Thermal Accelerated Ageing Test Method Development*by P. L. BÉGIN & E. KAMINSKAINTRODUCTIONA Research Workshop on the Effects of Ageing on Printing and Writing Papers organized in the summer of 1994 in Philadelphia by the American Society for Testing and Materials Institute for Standards Research (ASTM/ISR) brought together a large group of scientists from many countries. The goal of the workshop was to share results of research on the role of factors such as light, pollution, paper composition, etc. on the degradation of paper. Areas requiring additional research were identified in order to develop reliable methods of accelerated ageing which could be used to predict the permanence of paper under natural ageing conditions1. The Research Workshop, and many meetings that followed were at the origin of the ASTM temperature ageing research program undertaken in collaboration between two laboratories, the Conservation Processes and Materials Research Division of the Canadian Conservation Institute (CCI) in Ottawa, Ontario, Canada and the Preservation Research & Testing Office of the U.S. Library of Congress (LOC) in Washington, DC in the USA.The overall intent of the combined work was twofold. The first goal was to carry out accelerated ageing studies whose results could be used to guide the design of a thermal accelerated ageing test to predict the stability of paper under natural ageing conditions. The second goal was to determine the validity of such a test. The results of the two complementary studies provided the basis for a thermal accelerated ageing test which can be used to assess the relative stabilities of papers likely to be used for archival purposes. At the time of writing this paper, the final project reports from both the CCI and the LOC were under review, therefore, an overview of the CCI's project is presented. Recommendations for a thermal accelerated ageing test and a proposed test protocol are also outlined.

* Part of the ASTM/ISR research program on the effects of ageing on printing and writing papers.

OVERVIEW BackgroundFor the last century, many different accelerated ageing procedures and evaluation methods have been proposed for predicting and comparing the stability of various kinds of papers stored under ambient conditions. Most of these studies were based on the assumption that elevated ageing temperatures only accelerate the reactions responsible for natural ageing, without altering significantly their nature. Some researchers claim to find theoretical models capable of predicting rates of natural ageing of some papers2 or ranking their degradation rates3. Others remain convinced that such relationships are meaningless4 and that extrapolation of the behaviour of paper under elevated temperatures to ambient conditions is of little value because of substantial errors associated with this approach5,6.The only accurate way to confirm predictions based on results of accelerated ageing would be to evaluate the rate of degradation of paper under natural ageing conditions. This approach, even for the most unstable papers, would require waiting for many years to see changes, which could be assessed by the currently used testing methods. This is why today's artificial ageing tests, with their problems and limitations, seem to be indispensable to estimate the life expectancy of papers or at least to rank their relative stabilities.One approach in the assessment of the permanence of paper using accelerated ageing involves determining degradation rates of papers by monitoring changes in time of their physical and /or chemical properties at several predetermined elevated ageing temperatures and then analyzing the results in terms of the Arrhenius equation:

k = Ae-E/RT

where: k is the specific reaction rate constantA is the Arrhenius pre-exponential (frequency) factorE is the activation energy [kJ/mole] or [kcal/mole]R is the universal gas constantT is the absolute temperature [K]

For a valid Arrhenius model, a straight line is obtained when plotting the logarithm of the degradation rate of the studied property at given temperatures against reciprocals of these temperatures. The pre-exponential factor A, which may be obtained by extrapolating the straight line to zero value of the temperature reciprocal, appears to be affected by factors such as exposure to pollution7 and light8, relative humidity during the ageing9, sources of fibre10, additives present in the paper11, etc. The activation energy, directly proportional to the slope of the straight line, represents a measure of sensitivity of the degradation rate of the

studied property to temperature changes. The degradation rate of the paper at ambient conditions can be estimated by extrapolating the Arrhenius plot for a given property to natural ageing temperature. Combined with a kinetic equation describing changes of the property in time, this rate may then be used to estimate the life-expectancy of the paper.However, the linear relationship of the Arrhenius plot may not hold beyond the range of temperatures for which it was obtained, for example, due to a different degradation mechanism at low temperatures10, 12, 13. Extrapolation to ambient temperature and predictions based on the Arrhenius plot assume that the activation energy is independent of temperature. This assumption should be considered with caution since it has been shown in other research areas that it is not always the case, even for some relatively simple reactions14, 15.Another factor to consider is the low precision of methods used for measuring the monitored property of the aged paper, which can result in substantial errors in estimating parameters of the Arrhenius plot and the life-expectancy of the paper6'16.Regardless of its limitations17 . the Arrhenius equation is frequently used in studies of the degradation of paper as providing the most complete information about degradation kinetics. Studies have shown the importance of a number of factors on the degradation rate of paper, including its acidity18, moisture content19, manufacturing20, storage conditions21, exposure to light7, 8, and ageing conditions themselves (temperature, relative humidity, stack or free hung sheets, etc.)20,

