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Journal of Cultural Heritage 11 (2010) 19–26

Original article

Assessing stains on historical documents using hyperspectral imaging

Douglas Goltz a,∗, Michael Attas a,b, Gregory Young c, Edward Cloutis d, Maria Bedynski e

a Department of Chemistry, University of Winnipeg, 515 Portage Avenue, Winnipeg, MB, R3B 2E9 Canadab Atomic Energy of Canada Ltd., Pinawa, MB, R0E 1L0 Canada

c Canadian Conservation Institute (CCI), 1030 Innes Road, Ottawa, ON, K1A 0M5 Canadad Department of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, MB, R3B 2E9 Canada

e Library and Archives Canada, 625 du Carrefour Boulevard, Gatineau, QC, K1A 0N4 Canada

Received 2 December 2008; accepted 5 August 2009Available online 29 December 2009

bstract

Hyperspectral imaging can be an important tool for the assessment and documentation of the state of preservation of an object. Over time,ocuments that have experienced heavy usage will inevitably show evidence of handling, which can include staining. In this paper, the use ofyperspectral imaging is described for enhancing the assessment of the visual properties of stains. The use of imaging software (ENVI) is alsoescribed for quantitatively assessing the extent of staining in two different documents. Single 10 nm bandpass images can be useful assessingarker stains with well defined boundaries. In one document (a treaty), the faint discolouration on one page made the extent of staining difficult tossess visually. A false colour density slice (450 nm) provided a topographical image which was useful for enhancing the contrast between stainednd unstained paper. In this type of image, the degree of discolouration could be correlated to optical density and the amount of staining on a pageould then be related to the number of pixels for a given absorbance range. In a second document (a prayer book), the staining was more extensive

nd some of the stains were dark in appearance. This document also contained a lot of text that was written using a dark irongall ink, which limitedhe use of a density slice at a single bandpass. In this document, pixel unmixing was successfully used to quantitatively determine the extent oftaining. The measurement tool provided with the NuanceTM Imaging System made it possible to quantitatively describe the size of the stain inerms of the number of pixels as well as its appearance in terms of average optical density.

2009 Elsevier Masson SAS. All rights reserved.

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eywords: Hyperspectral imaging; Stain; Irongall ink; Treaty; Spectroscopy; P

. Introduction

Paper and parchment have been used for centuries to recordhe important historical or cultural heritage of mankind. Theseypes of materials are often exposed to usage or storage con-itions that affect their chemical stability. Deterioration ofocuments occurs naturally as a result of aging, but it can beccelerated by poor storage conditions (humidity) or chemicalffects such as the use of inks or sizing in paper. One indicatorf deterioration can be discolouration or staining of the surface.

stain may also be caused by a liquid that was spilled or it

ay be indicative of heavy usage and possibly careless storage

onditions of a previous user. Obviously, the extent of damageo the surface of a document will depend on the substance that

∗ Corresponding author.E-mail address: d.goltz@uwinnipeg.ca (D. Goltz).

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296-2074/$ – see front matter © 2009 Elsevier Masson SAS. All rights reserved.oi:10.1016/j.culher.2009.11.003

nmixing; Principal component analysis

aused the stain. Generally, all liquids under some conditionsill cause some change in the appearance of an object. A chem-

cally neutral solvent such as water can also have an adverseffect on written documents even if mould growth does not fol-ow the exposure since water spots or staining can arise as aesult of the dissolution and mobilization of ions or particles inaper or ink.

The causes of discolouration and staining in historicalocuments can be as diverse as the materials used and storageonditions applied to them. For paper, a common symptomf natural ageing is yellowing. Yellowing of paper has beenttributed to photochemical reactions, which can be especiallyroblematic for paper containing lignin [1,2]. Missori et al. [3]ompared the optical spectra of ancient and artificially aged

aper to show that discolouration in paper may be the result ofhemical changes in gelatin sizing that occur as a result of aging.iscolouration of paper used in historical documents can also beue to a type of stain known as foxing. Foxing is a well-known

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erm used to describe stains that are reddish-brown to darkellow in colour [4]. A number of researchers have studied theauses of foxing and have found evidence of bacterial [5] or fun-al [6–8] growth. Other workers have used atomic spectroscopyo examine the role of iron with this problem [9–11]. Buzio etl. [12] have reported on the use of attenuated total reflectancenfrared (IR) spectroscopy and atomic force microscopyor studying chemical and physical properties of foxingtains.

