AC1C2 C1 AC1C2 C2

I1I2I3 I1 I1I2I3 I2 I1I2I3 I3

YQ1Q2 Y YQ1Q2 Q1 YQ1Q2 Q2

HSI H HSI S HSI I

HSV H HSV S HSV V

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HSL H HSL S HSL L

Fig. 3. Bottom half of the inner paste down shown in forty different colour channels.

4.3 Watermark enhancement with Fast Fourier Transforms

The AC1C2 C1 colour channel was chosen for further enhancement as the features of the watermark were most clear in this colour space. The image was subjected to a series of commands in ImageJ to improve boundary clarity including Fast Fourier Transforms (FFT). 18 A bandpass filter 19 was applied to the AC1C2 C1 TIF channel image with varying settings to optimize the watermark image. Large structures were filtered down to 10 and 20 pixels, and small structures filtered up to 3 pixels. In this case most watermark detail is observed by filtering large structures down to 20 pixels and small structures up to 3 pixels (20/3). Horizontal stripes can also be suppressed which softens the laid lines allowing clearer visualization of the watermark (fig. 4).

Fig. 4. Fast Fourier Transform of the watermark with varying upper and lower structure limits and suppression of horizontal stripes. (i): AC1C2 C1 Colour Channel. (ii): FFT > Bandpass Filter (10/3). (iii): 20/3: Suppress horizontal stripes.

(i) (ii) (iii)

18 Fourier Transforms are used as an image processing tool to discern periodicity in an image. Performing filters on an image in the frequency domain as opposed to the image (spatial) domain is advantageous because 2D Fourier Transforms and a filter multiplication are computationally faster to perform than a convolution in the image domain. This is most noticeable as the filter size increases. However, spatial domain filtering is possible with the ‘Bandpass filter’ feature of ImageJ.

19 The application of a bandpass filter to an image removes high spatial frequencies (blurring the image) and low spatial frequencies (similar to subtracting a blurred image). The window of the bandpass filter can be set by the user who can select how many pixels to filter large structures down to, and small structures up to. Filtering large structures down to a certain number of pixels will smooth variations in the image with typical sizes larger than the number of pixels input. Filtering small structures up to a certain number of pixels attenuates objects in the image smaller than this size. There is also an option to suppress horizontal or vertical stripes which in effect is similar to subtracting an image that is only blurred in the horizontal or vertical direction in the original.

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A partial numeral (1.5 cm in height) was observed on the St Cuthbert Gospel inner paste down. Generally moulds are designed with the motifs aligned so that they appear in the correct orientation when viewed from the felt side,20 which is smoother for writing on. It is also usual for dates and initials to be placed below the main motif, which further suggests that this numeral is a 2 as opposed to a 5 which may be the case if the watermark was viewed from behind.

Fig. 5. The watermark could either be showing the numeral 2 or 5 depending on which way the mould was designed as the ‘right way up’. Assuming that the writing is on the felt side it is likely that this numeral is a 2 (i): Watermark with potential numeral 2 (ii): Drawn-over watermark with potential numeral 2. (iii): Drawn-over watermark with potential numeral 5.

4.4 Observation of chain and laid lines

Chain and laid lines are clearly visible in both the original image and the colour space processed images. The paper has been formatted with the laid lines running parallel to the Gospel’s spine and the chain lines running perpendicular. The inter-chain line distance is 25 mm (1 inch) and the inter laid line distance is 1 mm.

The Latin writing on the inner paste down only partially obscures the form of the watermark. It is the final ‘I’ in the word ‘Collegii’ which overlays the left hand side of the watermark.

Fig. 6. Laid lines, chain lines and watermark dimensions. (i): Laid lines parallel to spine with 1 mm interspacing. Chain Lines perpendicular to spine with 25 mm interspacing. (ii): Watermark angle and relative position to chain lines.

(i) (ii) (iii)

20 Hand-made paper has two sides: a mould side and a felt side. This is a result of the mould being inverted over a piece of new felt after the water with suspended paper fibres has been evenly distributed over the wire mesh of the mould. The mould side carries the impression of the wire and chain lines, and the felt side is smoother.

(i) (ii)

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Fig. 7. (i) Watermark on inner paste down. (ii) Final extracted watermark.

4.5 Discussion

Watermark identification is usually of great importance for dating paper and determining the location of the paper mill. However, in the St Cuthbert Gospel case the page had already been dated by the record of donation inscription as 1769, and is possibly from a paper mill in Liège as that is where the Jesuit College was situated. Although it was clear from colour space analysis that there was a watermark present, the exact form of the watermark design was not obvious and was difficult to interpret. An exact watermark match could not be determined, but great similarities to a post horn in a shield were found in the online Gravell Watermark Archive21 and in watermark catalogues by Briquet22 and Churchill23 held at the British Library (fig. 8).

Fig. 8. Examples of post horns surrounded by elaborate shields in Churchill, Hertzberger et al., Watermarks in Paper.

(i) (ii)

21 The Thomas L. Gravell Watermark Archive (http://www.gravell.org/) is an online database of photographic reproductions of over 7000 watermarks in paper dating from between 1400 and 1835.The watermarks were imaged by Gravell using the Dylux method and went online in 1995. The database has since been updated to a new platform allowing for more search options.

22 Charles Moïse Briquet, Les Filigranes: Dictionnaire historique des marques du papier dès leur apparition vers 1282 jusqu’en 1600 (Paris, 1907).