22.PurposeThe purpose of the CCI work was to study the degradation of two kinds of acidic wood-pulp papers (Table 1) aged in the form of free hung sheets and stacks at four elevated temperatures [65, 72, 80 and 90°C) using the Arrhenius model to try and determine which method best simulates natural ageing. For each temperature, the relative humidity for ageing was determined in such a way that the original moisture content of the papers did not change upon transferring them from the ambient conditions (23°C, 5O% RH) to the environmental chamber ageing conditions (Phase I). This ensured the same initial moisture content as at ambient conditions for the four elevated ageing temperatures. By maintaining the relative humidity associated with each temperature at a constant level, the moisture content could change during ageing as it would under natural ageing conditions. Ageing was monitored by measuring the degree of polymerization, some mechanical properties such as folding endurance, tear strength and zero-span tensile strength, and other properties such as pH, moisture content, brightness and colour (Phases II and III).

Table 1: The paper samples (made on a pilot papermachine at the Herty Foundation in Savannah, GA. USA).

RESULTS AND CONCLUSIONSMoisture Content StudyWhen paper is placed in the ageing chamber and conditioned at different temperatures, the relative humidity must be increased as the temperature increases to maintain the same moisture content as at ambient conditions (23°C and 50% RH)(Fig. 1).The relationship between the ageing temperature and relative humidity, required to keep the initial moisture content of the paper unchanged was found to be linear and similar for both experimental papers in the studied range of ageing temperatures (65 to 90°C) (Fig. 2). This relationship was used to determine the ageing conditions used in the Arrhenius study (65°C and 65% RH, 72°C and 67% RH, 80°C and 69% RH, and, 90°C and 72% RH). Moisture equilibrium data from the literature for the moisture content of wood shows a similar relationship between temperature and relative humidity23.Arrhenius studyThe rate of change in chemical and physical properties of paper aged at 65, 72, 80 and 90°C was greater for papers aged as stacks than as free hung sheets and the stack rate to sheet rate ratio increased with the ageing temperature (Table 2) as shown using DP (degree of polymerization) and tear strength (Fig. 3). These rates were used to prepare Arrhenius plots for papers aged in the form of single sheets and stacks.

Fig. 4: Arrhenius type plot. See text for discussion of the lines a, b, c and d.Fig. 4 represents Arrhenius type plots. As can be seen from the graph, the natural logarithm of the rate constants (Ink) is plotted against the inverse of the absolute temperature (K-1). If the scatterplot of the data points follows a straight line (line a), the single activation energy (reported in units of kJ mol-1 or kcal mol-1) of the process through the range of temperatures tested can be calculated from the slope of the line. The steeper the slope of the line, the higher the activation energy. Since the accelerated ageing of unstable papers is relatively slow, even at temperatures of 60°C, it takes a long period of time to observe significant changes in properties below this temperature. Therefore, the range of temperatures for which tests are performed usually extends from approximately 60 to 100°C. It is assumed that the line would continue to be straight and that the extrapolation of the line to ambient conditions (shown as the 23°C line) would produce an adequate estimate of the rate constant for the degradation of the paper at room temperature.Several factors could influence the slope of the Arrhenius plot and change the value of activation energy calculated from it. A factor that could increase the slope of the line is the possible trapping of volatile compounds whose influence on the degradation rate would increase with the increasing temperature. This would result in a higher activation energy for a process represented by line c. As-suming that the volatile compounds accumulating in the paper increase its degradation rate, the slope of the Arrhenius plot (and corresponding activation energy)

Table 3: Activation energies (kJ I mol) determined for various paper properties.