Assessing the state of preservation of written documents thatave historical or cultural value is important for developingbalanced approach to their conservation [13]. Multispec-

ral and hyperspectral imaging techniques are ideally suitedor the examination of historical documents because they areon-contact and generally non-destructive techniques [14].avermans et al. [15,16] have used false colour infrared pho-

ography (FCIR) for the examination of damage to the surfacef documents as a result of irongall ink. Their detection sys-em used photographic equipment for capturing multi-spectralmages between 320 and 1550 nm in seven bands. Goltz et al.17] applied visible (420–720 nm) hyperspectral imaging fornhancing the legibility of faint text in a copy book. Numerousathematical approaches were explored such as band sub-

raction and spectral classification for enhancing the contrastetween ink pixels and paper. For text that has become alteredr obscured by another ink, imaging in the near IR can be espe-ially powerful for distinguishing different inks [18]. For thisork, a liquid crystal tunable filter was used between 650 to050 nm with 7 nm bandpass resolution. Digital colour imagingas been used to successfully correlate the state of parchmentging and deterioration with visible (red, green, blue) reflectance19]. Forensic scientists have also explored the use of visiblend IR imaging for distinguishing inks in fraudulent documentsnd currency [20]. Brauns and Dyer [21] used a hyperspec-ral visible imaging system for examining red, blue and blacknks. The optical system they employed relied on a Michel-on interferometer rather than optical filters, which allowedhem to obtain spectra from 450 to 800 nm with a resolutionf 32 cm−1 or 1 nm. Recently, the feasibility of multispec-ral imaging has been applied to the examination of text onharred documents. Lin et al. [22] used a CCD array sensi-ive to IR interfaced with a digital camera for examining textn charred documents that was not legible in visible light.rown and Sin-David [23] have used a variety of imaging tech-iques including IR luminescence for recovering text from aharred crew notebook that survived NASA’s Space Shuttleolumbia.

. Research objectives

Optical spectroscopy and hyperspectral imaging can be pow-rful tools for assessing and characterizing historical documents.n this paper, we report on the application of visible hyperspec-

ral imaging for assessing surface stains in two different types ofocuments. One of the goals of this assessment is to determinestablishing the extent of staining on the surface of a given pagen historical documents.

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l Heritage 11 (2010) 19–26

. Experimental

Two historical documents were used in this study:

. Adhesion to Treaty No. 6 (1876), [(Docket title: “Treatybetween the Wood Cree Tribe of Indians and Her Majestythe Queen, re. Surrender of Certain Lands”) (GAD Ref. IT367)] on paper.

. A Jewish prayer book (Haggadah); illuminated manuscriptwritten and illustrated by Elkanah “Pituhei Hotam” b. MeirMalir of Altona. The date of execution is 1763 and themedium used is irongall ink on paper.

The Adhesion to Treaty No. 6 consists of machine-madeaper. The paper is at least 98% �-cellulose and it does notppear to contain any sizing. This treaty consists of single sheetshich were imaged on an easel. The paper used in the Haggadahanuscript is handmade, laid and medium weight (0.17 mm

hick) and does not contain sizing or filler. A watermark is vis-ble in the center of the last leave (a fleur-de-lys, measuring.5 × 3.0 cm). The large size and heavy weight of the Haggadahid not allow for its placement on an easel. Instead it was placedat on a photographing table in the National Archives of CanadaPreservation Centre, Gatineau). Spectroscopic imaging cam-ras were mounted directly in front of the objects at distancesf approximately 0.5 to 1.5 m. Lights were placed at 45◦ withespect to the surface of the objects.