23 W.A. Churchill, Watermarks in Paper in Holland, England, France, etc. in the 17th and 18th centuries and their interconnection (Amsterdam, 1935).

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4.5.1 Identifying the horn

In the first instance it was thought that this watermark was a wheel with an internal star (fig. 9 (i)), suggesting that the watermark is showing only about two-thirds of its total design. However, the thickness of the circular ‘outer wheel’ was greater in some parts, and was not consistent with other similar designs which dated to the fifteenth and sixteenth centuries. The inconsistencies of the design led the investigation to other watermarks, namely the horn, which in many cases dated to the eighteenth century.

Fig. 9. Example of watermarks which were used in comparison to the St Cuthbert Gospel watermark to aid in feature identification. (i): Wheel and star design from the online Gravell Archive. (ii): Watermark of a horn dated 8 October 1804. This watermark was found on a letter to Thomas Jefferson.

Fig. 9 (ii) shows an example of a horn watermark which occurs regularly in the on-line Gravell Watermark Archive. This horn watermark is dated 8 October 1804 by Ma-rie Joseph Paul Yves Roch Gilbert du Motier, Marquis de Lafayette on a letter. Its year of use was 1804 in La Grange, France, and is presumably from a French mill.

The watermark on the St Cuthbert Gospel was presumed to be a horn, yet there were several surrounding marks which were unaccounted for. Further comparisons with watermark collections led to Jane Austen’s letters at the Morgan Library and Museum24 where there were multiple examples of watermarks showing a horn inside a more complex watermark of a shield.

When the Morgan Collection watermarks were compared to the St Cuthbert Gospel watermark it was clear that there was also a surrounding shield on the Gospel’s watermark. However the top half of the shield is missing. This is not thought to be due to the folio being trimmed to fit the size of the Gospel since the shield on the

(i) (ii)

24 The Thaw Conservation Centre at the Morgan Library and Museum in New York owns fifty-one Jane Austen letters – more than any institution in the world. Curators recorded the watermarks and found numerous variations on the design of an elaborate shield surrounding a post horn. A post horn was used by a postman to announce delivery.

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Morgan watermark shows evidence of folding in a similar position (fig. 10 yellow arrow).

Fig. 10. Top: A comparison of the St Cuthbert Gospel watermark and MA 977.28 from the Morgan Collection. The yellow arrow shows an obscured line in the Morgan watermark which mirrors the cut-off point on the Gospel’s watermark suggesting the folio was not trimmed to fit the Gospel’s binding.

The 1.5 cm height of the partial numeral observed under the St Cuthbert Gospel watermark is consistent with numerals on other horn and shield watermarks. If used as a marker of date it would have to precede 1769, indicating it would end in either a five or a two i.e. 1765 or 1762, but there is no evidence of surrounding numerals.

4.5.2 The Meaning of the post horn

Concerning the post horn watermark, the Thaw Conservation Centre site states: ‘This style of watermark denotes the function of the paper – standardised according to a specific size and weight to be sent by mail. One of the watermarks seen on a letter dated 1804 was a copy of a Dutch watermark, which was appropriated by many English papermakers to denote a high quality paper.’

The horn as a symbol has great significance as described by the Local Post Collectors Society: ‘The horn, synonymous of the postal service for centuries, has been the most utilized insignia by postal administrations worldwide, particularly throughout Europe. It can be found as a central vignette on postage stamp issues; also on overprints, watermarks, and cancelling devices worldwide. Its configuration ranges from the early tasselled horn to a variety of stylized contemporary renderings.’

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5. Conclusion

This paper highlights the ease with which digital images can be manipulated to reveal hidden information without having to interact invasively or destructively with the physical item. This is particularly significant in cases where the paper is backed by another substrate, such as the case in the St Cuthbert Gospel, where the watermark would otherwise have not been found.

Colour space analysis offers a means of contact-free survey work in cases where watermarks or hidden inscriptions are predicted but not yet observed. Such work is often time-consuming and disappointing. A series of digital images can quickly be collected and processed with colour space analysis. If the digital images already exist then it is not required to even access the item. If a region of interest is located it can be recaptured for potentially greater detail using other technologies such as multispectral imaging, rather than using the instrument for the initial search. This is advantageous to users as it saves both time and money where instrument and collection item access is limited, as well as cutting down on handling time.

Although digitization is primarily used as a means of generating digital surrogates, the advantages of using these digital images as active educational resources via image processing are great. Historical documents and other collection items can be studied without the risk of damage to the primary source. Condition assessment images of collection items are useful to both conservators for monitoring changes and to researchers for detailed analysis and permanent access. These images can be enhanced and manipulated with image processing software to suit the specific needs of the user.

ImageJ is a useful resource for those working with cultural heritage materials. It is a reliable tool for the enhancement of images for condition assessments and research, for generating metadata documentation, for isolating regions of interest, and for the manipulation of images to reveal hidden material.

Colour Space Analysis is being used at the British Library to enhance faded designs on binding covers, disclose watermarks and hidden inscriptions and to reveal text which has been chemically treated or erased. In many cases applying colour space analysis to certain multispectral wavebands has proven successful. It is hoped that the application of colour space analysis will be adopted by scientists, researchers, conservators and curators working within the cultural heritage community.

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The Discovery of a Watermark on the St Cuthbert Gospel ... · The Discovery of a Watermark on the St Cuthbert Gospel using Colour Space Analysis eBLJ 2014, Article 2 2. Colour spaces