Accelerated aging and rate of degradation
Accelerated aging speeds the natural aging process of paper by subjecting it to extreme conditions in a climate chamber. Accelerated-aging tests are often used to determine the permanence (i.e., the rate of the degradation) of paper and to predict the long-term effects of a particular conservation treatment; however, there are many questions about the actual predictive value of these tests. Recently, Henk Porck reviewed the various methodologies for accelerated aging and current discussions in the preservation science community regarding this subject (Porck 1999). Several issues are worth mentioning.
There is a fundamental problem in the use of accelerated aging. While the Arrhenius principles apply to the kinetics of chemical transformations, the complex properties of paper that are often registered in accelerated aging (e.g., folding endurance, tear resistance, and paper discoloration) cannot be simply and unambiguously related to its chemical composition. Nonetheless, studies such as those at the Canadian Conservation Institute (CCI), in Ottawa, have indicated that, under certain conditions, the rate of the changes of such paper characteristics does relate to the chemical processes that take place during accelerated aging. On this basis, it is assumed that the principles of chemical reaction-kinetics do apply in the practice of accelerated-aging analysis (Zou 1996). Andrzej Baranski and others from the Jagiellonian University (Cracow, Poland) have reported on recent progress in the methodology of kinetic studies of cellulose degradation (Baranski et al 2000).
A complicating factor is the way in which the paper is exposed to the aging conditions. Confirming earlier studies from the Library of Congress (LC, Washington, D.C.) and the Netherlands Institute for Cultural Heritage (NICH, Amsterdam), investigations at the National Library of Australia (Canberra), the Slovak National Archives (Bratislava), and the Koninklijke Bibliotheek (The Hague) have shown that paper in stacks (i.e., books) ages differently than do single, loose sheets (Brandis and Lyall 1997; Hanus et al 1996; Pauk and Porck 1996). Some of these studies have shown that under both accelerated and natural aging conditions, the center of a stack of paper undergoes greater deterioration than do the regions located near the outside.
An interesting line of research in this context is the comparison of identical copies of books that, as part of separate library collections, have been stored under different conditions and show different stages of deterioration. Besides offering insight into the effects of environment on the rate of the natural aging of paper, the results of these studies may also indicate which environmental factors are responsible for the observed differences in aging, and thus in the rate of decay. Such comparative investigations can be of indirect value in developing reliable accelerated-aging methods. A good example is the study of pairs of books from the collections of the New York Public Library (NYPL) and the Koninklijke Bibliotheek, performed by the Koninklijke Bibliotheek in collaboration with the TNO Institute of Industrial Research (Delft, the Netherlands). This investigation concluded that the faster deterioration of the paper in books in the NYPL was caused by a higher concentration of the air pollutant sulfur dioxide, in combination with low or fluctuating high and low relative humidity (RH) in the NYPL storage rooms (Havermans 1997; Pauk and Porck 1996).
Accelerated aging of paper has commonly been done under a variety of temperatures and RHs. Because chemical paper degradation reactions vary according to these conditions, the validity of extrapolating results of accelerated aging to natural aging has severe limitations. A promising approach to the comparison between natural and accelerated aging is found in the ongoing research of David Erhardt and others at the Smithsonian Institution, Smithsonian Center for Materials Research and Education (Suitland, Maryland, USA). Their studies are based on the premise that the results of accelerated aging can serve as a basis for reliable predictions about natural aging only if the applied accelerated-aging method speeds the deterioration of paper without fundamentally changing the process. This means that every individual reaction involved in the decay ought to be accelerated by the same factor, and that the relationships among the reaction velocities must be kept constant. The investigations involved extended comparisons of the effects of different accelerated-aging methods and monitoring of the various degradation reactions by means of sensitive measuring equipment. It is expected that the results of these studies will form a basis for the formulation of more uniform and relevant accelerated-aging protocols (Erhardt et al 1999).
A research program at the Institute for Standards Research (ISR) of the American Society for Testing and Materials (ASTM) is focusing on the development of accelerated-aging tests (Arnold 1996). Its purpose is to develop testing techniques that will make it possible to develop standards for permanent paper that are based on performance rather than on composition. This program is committed to the development of tests in three areas: the aging of paper, the effect of light on paper, and the effect of environmental pollutants on paper. A wide range of acid and alkaline papers was prepared for the series of collaborative projects.
