investigations of nondestructive methods for the

A BS T RAC T

A large portion of printing and writing paper producedin Europe between the fourteenth and the eighteenth cen-turies is believed to contain gelatin sizing. Some of thesepapers have survived in exceptional condition after cen-turies of long-term natural aging. However, no survey ofthe gelatin content of a large representative sample of thesepapers has ever been accomplished. To undertake such as u r v e y, or to evaluate the gelatin content in important arti-facts on paper during conservation treatment, newnon-destructive analysis techniques must be developed.We are engaged in experiments employing near infrared(NIR) and ultrasonic (US) instrumentation for this pur-pose. In general we have found that NIR spectra and USvalues correlate with gelatin content in laboratory- m a d esamples and in historical paper specimens. As a result ofthis work, we believe both techniques deserve furtherresearch attention.

I N T R O D U C T I O N

Recent research provides increasing evidence thatgelatin plays a positive role in the permanence of paper.Dupont (2003) demonstrated that gelatin likely behaves asa sacrificial component in paper due to preferential hydrol-ysis of the protein molecules over those of the cellulose.Gelatin can also bind transition-metal ions such as iron(II) and copper found in iron-gall inks (Kolar et al. 2003;Kolbe 2004). Bowden and Brimblecombe found the sameeffect when they used gelatin to model the impact of cop-per on leather and collagen artifacts (2003). This isimportant because transition metals can enter the papervia contamination (such as in the water supply used tomake the paper) as well as from iron-gall inks. Very recent-

Investigations of Nondestructive Methods

for the Estimation of Gelatin Content in Pa p e r

M A R K O R M S B Y, T I M O T H Y B A R R E T T, A N D B A R R Y H O J JAT I E

The Book and Paper Group Annual 24 (2005) 37

Presented at the Book & Paper Group Session, AIC 33rd Annual

Meeting, June 8–13, 2005, Minneapolis, Minnesota. Received

for publication Fall 2005.

ly, Baty and Barrett have confirmed the anticipated role ofgelatin as a pH buffer in paper (2005).

Yet little is known about the actual quantity of gelatinpresent in historical papers and its relationship to the per-manence of those papers. In 1972 Barrow analyzed tenseventeenth- and eighteenth-century writing papers, andfound they contained 2.6–5.8 weight percent glue. To thebest of our knowledge, Barrett and Mosier’s “The Role ofGelatin in Paper Permanence” (1995) was the first articleto relate the present condition of historical paper speci-mens to the quantity of gelatin present. Specifically, usinga small-sample destructive test, they established a possiblysignificant correlation between higher gelatin content andbetter permanence (as indicated by lighter color). But theytested only forty specimens. To obtain a clear picture ofgelatin’s role in the stability of paper subjected to long termnatural aging, a survey of many hundreds of specimens isn e c e s s a r y. Since the sample pool must include some of themost rare, best-performing papers extant, the developmentof new non-destructive analysis techniques is essential.

This article summarizes our recent investigations ofnear infrared (NIR) and ultrasonic (US) testing methodsfor the estimation of gelatin content in paper.

NIR spectroscopy utilizes infrared radiation with wave-lengths in the range 700 to 2500 nm as opposed to themid-infrared (mid-IR) range (2500 to 200,000 nm) that ismore frequently used in conservation (Derrick et al. 1999).Because of advances with detectors and personal comput-ers, NIR is becoming more commonplace, and it isemployed by pulp and paper manufacturers specifically inthe areas of troubleshooting, diagnostics, condition mon-itoring, and moisture detection (Hojjatie et al. 2001), aswell as in many other industries. The peaks in NIR spec-tra are combinations of overtones and overlapping bands,so, unlike the mid-IR range, it is usually not possible toassociate specific chemical functional groups with certainpeaks in a NIR spectrum. While mid-IR is better suited toqualitative analysis of an unknown for comparison with adatabase, NIR has advantages for quantitative analysis of

certain materials. For example, cellulose absorbs so strong-ly in the mid-IR that peaks from a paper sample may bedistorted, and the signal from weaker-absorbing com-pounds and minor components may be overwhelmed. Inthe NIR range cellulose does not absorb as strongly, soquantitative analysis may be more feasible.