Visible imaging was performed using a NuanceTM Mul-ispectral Imaging System (Channel Systems and CRI). Thisnstrument is equipped with a liquid crystal tunable filter, opticsnd digital camera with a CCD detector. The image sensor pixelount is 1.3 megapixels and images were 1248 × 960 pixels.mages were acquired from 420 to 720 nm at intervalsf 10 nm.

Data acquisition by the instrument was controlled entirelyy a laptop computer. Spatial information is obtained in twoimensions (x, y) and spectral information is obtained in a thirdimension (z), which allows the storage of information in a-dimensional data cube. The exposure times for each wave-ength were entirely computer-controlled and varied accordingo the sensitivity of the system to specific wavelengths of light.or shorter wavelengths (e.g. 420 nm, visible camera), exposure

imes were as long as 5 s and for longer wavelengths (720 nm,isible camera), exposure times were approximately less thans.

For visible imaging, two 35 W Solux® bulbs (3500 K, 17◦)ere used. These incandescent bulbs are well suited for visible

maging as their output is identical to the electromagnetic spec-rum of sunlight, which allows for a more accurate presentationf colours. The Solux® bulbs did not emit a lot of heat and coulde operated at full power. For image cubes of the whole pages,colour corrected 50 mm lens was used. This lens was suppliedith the camera and did not have a setting for the f-stop. As such,

t was simply half stopped to minimize the effects of differentavelengths on size and focus of the object. For imaging por-

ions of text on individual pages that were of particular interest,n AF Micro Nikorr 105 mm f2D lens with a C-mount was used.

ultural Heritage 11 (2010) 19–26 21

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nless indicated, image cubes were acquired using a f-stop of.6.

In all imaging experiments, a data cube of a white reflectancetandard was collected first followed by an image cube ofach page. The reflectance standard used was a 20 × 20 cmpectalon® panel (Labsphere) with 98% reflectance at 550 nm.he data cube was flat-fielded by dividing the pixel intensity of

he sample by the corresponding pixel intensity from the whiteube reference image. This allows for a correction of varyingensitivity to light across the pixel grid of the detector in theamera. Flat fielding also corrects for variation in the intensityf the light source on the image. In order to perform the flat-fieldalculation for each pixel, light conditions (intensity, position)nd exposure times for each wavelength must be identical foroth the white cube and the image cube. The flat-fielded imagesere then processed to yield data in units of optical density by

atioing against the set of images obtained from the Spectralon®

anel:

ptical Density = − log(Iimage ÷ Iwhite)

where Iimage is the pixel intensity from the document andwhite is the intensity of the corresponding pixel from thepectralon® panel taken at the same wavelength as the sample

mage.

. Discussion

Digital imaging with a hyperspectral camera allows the usero collect a series of images at a number of well-defined wave-engths. The information is then stored in a 3-dimensional datale or image cube. The advantage of this approach is that spa-

ial features on the surface of an object can be created for eachavelength. Often, spectral or surface details that are not evidentisually can be significantly enhanced with a single wavelengthmage.

The Haggadah is an example of a historical document that hasxperienced handling over time and as a result has had a num-er of staining events on numerous pages. Most of the stains inhis prayer book appear to be a result of accidental spillage ofed wine. Apart from discolouring the support material (paper),iquids such as water or wine can also mobilize pigment or inkarticles, which in turn can result in their migration. Fig. 1 isvisible (420–720 nm) reflectance image of p. 10 (verso) of

he Haggadah. On this page, the spilled liquid, which was veryikely red wine, has covered much of the written text. The por-ion of text in this image is located at the bottom right handide of the page and the shape of the stain indicated that thepilled liquid has moved from the top of the image toward theottom.