The ASTM has also set up a natural-aging project. For the next 100 years, 10 North American institutions in different climates will store volumes of 50 test-paper types and submit monthly and yearly storage condition reports. Throughout this time, specimen pages will be extracted from each site and tested for optical and physical durability (McCrady 1999).
As part of the ISR project, the LC has developed an accelerated-aging test for paper that it believes offers several advantages over currently available tests. Instead of relying on expensive aging chambers that often lack the desired precision in maintaining preset RH levels at high temperatures, the LC investigators retain control of moisture concentration around paper at elevated temperatures by sealing paper samples inside airtight glass tubes. These tubes have the added advantage of retaining degradation products, as LC researchers believe books do as they age naturally under ambient storage conditions. They have compared the chemistry underlying the aging of paper in loose sheets, book-like stacks, and within airtight glass tubes with the natural aging process by chemical analysis of the degradation products that result in each case. The data demonstrate that aging paper within airtight glass tubes simulates natural aging better than does aging of paper in loose sheets or in stacks. Paper aged inside airtight glass tubes at 100C ages to about the same extent in five days as it does when single sheets are aged at 90C and 50 percent RH in a humid oven for 30 days. This test method is being evaluated at the CCI (Shahani 2000).
The CCI is also engaged in a parallel collaborative study of accelerated aging of paper under the ISR/ASTM framework for development of accelerated-aging tests for potential use in the development of performance-based standards for permanent paper. Elzbieta Kaminska, Paul Bégin, David Grattan, Donna Woods, and Anna Bülow from the CCI are examining the thermal-accelerated aging of paper in sheets and in stacks for some of the ISR papers (Kaminska et al 1999).
A review of historical sources can also provide a basis for insights into the natural aging rate of paper. Recent studies have drawn on historical sources from the mid-nineteenth century that document the inferior quality of Dutch paper at that time. These records can be compared with the findings of present-day examinations of the same material, traced in archival collections. Such comparisons should yield useful indications on the rate of paper decay (Grijn et al, in press; Grijn et al 1996; Porck et al 1996).
Air pollution and deacidification
Expert insight into the effects of air pollutants on paper-based collections is essential for the development of an adequate preservation policy. Although the problem of air pollutants is generally acknowledged, the mechanism of deposition and threshold concentrations-in particular, the impact of air pollutants on deacidified paper-is not well understood.
Useful information can be expected to emerge from a current research project of the Dutch General State Archives (The Hague, the Netherlands). In this study, conducted in cooperation with the TNO Institute of Industrial Research, identical archive and library materials and other test papers are being stored at two locations, one of which is equipped to filter air pollutants. Continuous monitoring of environmental conditions such as temperature, humidity, and concentrations of air pollutants, as well as frequent analysis of the quality of the stored material in both storage rooms, will yield useful data over time. Besides offering more insight into the long-term effects of air filtering, these data will give a realistic indication of the rate at which various papers deteriorate under different degrees of air pollution (Feber et al 1998).
Anna Johansson devoted her doctoral thesis research at the Göteborg University (Sweden) to the synergistic effects of air pollutants (sulfur dioxide, nitrogen dioxide, and ozone) and climate on the stability of paper. The effect of trace amounts of these pollutants on the degradation of paper was investigated by means of in situ DRIFT (diffuse reflectance infrared Fourier transform spectrometry) and reaction product characterization techniques. Different mass-deacidification processes were evaluated with respect to their ability to provide protection against further acidification of papers. They included the DEZ (diethylzinc), Battelle (magnesium titanium ethoxide), Bookkeeper (magnesium oxide), Wei T’o (methoxy magnesium methyl carbonate), and Sablé (carbonated magnesium methoxide and ethoxide) systems.
The investigators concluded that RH plays an important role in the uptake of the air pollutants. Clear synergistic effects were demonstrated in the deposition rate. Deacidification treatments did protect paper against the attack of acid air pollutants, although there were some quantitative differences. Deacidification did not provide adequate protection from oxidative degradation of the paper (Johansson 2000).