In previous work, historic paper specimens with knowngelatin content were analyzed quantitatively and nonde-structively using mid-IR (Ormsby and Barrett 2002).Preliminary work during that project using laboratory-made controls indicated that NIR might be better suitedto this application. In the present research, a portable UV-VIS-NIR spectrometer was used to analyze the controlsand historical specimens.

Ultrasonic (US) techniques offer an alternative non-destructive technique for estimating gelatin content. In the

paper industry US measurements are used to determinepaper strength non-destructively both in the laboratory andonline—on a rapidly moving web of new paper. Researchhas shown that increasing amounts of gelatin content inlaboratory samples or historical papers correlate withincreasing z-direction (through the thickness of the sheet)US properties (Barrett 1989; Waterhouse and Barrett 1991;Barrett et al. 2000). Based on this earlier work we have con-tinued testing on the US properties of both paper typesusing new instrumentation (SoniSys 3D-UTI) developedby SoniSys, Inc. of Atlanta, GA.

P R O C E D U R E S

L a b o r a t o r y-prepared Whatman and flax handmadepaper samples used in our experiments were sized in 0.5%,

38 The Book and Paper Group Annual 24 (2005)

Fig. 1. Table of historical specimens used in the experiments. Some specimens were excluded from the NIR model calculations because

of their large standard deviation relative to the gelatin concentration or because further analysis showed that they were being given too

much weight in the calculations.

1%, 2%, 4%, 6%, and 8% gelatin solutions (wt/wt) pre-pared using a photographic-grade gelatin and hard water asdocumented by Barrett and Mosier (1992). Percent offinal-weight gelatin pick-up values shown in the fig u r e swere obtained by preconditioning and conditioning allspecimens and then using the first bare sheet as the zerogelatin content baseline.

Figure 1 lists all historical paper specimens used in ourexperiments. Hydroxyproline determinations were under-taken as described by Barrett and Mosier (1995).

N I RReflectance NIR spectra were gathered with a LabSpec

Pro spectrometer (Analytical Spectral Devices, Inc.) and awide-angle, non-contact probe as shown in figure 2. Theprobe illuminated an area approximately one square cen-timeter on each paper specimen (fig. 3). Samples wereplaced on top of a sheet of Tyvek (DuPont), which pro-vided a uniform background. For each specimen spectrawere taken at random locations that did not have ink orobvious signs of staining or other damage. A total of sixspectra were taken from each specimen, three each fromthe front and back. Each spectrum was acquired in approx-imately five seconds. The spectra were averaged to helpcompensate for variation across the surface of the page dueto possible non-uniform gelatin coating. The data wereanalyzed using GRAMS/PLSplus IQ software(ThermoElectron).

Figure 4 shows the spectra from a group of historicspecimens dating from 1483–1798. The variation in thepeak heights, particularly near 1400–2400 nm, is presum-ably related to differences in the concentration of gelatinsizing, which were known to range from 0–7% by weightbased on Barrett and Mosier’s work. To evaluate this rela-tionship, statistical analysis was performed to calculate amodel relating the NIR data to the gelatin content.

Although spectra were collected in the UV and visibleranges, as shown, these points were not included in calcu-lating the model. Previous work using a fluorescencemicroscope had indicated that the UV range might be use-ful because of gelatin’s fluorescence (Ormsby 2001), butwith the UV-VIS-NIR spectrometer the UV signal-to-noise ratio was too low to supplement the NIR data. Thevisible range was also not used in the model, but the dataare available for possible future analysis such as calculatingcolorimetric values.

The statistical analysis software offers several methodsfor developing the model, and each option has a numberof variables that must be selected. Once the calculationsare completed, a variety of techniques is used to evaluatethe quality and robustness of the model. For example, byplotting various statistics it may become apparent that aparticular spectrum skews the results or that the model fit s

Ormsby, Barrett & Hojjatie Investigations of Nondestructive Measurement of the Gelatin Content of Historic Papers 39

Fig. 2. Reflectance spectra were collected with a portable UV-

VIS-NIR spectrometer. The historic paper specimen was placed

on an adjustable stage for focusing. Fig. 3. Spectra were taken from an area of approximately one

square centimeter. The missing rectangular area was previously

removed to make destructive hydroxyproline measurements.