could be reduced by the effect of diffusion of the volatile compounds from the paper. This effect would be more pronounced for single sheets than for stacks and would increase with the increasing temperature. Another factor that could decrease the slope and reduce the activation energy (line d) is a reduction in the moisture content of the paper as the ageing temperature increases. This would occur if the relative humidity was set at the same level (e.g. 50%) for all ageing temperatures, with the degradation rates being reduced the most for the highest applied temperature.Another point to remember is that two papers having the same or similar activation energy of degradation doo not necessarily have the same degradation rate (the same stability) at the given ageing conditions. For example, lines a and b (Fig. 4) have essentially the same slope but intersect the 23°C line at different points representing actual degradation rate constants at this temperature, with line a corresponding to a lower rate constant (greater stability) than line b.Linear Arrhenius plots were obtained in the studied range of ageing temperatures, for degradation rates based on cellulose depolymerization and the loss of mechanical strenght, measured as folding endurance, tear index and zero-span breaking length (Fig. 5). Similar activation energies for cellulose depolymerization were observed for both experimental papers (Table 3). Activation energies for the loss of mechanical strenght were generally higher than for cellulose de-polymerization. In most cases, activation energy for stacks was higher than for free hung sheets and the lines converge as they approach ambient temperature (23°C). The fact that the lines for stack and single sheet actually cross before they reach ambient temperature could be the result of the diffusion and/or trapping of volatile compounds, as explained above.The values for activation energy for cellulose depolymerization (103-118 kj mol-1) are similar to those determined in other studies. Zou et al found activation energies for cellulose depolymerization from 104 to 113 kj mol-1 for papers made from bleached bisulfite and bleached kraft pulps24. Another study by Daruwalla and Narsian produced activation energies of 113-117 kJ mol-1 for cellulose degradation in dilute acid solution in the temperature range of 30-70°C25.

Fig. 5: Arrhenius plots for several properties of paper no. 1. Solid symbols: aged in stack; outline symbols: aged as free hung sheets.For mechanical properties including fold endurance, tear strength, and zero-span tensile strength, a wider range of activation energies was observed than for cellulose depolymerization (Table 3). This could be caused by the fact that the mechanical properties are affected not only by fibre strength, but by other factors as well, such as interfibre bonding, thickness of paper sample, etc. These tests are also subject to lower precision than the determination of the degree of polymerization.It is difficult to estimate the effect of the diffusion of the volatile compounds, which appears to affect the degradation rate at the elevated ageing temperatures, on the ageing rates at ambient conditions. The results of the Arrhenius study seem to indicate that this effect would be much smaller or possibly negligible at ambient temperature, as demonstrated by the convergence of regression lines for papers aged in the form of sheets and stacks. These results also seem to suggest that the most accurate simulation of accelerated ageing would be for samples between the single sheet and stacks evaluated in this study. The single sheet sample

might release too much volatile compounds that would otherwise accelerate the degradation of paper, whereas the stack might exaggerate the effect of trapping the volatile compounds, thus producing too high degradation rates. Work carried out at the U.S. Library of Congress comparing degradation products of papers artificially aged in various forms to those found in naturally aged samples suggests that stack and tube ageing correspond better to natural ageing26.Life expectancy predictionsThe life expectancy of a paper can be defined as the amount of time required for its properties to deteriorate to the point where the paper becomes unusable. The amount of loss of properties, or the minimum acceptable levels of strength of paper have never been established. These levels would have to be determined individually because there are many different types of paper that serve different functions, not all requiring the same retention of properties.

In the absence of commonly accepted deterioration levels, an arbitrary end-point was selected to determine if life expectancy predictions were possible. The endpoint selected was the time required to reduce some paper properties (degree of polymerization and folding endurance) to half their initial value. This time to 50% property loss (time to PL50%) can be evaluated from a kinetic equation linking the value of the property, the ageing time and the rate constant of the degradation process at a given temperature.To take into account random errors with which the values of the rate constants are determined, an extrapolation procedure can be used. In this case, the time to PL50% is calculated according to the equation for each ageing temperature, based on the corresponding value of the rate constant, plotting the logarithm of the time to PL50% versus the inverse of the corresponding absolute temperature (K-1) and extrapolating the regression line to ambient temperature (Fig. 6 and 7). This method provides an estimate of error of prediction based on dispersion of the data points with respect to the regression line, due to random errors in determining the rate constants.