Dr. Johansson’s thesis was based on work carried out within the framework of an extensive research project, “Effects of Air Pollutants on the Accelerated Aging of Cellulose-based Materials.” The research was funded by the European Union (STEP program) and coordinated by John Havermans of the TNO Institute of Industrial Research (Havermans et al 1994; Havermans 1995). The effect of air pollution on deacidified paper remains a subject of interest, and it is being addressed in one of the development projects of the Helsinki University Library (Finland).
Formation of acids
While it is well known that papers become more acid with age, it is generally assumed that this declining pH does not significantly contribute to the degradation of paper. It is often presumed that only the acids introduced in the manufacture of paper and those absorbed from the environment are responsible for the deterioration of paper. In this context, the term “acid-free,” which in effect equates neutral and alkaline papers, is often used to imply permanence. However, the spontaneous formation of acids in cellulose during aging cannot be overlooked as a cause of paper degradation.
The LC investigated the role of acid formation in the process of paper aging. The researchers used capillary electrophoresis to establish the spontaneous generation of formic, acetic, lactic, oxalic, and several other weak aliphatic acids in acid, neutral, and alkaline papers at room temperature at a rate that is fast enough for detectable concentrations of these acids to form in a few months. Thus, neutral papers cannot remain acid-free for long. Weak acids formed in the degradation of cellulose and hemicelluloses have generally been considered not to pose as significant a threat as do stronger acids introduced from acidic alum-rosin size or those formed by absorption of oxides of nitrogen and sulfur from the environment. However, the present findings suggest that these weak acids accumulate at a sufficiently high rate to contribute significantly to the increasing acidity in paper as it ages. Alkaline papers showed appreciably higher rates of accumulation than did other papers, since the acids formed are immediately neutralized and cannot enter into other reactions or dissipate. It was also shown that these weak acids attach themselves strongly enough to paper, probably by hydrogen bonding, that they are not easily dislodged from the paper matrix, even upon airing. Because of this tenacity and because they catalyze their own formation, these acids present a constantly escalating source of damage that can be dealt with only through deacidification (Shahani 2000).
Local yellow or brown discolorations of paper, often referred to as “foxing stains,” have been the subject of investigation; however, preservation science research has not yet reached a consensus on the cause of this phenomenon. Several factors presumably are involved in this form of paper damage.
In a joint project of the Université de la Rochelle, the Université de Technologie de Compiègne, and the Musée du Louvre (Paris), two noninvasive techniques-fluorescence and FTIR (Fourier transform infrared spectrometry)-were used to identify chemicals in 154 samples of foxed papers from the seventeenth to the twentieth centuries. The study aimed to define objective criteria for a taxonomy of the foxing stains.
Although fluorescence appeared to produce little chemical information, these researchers maintained that the quantitative measurement of fluorescence would be of significant interest if fluorogenic compounds were the precursors of the brown stains. FTIR provided more insight into the chemical characteristics of the foxing stains than did fluorescence. The use of FTIR spectra as a tool for categorizing foxing stains is discussed in detail by Choicy et al (1997).
Indoor air pollutants
One cause of paper degradation is indoor organic pollutants that are generated from certain storage and exhibit materials. Anne-Laurence Dupont and Jean Tétreault from the CCI are assessing the potential impact of acid-emissive materials on cellulose-containing materials. They use cold extraction pH measurements and determination of the degree of polymerization (DP) of cellulose to assess the effects of acetic acid vapor on various test papers (Dupont and Tétreault, submitted).
Coatings are often used as a means of passive conservation; however, direct contact with unsuitable coatings or the emission of harmful volatile compounds from coatings can damage artifacts. Tétreault has reviewed and updated the knowledge of the various coatings used in museums and other institutions. Different spot tests are included in his final report (Tétreault 1999).
Rapid aging of poor-quality paper materials, such as acidic mat boards, lignin-containing papers, and file covers are known to affect the aging of higher-quality unbuffered paper that is in contact with or in close proximity to them. John Bogaard and Paul Whitmore from the Carnegie Mellon Research Institute (Pittsburgh, Pennsylvania, USA) are studying the migration of degradation products from poor-quality materials into higher-quality papers by determining chemical properties, such as DP, carbonyl and carboxyl groups, and pH.