Fig. 4. U V-VIS-NIR spectra collected from papers produced

between 1483 and 1798 are shown. The UV range is at the left

and the NIR range is at the right.

the data poorly in a certain range. Through a process oftrial and error the model’s variables are adjusted to opti-mize it. In this project, a partial least squares method wasapplied to the first derivative of the spectra. Further detailsare available upon request.

U SUsing the Whatman laboratory-prepared samples and

historical specimens assayed for gelatin content by Barrettand Mosier (1995), a number of tests were run. The appa-ratus employed in all tests was a SoniSys 3D- U T Iultrasonic testing instrument set up for paper stiffness mea-surement through the out-of-plane (“through thethickness of the sheet”) or z-direction. The instrument wasused with a transducer at an excitation frequency of 1 MHzand a loading pressure of 50 kPa. A neoprene platen 19 mmin diameter was used for all tests. Unless noted otherwise,paper was tested at 50% relative humidity (RH) after con-ditioning for 24 hours at 50% RH. In all experimentsultrasonic data were collected at a total of ten randomlyselected points along the surface of each specimen awayfrom printed areas whenever possible. Mean and standarddeviations of the measured data were stored in a spread-sheet file. Figure 5 is a schematic diagram of the SoniSysout-of-plane stiffness tester. Figures 6a–d shows differentviews of the SoniSys 3D-UTI ultrasonic testing instru-ment in its out-of-plane configuration including a photoof a paper sample in place during testing.

R E S U LT S A N D D I S C U SS I O N

N I RPreliminary work focused on samples of gelatin-sized

flax paper controls. The gelatin content was determined byweighing the paper before and after the sizing bath and cal-culating the percent weight gain. These data were used bythe analysis software to calculate a model relating thegelatin content to the NIR spectra. The results are plottedin figure 7. If the model were perfect then all the pointswould fall along the straight line and the R2 value wouldequal 1. If there were no relationship between the gelatincontent and the NIR spectra then the points would be scat-tered randomly around the graph and the R2 value wouldbe close to 0.

The results from the flax controls are promising sincemost points are near the line and the R2 value is close to 1.With the historical specimens there is much more varia-tion not only in age but also in fiber type, productionmethods, additives, contaminants, etc. The model for these


Fig. 5. Schematic diagram of the SoniSys out-of-plane (through

the thickness of the sheet) stiffness tester showing principles of

ultrasonic measurement. In this dry-contact technique a paper

specimen is sandwiched between two piezoelectric ceramic

transducers and the speed of ultrasonic waves traveling through

the sample is measured. A 50 kPa loading pressure and

Neoprene-terminated plastic delay lines are used to enhance

acoustic coupling.

Fig. 6a.

Fig. 6b.

specimens is likely to be less successful than with the con-trols, and figure 7 shows the results. The points are spreadfurther from the line, and the R2 value is 0.70 compared to0.94 with the controls.

With the control samples in figure 7 the gelatin contentwas determined by the weight gain. For the historical spec-imens the gelatin data plotted on the horizontal axis infigure 8 are from Barrett and Mosier’s 1995 destructivemeasurements of hydroxyproline, an amino acid compo-nent of gelatin. The hydroxyproline procedure is difficult

and time-consuming. In addition, the precision of somemeasurements was poor, as indicated by the error barsshown in figure 8. These bars represent the standard devi-ation of nine measurements from each specimen (fewermeasurements were made on some specimens because ofthe limited sampling area available). As illustrated in fig u r e8, the error bars tend to become larger at the higher gelatincontent. Unfortunately, in this range there were also fewerhistorical specimens to work with. Therefore, the modelhas difficulty in fitting the data at higher concentrations.

It might be possible to fine-tune the model by experi-menting with different variables or algorithms, but themodel can only be as good as the data used to calibrate it.


Fig. 6c.

Fig. 6d.

Fig. 6. Photographs of the SoniSys 3D-UTI ultrasonic instru-

ment with (a) a metallic shield and computer used for data

acquisition and monitoring, (b) a close-up view of the base sup-

port, and also close-up views of the transducer-delay line

assemblies (c) without and (d) with a test specimen. By measur-

ing the sample thickness according to a TAPPI standard and also

measuring the time of flight of ultrasound through the paper

specimen, the ultrasonic velocity is determined. From that the

paper specific stiffness along the thickness direction is calculat-

ed.