Spectral differences between the irongall ink and the winetain allow one to distinguish them using single bandpassmages. In general, shorter wavelengths (∼450 nm) were foundo be more useful for enhancing the appearance of the stain while

onger wavelengths (∼720 nm) were more useful for enhancinghe contrast between the ink and paper. Fig. 2a shows a single0 nm bandpass image at 450 nm of p. 10 (verso) of the Hag-adah. At these wavelengths, the light absorbing properties of

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ig. 1. Visible (420–720 nm) reflectance image of bottom of p. 10 of the Hag-adah (verso).

oth the wine stain and irongall ink are similar, which makest difficult to assess the effects of the staining liquid on the inktself. A 10-nm bandpass image at 720 nm of p. 10 (verso) ofhe Haggadah is also shown in Fig. 2b. In this figure, the lightbsorption property of the wine stain and ink is significantly lesshan that of the irongall ink. This can be useful for minimizinghe visual appearance of the wine stain relative to the irongallnk. Close inspection of Fig. 2b shows individual lines of the

obilized ink, which clearly indicate the movement of the liq-id when this staining event occurred. It would be logical toonclude that when wine spilled on the surface of this page andoved toward the top of the image it dissolved and mobilized a

mall quantity of ink, causing it to spread toward the top of theage.

Although single bandpass images can be useful for enhanc-ng the contrast between stained and unstained areas of paper,nother useful application for hyperspectral imaging is quanti-ying the degree of staining on a page. Fig. 3 shows a visible420–720 nm) reflectance image of p. 4 (verso) of the Adhesiono Treaty No. 6. This page illustrates a different example of stain-ng or discolouration of the surface of a paper document. Theaint discolouration that appears on this page appears to haveeen caused by exposure to moisture, possibly from rain. Thetructure of the discolouration also suggests that it may haveome in contact with another page or another document at someime. Some of the boundaries are faint in appearance and distin-uishing the discolouration from the unstained paper is furtheromplicated by the presence of text which can be distracting to

he eye. Also shown in Fig. 3 are three regions of interest thatre identified as ‘Paper’, ‘Dark Paper’ and ‘Discolouration’. Theegion of interest without obvious discolouration is ‘Paper’ andt is lighter in appearance than the region of interest defined as

22 D. Goltz et al. / Journal of Cultural Heritage 11 (2010) 19–26

Fig. 2. a: 10 nm bandpass image (at 450 nm) of bottom of p. 10 Haggadah (verso); b:

Fig. 3. Visible (420–720 nm) reflectance image of p. 4 (verso) of the Adhesionto Treaty No. 6.

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10 nm bandpass image (at 720 nm) of bottom of p. 10 Haggadah (verso).

Dark Paper’. The region of interest that represents the faintlytained or discoloured area is labeled ‘Discolouration’.

Visible reflectance spectra of the three regions of interestPaper, Dark Paper and Discolouration) were collected and arehown in Fig. 4. Apart from the stained area having a higherbsorbance, the visible spectra of the stained and unstained areao not have any important spectral features to distinguish them.herefore, it is not surprising that single bandpass images aref limited use for enhancing the visible appearance of the faintiscolouration on this page. In order to quantify the amount ofiscolouration on this page, pixels from each specific regionf interest (n = 1800 pixels) were selected for the spots labeleds ‘Paper’, ‘Dark Paper’ and ‘Discolouration’ shown in Fig. 3.

he visible spectra (Fig. 4) indicate that the greatest difference inptical density between the paper and the discoloured paper wast shorter wavelengths (e.g. 450 nm). Therefore distribution ofptical density values for these pixels in each region of interest is

ig. 4. Visible spectra of ‘paper’, ‘dark paper’ and ‘discoloured’ paper on p. 4verso) of the Adhesion to Treaty No. 6.

D. Goltz et al. / Journal of Cultural Heritage 11 (2010) 19–26 23

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aThe end result can be migration of ink through the paper orparchment, which makes the text difficult to read. This typeof situation can become worse when the ink on both sides ofthe paper becomes visible. Fig. 7 shows a visible (420–720)

Fig. 5. Distribution of pixels at the 490 nm bandpass for paper a

hown for the 450-nm band in Fig. 5. The area of the page whichas not discoloured (‘Paper’) had an average optical density of.140 ± 0.027. Darker areas of the paper which did not appearo be discoloured (‘Dark Paper’) had an average optical densityf 0.210 ± 0.026 and the faintly discoloured areas (‘Discoloura-ion’) had an average optical density of 0.232 ± 0.026. Theseverage values of optical density are useful for describing howuch darker the stain is relative to the paper. The distributions

f optical density values of both ‘Dark Paper’ and ‘Discoloured’aper indicate a lot of overlap in the light absorption propertiesor these regions. More importantly, these values for optical den-ity provide a very good estimate for the range of values requiredo produce a density slice image.