Two ingredients in iron-gall inks are known to cause degradation of paper artifacts: sulfuric acid, which catalyzes the hydrolysis of cellulose; and iron (II) sulfate, which catalyzes the process of cellulose oxidation. Because both sulfuric acid and iron (II) sulfate are water-soluble, these ingredients are able to migrate and could spread ink corrosion throughout the paper. However, this migration process is not understood.
Using scanning electron microscopy and X-ray fluorescence analysis techniques, Hans Neevel (Netherlands Institute for Cultural Heritage) and Cornelis Mensch (Shell Research and Technology Centre, Amsterdam, the Netherlands) studied the presence of iron and sulfuric acid outside the inked areas in test samples onto which iron-gall ink lines had been applied. The paper was subjected to accelerated thermal aging at fluctuating RH conditions to simulate the ink-corrosion process. The distributions of iron and sulfur across the paper were then determined, and the results were compared with the levels and distribution of iron and sulfur found in a sixteenth-century manuscript. The researchers discovered that in the artificially aged samples, sulfur (sulfuric acid) had moved out of the inked areas, whereas iron had not. Iron migration could likewise not be observed in the naturally aged samples, while contradictory results were found with respect to the migration of sulfur (Neevel and Mensch 1999).
Before we can fully understand the ink-corrosion mechanism, we must answer a question concerning the release of contaminants from the paper material during the aging process. Researchers from the TNO Institute of Industrial Research and the Shell Research and Technology Centre studied the effects of iron-gall inks on the emission of volatile organic compounds (VOCs) from paper artifacts. The VOC emission of test papers onto which lines of an iron-gall ink preparation were plotted was determined during accelerated aging by means of gas chromatography coupled to mass spectrometry (GC/MS).
The findings showed that the presence of iron-gall ink increases the rate of formation of formic acid, acetic acid, and furan derivatives as main volatile compounds. The research findings indicate that the presence of iron in the ink appears to stimulate certain paper-degradation processes, namely acid-catalyzed hydrolysis and dehydration. The harmful effects of some of the released VOCs have been discussed in relation to the conservation of ink-corroded paper (Havermans et al 1999a).
Charlotte Ahlgren (National Museum, Department of Paper Conservation, Stockholm, Sweden) is investigating the role of oxygen in the ink-corrosion process. An iron-gall ink preparation is applied to handmade rag and newsprint papers that are housed in encapsulations at 30 percent or 65 percent RH, with or without oxygen absorbers. The effect of oxygen will be determined by means of Raman spectrometry and/or by accelerated aging and measurement of the bursting strength of the paper samples. The aim is to determine whether an oxygen-free microclimate could retard the ink-corrosion process, which involves oxidation.
Monitoring of paper degradation
Monitoring the degradation of paper is essential for improving our understanding of how paper ages. At the CCI, Elzbieta Kaminska is determining, by means of statistical analysis, which methods are most useful for describing the chemical and physical changes that occur as paper ages. More than 20 different tests were conducted on various kinds of new and naturally aged papers after different treatments or accelerated aging or both.
The preservation community needs a suitable instrument for diagnosing the state of paper deterioration. Existing standardized testing methods often cannot be applied because of the large number of test specimens required. A research project by J. Luiz Pedersoli, initiated at the NICH, aims to develop microanalytical methods for characterizing the condition of paper. Investigators will evaluate a number of chromatographic, spectroscopic, thermal, and microscopic techniques to determine whether they are able to assess several paper properties using smaller samples than those that are currently required. The properties to be assessed include acidity, DP, transition metals content, and oxidative stability, as well as the nature and amount of degradation products. The evaluation of the microanalytical methods will be based on accelerated aging of representative standard reference papers and on comparisons of the results obtained with those of related standardized testing methods (Pedersoli 1999).