Fig. 7. The horizontal axis shows the gelatin concentration of

flax control samples as determined by the weight gain from siz-

ing. The vertical axis is the gelatin concentration as predicted by

the model based on the NIR spectra. If the model were perfect

then all the points would fall on the line.

Fig. 8. The horizontal axis shows the gelatin concentration deter-

mined by destructive hydroxyproline measurements of the

historic specimens. The vertical axis is the prediction from the

NIR model. Compared to figure 7 the points are farther from

the line. Note the different scale for this graph as well as the

wide error bars on some hydroxyproline measurements.

Given the limitations of the hydroxyproline measure-ments, it is unlikely that the model’s performance can beimproved substantially. An alternative is to find a moreaccurate method to measure hydroxyproline. One promis-ing candidate is gas chromatographic analysis of aminoacids (Schilling et al. 1996). Designed for working withmicrogram-sized samples from paintings, the sensitivemethod was evaluated by studying the effect of pigmentsand accelerated aging on the analysis of gelatin (Schillingand Khanjian 1996). Future work will explore using thistechnique to analyze the historic paper specimens.

U SFigure 9 shows normalized mean ultrasonic specific

stiffness values obtained on the laboratory- p r e p a r e dWhatman papers. Measurements were repeated three timesduring a three-week period and comparison of the threemeasurements resulted in a maximum coefficient of vari-ation of 6.1% for all specimens. Figure 10 confirms thetrend of increasing US value with increasing gelatin con-centration reported in earlier publications (Waterhouse andBarrett 1991; Barrett, et al. 2000). In the paper industry,increasing US value is generally associated with increasingdegree of refining or beating. This is because increasedbeating produces more bonding, a denser sheet, and anincreased capacity to transmit sound waves. The increasedbeating and bonding also produce generally higher strengthcharacteristics. It has therefore been possible to obtain USestimates of strength properties with no destructive test-ing. Increased beating might be considered as one source ofhigher US values in historical papers as well; however,high-quality book and writing papers were made from verywell-worn and carefully sorted old rags. In the stamper or

beater they responded to beating rather quickly (comparedto new fiber), shortening to the point where a pulp formaking well-formed sheets was possible in a short periodof time. With such brief beating times, the finished sheetsafter drying were not especially strong. We believe themajority of their strength and high US values came fromthe addition of gelatin.

US analyses are normally done on papers that have beenconditioned in a controlled atmosphere prior to the test,whereas any regime for analyzing large numbers of booksin special collections libraries will require testing undervarying RH conditions. The impact of defacto condition-


Fig. 9. The horizontal axis shows the weight percent gelatin

concentration in the Whatman laboratory prepared samples. The

vertical axis shows z-direction out-of-plane specific stiffness as

dimensionless quantities.

Fig. 10. The horizontal axis shows the Whatman samples with

gelatin content increasing from left to right. The vertical axis

shows z-direction out-of-plane specific stiffness values in (km/s)2

units.

Fig. 11. The horizontal axis shows the gelatin content for the

historical specimens increasing from left to right. The vertical

axis shows z-direction out-of-plane specific stiffness values in

(km/s)2 units.

ing “in the stacks” immediately prior to testing has yet tobe investigated. In the paper industry, changes in RH pro-duce a predictable variance in US data output for papersnot sized with gelatin. The very preliminary results shownin figure 10 suggest the effect is the same for gelatin-sizedpapers, and we expect to be able to devise a formula tocompensate for tests on historical papers executed in ambi-ent conditions at RH values above or below the normal50% RH. The ideal correction factor may eventually bebased on the moisture content of the paper itself—a prop-erty which can be measured non-destructively.

Figure 11 shows relatively good agreement between USvalues and the known hydroyxproline assayed gelatin con-tent of twenty-one historical specimens.

As indicated in the NIR discussion, our future effortswill focus on new amino acid analyses of a larger pool ofhistorical specimens in order to arrive at “standards” withmore precisely known gelatin content. This will permitthe development of more refined US as well as NIR meth-ods.