The density slice image resembles a topographical surfacend can be created using ENVI. This two-dimensional imagehows the spatially resolved areas of different absorbance forsingle bandpass. Areas that have a higher optical density are

eadily distinguished from areas with lower optical density bysing a false colour image. Fig. 6 shows a false colour imagef a density slice for a single bandpass (450 nm) of p. 4 (verso)f the Adhesion to Treaty No. 6. For this type of faint staining,he density slice image is useful for quantifying the number ofixels associated with optical density at a specific bandpass. Theanges for each optical density at 450 nm were chosen using theverage values obtained from the data used to generate the distri-ution plot (Fig. 5). The different colours in Fig. 6 are indicativef the different values for optical density. In this image, the pix-ls representing areas of the paper that were not discolouredre white, ink pixels are black. The dark areas of paper areight blue and finally the faintly stained or discoloured areasre light purple. By choosing a small number of colours, themage is greatly simplified allowing for easy visualization of thextent of discolouration on this page. Determining the number ofiscoloured pixels can be a useful tool for calculating the approx-mate amount, as a percent, of the discoloration on this page. Forxample, the total number of pixels for one page is 1,095,500.

he number of pixels that could be classified as paper that isot discoloured would be 870,308 (white) and 249,089 (blue).inally, the area of the paper that was classified as discoloured

s 162,597 pixels (purple). When the ink pixels (28,674 pixels)FA

scoloured areas of p. 4 (verso) of the Adhesion to Treaty No. 6.

re subtracted, the proportion of this page that is discoloured byhe faint stain is approximately 12.4%.

In some cases, a liquid that causes a stain may also have andverse effect on the transparency of the paper or parchment.

ig. 6. False colour image of the density slice (450 nm) of p. 4 (verso) of thedhesion to Treaty No. 6.

24 D. Goltz et al. / Journal of Cultural Heritage 11 (2010) 19–26

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Fig. 8. False colour image of the density slice (500 nm) showing dark staining onpc0

ccttstoswppothe user can select as many pixels as are practical for separatingpaper and the inks into each spectral class. A grayscale imageis then created for each spectrum and each pixel is assigned afraction of the total pixel intensity. Fig. 9 shows a false colour

Fig. 9. Colour-enhanced view of stains on p. 3 (recto) of the Hag-gadah after pixel unmixing. The size and optical density for each stain

ig. 7. A visible (420–720 nm) reflectance image showing a portion of p. 3recto) of the Haggadah.

eflectance image of p. 3 (recto) of the Haggadah. On this page,dark stain which was caused by a wine spill as indicated by

he arrow is quite visible. This wine spill also caused some ofhe ink to diffuse into the paper, which makes the ink on thether side of the page visible. The wavelength dependence ofhis stain can be shown with point spectra; however, a spatiallyesolved image of the wavelength dependence is possible using aensity slice image from a single wavelength of the image cube.he density slice image is a powerful tool for revealing spatialetails of a stain or ink as it relates to absorbance. Therefore, theensity slice image provides a quantitative measure to describeow dark a stain would appear, relative to paper or ink. Fig. 8hows a density slice (500 nm) for a portion of the dark stainn p. 3 (recto) of the Haggadah. This rather dramatic image isseful for illustrating structural details with regard to the amountf staining as well as the extent of dark staining, particularlyround the letters in the text.