Chemiluminescence was put forward as a means of monitoring the aging of paper at an International Council of Museums, Conservation Committee Working Group meeting in Ludwigsburg in 1998 (Pedersoli and Hofenk de Graaff 1998). NICH began to work on chemiluminescence as part of Pedersoli’s research. NICH’s work on chemiluminescence will be continued within the framework of an international project that was recently accepted by the European Union and coordinated by Matija Strlic, of the University of Ljubljana, Faculty of Chemistry and Chemical Technology, in Slovenia (Kolar 2000).
To help better understand the complex chemical processes of paper deterioration, Matija Strlic, Boris Pihlar, Jana Kolar, and coworkers from the University of Ljubljana’s Faculty of Chemistry and Chemical Technology, and the National and University Library’s Conservation Department are trying to develop new analytical approaches to the elucidation of cellulose degradation. The studies concentrate on the following methodologies:
- quantification of the presence of oxidized functional groups in cellulose
- determination of molecular weight and its distribution and evaluation of errors
- development of models for testing antioxidant formulations
- determination of early oxidative degradation pathways
- studies on lignin models
- determination of low-molecular-weight degradation products (Levart et al 1999; Rychl et al, in press; Strlic and Pihlar 1997; Strlic et al 1998; Strlic et al 1999; Strlic et al, in press).
Oxidative paper-degradation processes have become the subject of increased attention in preservation science research. This new focus on oxidation is not only confined to specific problems such as ink corrosion and photodeterioration, but also concerns the study of paper decay in general. Jana Kolar and Matija Strlic are studying oxidative processes in paper. The main factors leading to the deterioration of deacidified paper made from bleached pulp were identified and their importance in the degradation of paper considered. These studies also clearly demonstrate the protective effect of antioxidants (Kolar 1997; Kolar et al 1998; Kolar and Strlic 1999; Strlic and Kolar 1999).
To secure the preservation of the written and printed cultural heritage, Canada is preparing a Canadian Permanent Paper Standard. This enterprise is offering new insight into several factors responsible for the degradation of paper. Paul Bégin, David Grattan, and Joe Iraci from the CCI are participating in the preparation of a draft permanent paper standard for adoption as the Canadian General Standards Board (CGSB) standard. The Canadian Co-operative Permanent Paper Research Project provided valuable information about the impact of lignin and air pollutants on the stability of paper. After undergoing several revisions, the draft standard is in its final stage. An important conclusion is that the fiber composition of paper is of minimal importance to its permanence, as long as the paper is buffered with at least 2 percent calcium carbonate (Bégin et al 1998, 1999; Zou et al 1998).
Although papers in archives and libraries are generally well protected from light, the effects of light on paper should not be underestimated. In a review article, John Havermans and Javier Dufour (Complutense University of Madrid, Spain) identify the serious risk that, during consultation of archival documents and books, certain compounds, called initiators, may be formed that cause further deterioration of the paper (Havermans and Dufour 1997). The extent of this risk was unknown until now, and in recent years only a few research attempts have been made on the topic. These include studies on the role of oxygen and on the effectiveness of certain inhibitors (Destiné et al 1996; Wang et al 1996). To improve our understanding, and in connection with the fact that alkaline compounds may promote photo oxidation, research into the effects on the oxidative/alkaline deterioration process, especially with respect to naturally aged papers that have been deacidified, is recommended.
The wet/dry interface phenomenon concerns a specific form of paper discoloration (generally resulting in a brown line) that takes place at the border between (formerly) wet and dry regions in a sheet of paper. The phenomenon has been known since the mid-1930s and has drawn recent attention because it might be part of the cause of several types of local paper discoloration.
At the NICH, Anne-Laurence Dupont used a variety of solvents to study the formation of brown lines on filter paper at the wet/dry interface. She also investigated the effects on aging and conservation treatments of washing and bleaching with sodium borohydride (Dupont 1996a). In additional studies on the nature of the brown-colored oxidation compounds formed at the wet/dry interface, the use of analytical tools, including TLC (thin-layer chromatography), FTIR, and GC/MS, has been evaluated (Dupont 1996b). Frank Ligterink and J. Luiz Pedersoli of the NICH plan to continue this research.