C O N C LU S I O N S A N D F U T U R E W O R K

NIR and US methods deserve additional researchattention for use in the non-destructive analysis of histor-ical papers. US data combined with NIR data may be ableto eventually give a composite picture of gelatin quantityand quality. In addition, NIR is used in several industriesfor moisture content determination, and a model mightbe developed for historical paper specimens. Likewise,because US methods are used to accurately predict thestrength properties of modern-day papers, with researchthey may eventually be used to estimate the strength ofhistorical papers.

R E F E R E N C E S

Barrett, T. 1989. Early European papers, contemporaryconservation papers: A report on research undertakenfrom fall 1984 through fall 1987 by Timothy D. Barrett.The Paper Conservator 13:1–108.

Barrett, T., and C. Mosier. 1992. A review of methods forthe identification of sizing agents in paper. In C o n f e r e n c ePa p e r s Manchester 1992, ed. S. Fairbrass, 207–213.London: Institute of Paper Conservation.

Barrett, T., and C. Mosier. 1995. The role of gelatin inpaper permanence. Journal of the American Institute forConservation 34(3): 173–186.

Barrett, T., P. Lang, and J. Waterhouse. 2000. Non-destruc-tive measurement of gelatin and calcium content ofEuropean book papers: 1400–1800. In April 1996 EriceBook Conservation Conference Postprints, Sicily, Italy.605–624.

Barrow, W. J. 1972. Manuscripts and documents: Their deterio-

ration and restoration. 2nd ed. Charlottesville, Va . :University Press of Virginia.

Baty, J., and T. Barrett. 2005. The pH and moisture con-tent buffering effects of gelatin size in paper. MSsubmitted for publication.

Bowden, D. J., and P. Brimblecombe. 2003. The rate ofmetal catalyzed oxidation of sulfur dioxide in collagensurrogates. Journal of Cultural Heritage 4(2): 137–47.

Derrick, M.R., D. Stulik, and J.M. Landry. 1999. Infraredspectroscopy in conservation science. Los Angeles: GettyConservation Institute.

Dupont, A.L. 2003. Gelatine sizing of paper and its impacton the degradation of cellulose during aging: A studyusing size-exclusion chromatography. PhD dissertation,Universiteit van Amsterdam, Amsterdam.

Hojjatie, B., J. Abedi, and D. Coffin. 2001. Qualitativedetermination of in-plane moisture distribution inpaper. Tappi Journal 84(5): 1–11, 71.

Kolar, J., M. Strlic, M. Budnar, J. Malesic, V. S. Selih, andJ. Simcic. 2003. Stabilization of corrosive iron-gall inks. A c t aChimica Slovenia 50: 763–70

Kolbe, G. 2004. Gelatin in historical paper production andas an inhibiting agent of iron-gall ink corrosion onpaper. Restaurator 25(1): 26–39.

Ormsby, M. 2001. Methods for analyzing the gelatin con-tent of historical papers. Presentation at EasternAnalytical Symposium, Atlantic City, NJ.

Ormsby, M., and T. Barrett. 2002. Quantitative measure-ments of the gelatin content of historic papers usingmid-IR and NIR. Poster presented at Fi f t hInternational Infrared and Raman Users GroupConference, Los Angeles, CA.

Schilling, M. R., H. P. Khanjian, and L. A. C. Souza. 1996.Gas chromatographic analysis of amino acids as ethylchloroformate derivatives: Part 1, composition of pro-teins associated with art objects and monuments. J o u r n a lof the American Institute for Conservation 35: 45–59.

Schilling, M.R., and H.P. Khanjian. 1996. Gas chromato-graphic analysis of amino acids as ethyl chloroformatederivatives: Part 2, effects of pigments and acceleratedaging on the identification of proteinaceous bindingmedia. Journal of the American Institute for Conservation 3 5 :123–44.

Waterhouse, J., and T. Barrett. 1991. Aging characteristicsof European handmade papers: 1400–1800. Ta p p iJournal (1): 207–212.

MARK ORMSBY

Conservation ScientistNational Archives and Records AdministrationCollege Park, [email protected]


TIMOTHY BARRETT

Research Scientist, Adjunct ProfessorUniversity of Iowa Center for the BookIowa City, [email protected]

BARRY HOJJAT I E

Associate Professor and Engineering CoordinatorValdosta State UniversityValdosta, [email protected]


investigations of nondestructive methods for the

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