Using the extrapolation method and 95% confidence intervals for paper no. 1 aged in stacks as an example, the predictions of time to PL50% range from 34 to 387 years for cellulose depolymerization (Fig. 6) and 31 to 6133 years for loss of folding endurance (Fig. 7).This example demonstrates the difficulty associated with the prediction of the life expectancy of paper. Small errors in determining the degradation rates at elevated temperatures translate into large errors in the prediction of life expectancy at ambient temperature. To reduce errors in the prediction of life expectancy, it would be necessary to measure the degradation rates with a precision significantly higher than that achieved for the tests carried out in this study or it would be necessary to determine degradation rates at lower temperatures (below 60°C) approaching ambient levels. Unfortunately, this would be very difficult to accomplish because of the ageing time required to produce measurable changes in properties at such low temperatures.RECOMMENDATIONS FOR AN ACCELERATED AGEING TESTThe data collected in the CCI Thermal Accelerated Ageing Research Project provided information on the accelerated ageing of paper in the form of single sheets and stacks27. Results indicated that the preferred ageing method would be to age paper samples in some form that would give results that lie between the ones for sheet and stack ageing. However, this information was not sufficient to design a thermal accelerated ageing test. A key component for the design of such a test was the identification of degradation products from both artificially and naturally aged papers. The Library of Congress provided information on sheet, stack and tube ageing, as well as information on the degradation products resulting from these accelerated methods26.Data presented by the LOC regarding degradation products of accelerated ageing for six custom made papers and several naturally aged papers, suggests that the paper samples aged in stacks and in sealed glass tubes approximate the natural ageing of paper. Even though the CCI had no experience with ageing paper in sealed glass tubes, this method was selected based on the results of the LOC work and because it has the advantages of providing greater convenience, potential for improving reproducibility of results from run to run within a lab as well as between labs, and avoiding troublesome calibrations of humid chambers. However, we are aware that this approach limits the amount and size of aged paper samples, requires properly sealed tubes and limits the ageing time to a time before which differences between the degradation products observed for tube and other forms of ageing begin to appear as described in the LOC report26.

Although linear Arrhenius relationships can be extrapolated to ambient temperatures, predictions of life expectancy of die studied papers under natural ageing conditions fall, in most cases, into ranges which are too broad to be of significant practical value. We believe that papers that are most stable at elevated temperatures will retain their resistance to degradation at ambient conditions as well. At the same time, papers that are least stable under accelerated conditions are least likely to survive the vagaries of time.In view of the above, both labs agreed to propose a single temperature ageing test where samples of paper are aged for one predetermined time period and, based on the results, classified into two main stability groups; stable and unstable. This single ageing time provides limited information regarding the rate of change of a measured property, however, it should allow classification of papers into the two broad classes of stability based on the retention of the property. The ageing conditions suggested for this purpose are 5 days at 100°C or 14 days at 90°C. Results of the ageing test could be then combined with those of the pollution and light ageing tests to classify the tested papers into one of three categories; stable, moderately stable and unstable.The accelerated ageing protocol was drafted using the results from both the CCI and the LOC projects. Additional work was carried out at the CCI where a large variety of papers was evaluated using the proposed thermal accelerated ageing test protocol to determine levels of acceptable retention values of properties (benchmarks) in order to divide papers into the two groups of stability27. There was also an attempt to determine the ageing conditions within the sealed tubes. Results of this study provided additional information required to finalize the parameters of the thermal accelerated ageing test protocol outlined below.PROPOSED THERMAL ACCELERATED AGEING TEST PROTOCOLObjectiveThe proposed test is designed to compare different cellulose-based papers for their inherent stability, or their resistance to natural ageing over long-term storage under ambient conditions. It is important to note that it is not designed to predict how long a paper will last. Life expectancy will depend in large part on the limits of acceptability beyond which various paper properties are no longer useful as defined by end-users. For a given paper, those limits will be different for different end-users.The usefulness of this test is in guiding users in selecting papers that will best meet their needs for stability. Like its predecessors, this test relies on the measure-