Aqueous treatments have always been important in paper conservation, and there is an extensive literature on their benefits, especially with respect to the improved appearance of the treated papers. Although it is acknowledged that treating paper with water also brings about profound, and often permanent, structural and mechanical changes, less attention has been paid to the characterization and quantification of these influences, particularly with a view to optimizing conservation procedures.
In 1997, Anthony W. Smith reported on a long-term preservation science project entitled “Paper Substrates and Graphic Media” that was undertaken at the Camberwell College of Arts and funded by The London Institute. The purpose of the project was to investigate the effects of aqueous conservation treatments on the mechanical properties of paper. A preliminary study on the effects of “washing” showed several main changes, including a reduction in the elastic modulus and an increase in the extensibility, compared with untreated paper. No significant differences were observed between tensile strength before and after washing. These findings provide a better understanding of the “improvement” that is generally observed by conservators as a consequence of the washing of paper; that is, the changes detected have less to do with an increase in the strength of a sheet than with an increase in its flexibility. The report gives special attention to the need for careful specimen preparation and control of test conditions. It also describes future research plans, including studies into the effects of repeated washing and of washing brittle paper, and the influence of drying methods (Smith 1997).
Disinfection with ethylene oxide
The vacuum fumigation system, using the gaseous sterilizing agent ethylene oxide (EtO), is considered the most effective means of protecting documents from the harmful effects of microbiological damage. Although EtO is a significant health hazard, many institutions still use this system to sterilize archival and library materials. In such sites, strict regulations are established to govern the permissible level of exposure.
To make a comprehensive comparison of the techniques of EtO sterilization and methods of determining the residual EtO in the material treated, an international project has been set up among the Centre de Recherches sur la Conservation des Documents Graphiques (CRCDG, Paris, France), the Slovak National Archives, the State Central Archives (Prague, Czech Republic), and the Chemical-Technological University (Prague, Czech Republic). The results of the different sterilization equipment and procedures employed in Paris, Bratislava, and Prague were compared using different sorts of test samples, including Whatman, Xerox, handmade, and notebook papers. Independent determinations of residual EtO were carried out by GC (gas chromatography) in two laboratories (Paris and Prague) using different GC systems.
Calculations of the content of residual EtO indicated that the samples tested by the method used in Paris contained two to nine times higher levels of EtO than did those tested by the method used in Prague. Such discrepancies could be explained by differences in technical procedures and time shifts among the various tests. Nonetheless, the differences underscore the need for a detailed comparison of different techniques and methods and indicate that a standardized method for measuring residual EtO in sterilized materials would be very useful (Hanus et al 1999).
Disinfection with beta radiation and microwaves
The treatment of microbiological damage is seriously hampered by the fact that the use of ethylene oxide gas is restricted, and in many countries forbidden, because of its high risk of harmful health effects. Consequently, research is under way to develop suitable and safe alternative fungicides.
Malalanirina Rakotonirainy and other researchers at the CRCDG investigated the disinfecting capacity of beta radiation and microwaves. The test material consisted of different sorts of paper that were artificially contaminated with various fungi from the “mycothèque” of the Natural History Museum in Paris. In addition to the fungicidal effect, the influence of radiation on the physicochemical characteristics of the paper samples was determined using accelerated-aging tests.
Although beta radiation, in a sufficiently high dose, was found to be effective in attacking the fungi, a strong dose-dependent depolymerization of the cellulose molecules was observed in all cases. Consequently, beta radiation, like gamma radiation, which previous studies of the CRCDG and others had found to produce similar adverse effects, cannot be recommended. A fungicidal effect of the microwaves was also demonstrated; however, the microwave treatment did not show significant negative side effects on the paper itself. Though the practical limitations of the microwave equipment used do not yet allow the possibility of large-scale treatment, the study has clearly indicated the applicability of microwave treatment (Rakotonirainy et al 1999).
Paper that has been heavily damaged by water (e.g., by a flood or other disaster) can be treated in different ways. A popular method, which is often used in commercial settings, is to freeze-dry the damaged documents. Possible negative influences of this drying procedure have not yet received full attention.