On p. 3 (recto) of the Haggadah, a number of smaller stains

ith a range of optical densities are also visible. Quantifying

he amount of staining on this page using a density slice imageas not possible since the light absorbing properties were too

imilar to large areas of ink on this page. The software used to

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. 3 (recto) of the Haggadah. Different colours represent different values for opti-al density. White: 0.000–0.346; blue: 0.346–0.520; yellow: 0.520–0.693; green:.693–0.867; red: 0.867–1.040; maroon: 1.040–1.214 and black: 1.214–1.387.

ontrol the NuanceTM imaging camera had a useful feature forlassifying pixels and unmixing them according to their spec-ral response. Spectrally mixed pixels result when there is morehan one substance located along the optical path, so that thepectrum at a specific pixel is a combination of the spectra ofhe materials. Since we are using reflectance images, the valuesf pixel intensity in the image cube are converted to optical den-ity. The software then uses a linear combinations algorithm,hich assumes that each pixel is made up of a combination ofure spectra. This is achieved by constructing a matrix with ‘n’ure spectra and m spectral channels as well as the contributionf each pure spectrum to a pixel [24]. Instead of pure spectra,

re: stain 1 (81,902 pixels)–0.3593; stain 2 (6438 pixels)–0.2995; stain 32121 pixels)–0.3663; stain 4 (1781 pixels)–0.3855; stain 5 (985 pixels)–0.3690;tain 6 (908 pixels); stain 7 (888 pixels)–0.2931; stains 8–12 (mean = 696 pixels)mean = 0.23–0.27).

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mage of p. 3 (recto) of the Haggadah that was reconstructedrom the unmixed pixels. In this type of false coloured image,pectral detail (i.e. absorbance) is not possible and only the shapef the stain can be shown in this type of image. A measurementool with the NuanceTM pixel unmixing software is availablehat allows for the determination of the average optical den-ity of the stains. Values for average optical density and theumber of pixels can be calculated for individual stains. Byomparing the number of pixels that are classified as stainedelative to the total number of pixels for a page (1,095,500),he proportion of staining on this page can also be determined.or example, stain #1 which has an average optical density of.3593 is the largest in size (81,902 pixels), comprising approx-mately 7.47% of the page. Stains 3 and 4 are much smallern size (21,221 and 1781 pixels respectively) but their meanptical densities in the visible spectrum are 0.3663 and 0.3855,hich indicates that they are darker in appearance. A large num-er of other stains (5 to 12) on this page are noticeably fainterOD = 0.23–0.26) in appearance and their total area on the pageas calculated to represent less than 1% of the surface of thisage.

. Conclusion

In this paper, we have attempted to show how visible hyper-pectral imaging can be useful for quantitatively assessing stainsn the surface of written documents. The darkness of the stainay be associated with the amount and the type of liquid spilled.ince the absorption of light is generally proportional to concen-

ration, then describing a stain in terms of optical density can beore useful than a qualitative description (i.e. light or dark). For

arker stains with well defined boundaries, the single bandpassmage is useful for assessing their spatial distribution. Softwareools such as density slice images for a single 10 nm bandpassan be useful for enhancing visual appearance of a faint stainn a document, which in turn makes it possible to quantitativelyssess the spatial distribution of discoloured or faint stains. Inhis study, the density slice image worked best when a smallumber of colours were used to show areas of discolouration,nk and paper. For imaging full pages in a document, the amountf text, the presence of drawings and large numbers of differenttains can limit the usefulness of single bandpass images or evenensity slice images. In this case, we have shown the applicationf pixel unmixing to determine the extent of staining on an entireage of text.

By itself, the presence of a stain is probably not a good indi-ator of physical damage to the surface of an object. However,f a stain can be associated with damage (e.g. depolymerization)o an object, then having a semi-quantitative description of atain’s spatial distribution on the surface could be beneficial tohe conservator. For example, if water causes a faint stain on theurface of a document, then enhancing its visibility using pixelnmixing or single bandpass density slice images can be a use-

ul predictive tool for future problems such as foxing. Similarly,hanges in the appearance of a document may also be indicativef instability which could occur as a result of storage conditions.ne possible application of determining the amount of staining

[

l Heritage 11 (2010) 19–26 25

nd absorbance could be the long term monitoring of documentsver time.

cknowledgements

Funding from the Canada Foundation for Innovation (CFI),he Manitoba Research and Innovation Fund (MRIF), theational Sciences and Engineering Research Council of Canada

NSERC) is gratefully acknowledged by the authors. Theuthors would also like to thank Mary Murphy and Catherineraig-Bullens of the Library and Archives Canada (Preservationentre) for their support and assistance with this project.

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