ment of physical properties of the artificially aged samples. Given the high standard deviation generally associated with such tests, subtle differences in the stability of different papers cannot be discerned. A classification of tested papers into broad classes of stability provides a much more reliable approach, and is highly recommended. Here we have suggested division of papers into two classes, stable and unstable, based upon the papers passing or failing an arbitrary benchmark based upon our experience with the application of this test to several papers. However, other classifications, that are better suited to a user's needs, are entirely possible.It is also important to test the papers for their resistance to the effects of light28, 29 and air pollutants30

as these, combined with the thermal ageing test, will provide information to classify the papers into three categories; stable, moderately stable, and unstable. For example, a paper may be stable under thermal ageing conditions, but could be destabilized by either light or air pollutants. The final classification of such a paper would be moderately stable or unstable.The CCI and the LOC joint proposal for an accelerated ageing test protocol is as follows:1. Setup• A laboratory oven that can provide temperature control within ±0.5°C is required. Control of oven relative humidity is not required.• Glass tubes fitted with thermally stable O-rings or gaskets and screw-on caps that ensure a hermetic seal during the ageing are required.• The ratio of the paper sample mass to the internal volume of the tube is maintained at 1.0 g (oven dry) of paper per 36 ml of volume.• Paper samples are conditioned at 23°C and 50% RH for ≥24 hours both immediately prior to and following the accelerated ageing period.2. Procedure• Five sheets from a randomized paper sample are used for the preparation of three sets of test strips for folding endurance measurements, and 8 or 16 (depending on grammage) sheets are used for the preparation of three sets of test strips for tear strength measurements. The purpose is to have three sets of test strips (one unaged control plus two sets for ageing) from the same source of paper.• Paper samples are cut to the appropriate dimensions in the machine direction for folding endurance tests and in the cross direction for tear strength tests that are performed following the ageing procedure. For the tear strength test, the samples are rolled for tube insertion.

• Four ageing tubes (145 ml) are required to age one sample of paper with the fold and tear strips divided evenly between the four tubes (4.0 +0.1 g oven dry weight is prepared for insertion into each ageing tube). Therefore, it is necessary to determine the paper moisture content at the preconditioned equilibrium conditions developed at 23°C and 50% RH.• While still within the paper testing room, the preconditioned paper samples are inserted into the ageing tubes, the caps put in place and secured (hand-tight) to ensure a complete seal.• Loaded tubes are inserted into the heated oven on their sides and placed one layer deep on each oven rack. They are then maintained at 100.0°C ±0.5°C for 120 ±0.5 hours in one continuous period.• An alternate acceptable procedure is to age the paper at 90.0°C ±0.5°C for 336 ±0.5 hours in one continuous period. However, papers aged at 90°C for 336 hours should not be compared with those aged at 100°C for 120 hours.• Upon removal from the oven, each tube is returned to the environmentally controlled paper testing room (23°C, 50% RH) and allowed to equilibrate to room temperature for 1 hour. The tubes are then uncapped and the sample equilibrated for 24 hours. Tear samples are flattened according to the procedure outlined in the CCI Final Report27.• MIT fold endurance and tear strength tests are then performed on the samples and results calculated according to the relevant ASTM or TAPPI test method. These tests are performed at the same time (when possible) on both unaged and aged samples of the same paper.3. Report• Paper stability is reported according to the percent retention benchmarks listed in Table 4. The worst result from the test is used to make the stability prediction. That is to say that if the tear strength is unstable and fold endurance is stable, the paper would be classified as unstable.Table 4: Suggested benchmarks for stability classification: % fold number and tearing force re-tention after 5 days of ageing at 100°C.

ACKNOWLEDGEMENTSThe authors wish to thank the members of the research teams from the other four laboratories involved in this ASTM/ISR Research Program (Image Permanence Institute at Rochester Institute of Technology in Rochester, NY., KCL (Finnish Pulp & Paper Research Institute) in Espoo, Finland, USDA Forest Products Laboratory in Madison, WL, and U.S. Library of Congress Preservation Research & Testing Office in Washington, DC.) for the valuable cooperative effort.We would also like to thank Bruce Arnold from R.B. Arnold Associates Inc. (Chair: ASTM Paper Ageing Research Program) and Dr. David Grattan, Joe Iraci, Tom Strang and Stefan Michalski from the Canadian Conservation Institute for helpful discussions. We also wish to thank Donna Woods and Anna Biilow for their assistance during this project.We are also indebted to the organizations that provided financial and in-kind contributions.SUMMARIESThermal Accelerated Ageing Test Method DevelopmentA study of the thermal ageing of paper was undertaken jointly by the U.S. Library of Congress (LOC) and the Canadian Conservation Institute (CCI) as part of the ASTM/ISR Research Program on the Effects of Ageing on Printing and Writing Papers. An overview of the CCI's part of the project and the joint CCI/LOC recommendations for an accelerated ageing test are provided. Advantages and limitations of the proposed test protocol are also discussed.Developpement me'thodique du test de vieillissement accelere par la chaleurUne etude relative au vieillissement du papier accelere par la chaleur a ete faite conjointement par le U.S. Library of Congress (LOC) et ITnstitut canadien de conservation (CCI) dans le cadre d'un programme de recherche ASTM/ISR sur les effets du vieillissement sur des papiers a ecrire et a imprimer. On donne un aperçu sur l'etude realisee par l'Institut canadien de conservation et on communique egalement les recommandations conjointes du CCI/LOC concernant le test de vieillissement artificiel. Les avantages et les limites du test propose font l'objet d'une discus-Methodische Entwicklung der beschleunigten HitzealterungAls Teil eines Forschungsprogramms des US Institut für Normenforschung in der Gesellschaft für Materialuntersuchung zur Wirkung einer Alterung auf Druck- und Schreibpapier haben die

Library of Congress und das kanadische Konservierungsinstitut eine Untersuchung über die Hitzealterung durchgeführt. Es wird ein Überblick über den vom kanadischen Konservierungs-institut erbrachten Anteil gegeben, und es werden die von beiden Partnern erarbeiteten Empfeh-lungen zur beschleunigten Alterung sowohl mitgeteilt als auch nach ihren Vor- und Nachteilen diskutiert.REFERENCES1. Proceedings. Workshop on the effects of aging on printing and writing papers. ASTM Institute for Standards Research, Philadelphia, PA., July 6-8, 1994.2. Zou, X., T. Uesaka & N. Gurnagul: Prediction of paper permanence by accelerated aging II. Comparison of the predictions with natural aging results. Cellulose 3 (1996): 269-279.3. Scribner, B. W.: Comparison of accelerated aging of record papers with normal aging for 8 years. Journal of Research of the National Bureau of Standards 23 (1939): 405-413.4. Bansa, H. & Hofer, H. H.: Artificial aging as a predictor of paper's future useful life. Abbey Newsletter Monograph Supplement 1 (1989).5. Erhardt, D.: Paper degradation: A comparison of industrial & archival concerns. TAPPI Pro-ceedings. Paper Preservation Symposium 1988: 83-90.6. Ströfer-Hua, E..1 Experimental measurement: Interpreting extrapolation and prediction by accelerated aging. Restaurator 11 (1990): 254-266.7. Atherton, J. B., F. L. Hudson &J. A. Hockey: The effect of temperature, light and some transitional metal ions on the sorption of sulphur dioxide by paper.]. Appl. Chem. Biotechnol. 23 (1973): 407-414.8. Oye, R., T. Okayama, H. Matsubara & K. Hanamura: Studies on degradation of paper and con-servation II: Degradation ofmechanical pulp-containing paper. Japan TappiJ. 45 (1991): 277.9. Block, I.: Temperature and humidity in the accelerated aging ofcellulosic textiles. ICOM Committee for Conservation Preprints, 7" triennial Meeting, Copenhagen (1984): 84.3.7-84.9.10.10. Luner, P.: Evaluation of Paper Permanence. Wood Science and Technology 22 (1988): 81-97.11. Baer, N. S., & N. Indictor: Use of Arrhenius equation in multicomponent systems. Advances in Chemistry Series 164. Preservation of Paper and Textiles of Historic and Artistic Value. American Chemical Society 1977: 336-351.12. Arney, J. S. & A.J.Jacobs: Newsprint deterioration. The Iniluence of temperature on the relative contribution of oxygen-independent and oxygen-dependent processes in the total rate. Tappi J. 63 (1980): 75-77.13. Bilz, M., & D. W. Grattan: The aging of parylene: Difficulties with the Arrhenius Approach. Preprints Vol II. ICOM Committee for Conservation, 11" Triennial Meeting, Edinburgh, Scotland 1996: 925-929.14. Legan, S. R.: The origin and status of the Arrhenius equation. Journal of Chemical Education 59 (1982): 279-281.15. Laidler, K. J.: Theories of chemical reaction rates. McGraw-Hill Series in Advanced Chemistry McGraw-Hill Book Company Inc. 1969: 59-63.16. Browning, B. L. & W. A. Wink: Studies on permanence and durability of paper I: Prediction of paper permanence. TappiJ. 51 (1968): 156-163.

17. Priest, D.J.: Artificial aging of Paper: Correlation with natural aging. Proceedings. Workshop on the Effects of Aging on Printing and Writing Papers. ASTM Institute for Standards Research, Philadelphia, PAJuly 6-8, 1994: 59-71.

18. Arney,J. S. & H. A. Chapdelaine: A kinetic study of the influence of acidity on the accelerated aging of paper. Advances in Chemistry Series 193. Preservation of Paper and Textiles of Historic and Artistic Value II. American Chemical Society 1981: 189-204.19. DuPlooy, A. B. J.: The influence of moisture content and temperature on the aging rate of paper. Appita 34 (1981): 287-292.20. Hendriks, K. B.: Permanence of paper in light of six Centuries ofpapermaking in Europe. Proceedings. Environnement et Conservation de PEcrit, de PImage at du Son. Actes des Deuxiemes Journees Internationales d'Etudes de PARSAG. Paris, May 16-20, 1994: 131-137.21. Shahani, C. J., F.H. Hengemihle & N. Weberg: The effect of variations in relative humidity on the accelerated aging of paper. Historic Textiles and Paper Materials II: Conservation and Characterization. American Chemical Society Symposium Series 410 (1989): 63-80.22. Shahani, C. J.: Accelerated aging of paper: Can it really foretell the permanence of paper. Proceedings. Workshop on the Effects of Aging on Printing and Writing Papers, ASTM Institute for Standards Research. Philadelphia, PA, July 6-8, 1994: 120-139.23. Kollman, F. P. & W. A. Cote: Steaming and seasoning of wood. Principles of Wood Science and Technology 1: Solid Wood. Berlin: Springer 1984: 442-444.24. Zou, X., T. Uesaka & N. Gurnagul: Prediction of paper permanence by accelerated aging I: Kinetic analysis of the aging process. Cellulose 3 (1996): 243-267.25. Daruwalla, E. H. & M. G. Narsian: Detection and identification of acid-sensitive linkages in cellulose fiber substances. TappiJ. 49 (1966):106-lll.26. Shahani, C, S. B. Lee, F. H. Hengemihle, G. Harrison, P. Song, M. L. Sierra, C. C. Ryan & N. Weberg: Accelerated Aging of Paper: I. Chemical Analysis of Degradation Products; II. Application of Arrhenius Relationship; III. Proposal for a New Accelerated Aging Test: ASTM Research Program into the Effect of Aging on Printing and Writing Papers. Preservation Research and Testing Division, Library of Congress, February 2001.27. Kaminska, E., P. Begin, D. Grattan, D. Woods & A. Biilow: ASTM/ISR Research Program on the Effects of Aging on Printing and Writing Papers: Accelerated Aging Test Method Development. Canadian Conservation Institute, November 2001.28. Atalla, R,J. Bond, C. Hunt & U. Agarwal: Quantification and Prediction of Aging of Printing and Writing Papers Exposed to Light: ASTM Research Program into the Effect of Aging on Printing and Writing Papers. USDA Forest Service, Forest Products Laboratory, August 2000.29. Forsskåhl, I.: Light Aging Test Method Development: ASTM Research Program into the Effect of Aging on Printing and Writing Papers. KCL, August 2000.30. Reilly, J. M., E. Zinn & P. Adelstein: Atmospheric Pollutant Aging Test Method Development: Report to ASTM Image Permanence Institute at Rochester Institute of Technology, June 2000.

P. L. BéginE. KaminskaCanadian Conservation Institute1030 Innes Road,Ottawa, K1A 0M3

Canada