Søren Carlsen and colleagues from the Royal Library, Department of Preservation (Copenhagen, Denmark) investigated the effects of freeze-drying on the mechanical strength and aging stability of paper. The authors used three types of paper: groundwood, cotton, and coated. All were freeze-dried, air-dried, and exposed to accelerated aging. They found that freeze-drying primarily influences characteristics such as moisture content, folding endurance, and tear strength. Freeze-drying particularly affected the mechanical strength of paper with low initial strength; its effect on paper with high mechanical strength was relatively small. In general, freeze-drying influenced paper more than did air drying (Carlsen 1999).
The treatment of ink-corroded paper artifacts remains a concern in the field of paper conservation. The effectiveness of treatments and their possible negative long-term side effects are often a reason for particular anxiety.
Iron-gall ink corrosion has become an important research priority at the NICH. The NICH Ink-Corrosion Project includes four components:
- investigations into the causes and mechanisms of ink corrosion
- the development of early-warning and condition-rating methods
- the development of suitable methods to accelerate and measure the corrosion process
- the testing and optimization of the treatment of ink corrosion by means of phytates (Neevel and Reissland 1998).
The NICH’s work on nonaqueous treatment of ink corrosion will be continued within the framework of an international project recently accepted by the European Union and coordinated by Jana Kolar (National and University Library, Conservation Department, Ljubljana, Slovenia). The State Central Archives in Prague (Czech Republic) are also contributing to this project (Durovic 2000; Kolar 2000).
Birgit Reissland and Suzanne de Groot from the NICH studied the effectiveness of nine commonly used aqueous treatments for iron-gall ink corrosion. Standard reference papers with an applied corrosive iron-gall ink preparation and four original seventeenth- and nineteenth-century iron-gall ink written manuscripts were immersed in different treatment solutions. The effect on the degradation process was determined by measuring the bursting strength of the paper samples after accelerated aging. Side effects, such as mechanical damage, color changes of paper and ink, and ink bleeding, were determined by visual examination. Results of this study indicated that a combined calcium phytate/calcium bicarbonate treatment, as well as a single treatment with calcium bicarbonate, could effectively delay ink corrosion and showed minor side effects (Reissland and Groot 1999).
At the LC, Heather Wanser and others have studied the effect of several aqueous and non-aqueous deacidification treatments on manuscripts written in iron gall inks. The effect of treatments on inks was evaluated by X-ray microanalysis to monitor changes in metal content and by colorimetry to measure changes in color. A potassium peak frequently found in the untreated samples was invariably lost after aqueous deacidification treatments. The presence of the potassium peak in the untreated ink samples has been tentatively ascribed to the presence of potassium salts in gum arabic, one of the essential ingredients of iron gall inks. The loss of potassium salts resulting from aqueous deacidification was not related to the appearance of the inks. Magnesium bicarbonate solutions prepared in 70 percent alcohol retained the potassium peak, as did the Bookkeeper and Wei T’o-like methyl magnesium carbonate treatments. These treatments had no noticeable effect on the color of any of the inks; aqueous treatments, by contrast, changed the appearance of most of the inks with dark-brown tints changing to light or even orange-brown. The appearance of the inks was also judged by visual examination by a panel of paper conservators.
Laser cleaning is a relatively new technique in the field of conservation. More and more museum artifacts are now being cleaned with laser techniques; however, the possible harmful effects of this process are not yet known.
Carole Dignard, Paul Heinrichs, Tom Stone, and Gregory Young from the CCI have undertaken an in-depth study of the photothermal, physical, and chemical effects of laser radiation on the surfaces of natural organic materials. The goal is to contribute to the establishment of guidelines for the appropriate use of lasers on museum artifacts (Dignard et al 1997a, 1997b).
Nd-YAG laser cleaning has been practiced since the early 1990s. Its effects on the materials are still not well understood. Little information is available on the assessment of Nd-YAG laser-cleaned organic materials, in particular. Carole Dignard, Paul Heinrichs, Tom Stone, and Gregory Young from the CCI are developing expertise and experience with analytical methods to assess the results of laser cleaning. They have performed tests on a variety of soot-covered organic materials. Fluence (energy density) and repetition rate (frequency) were varied incrementally (Dignard et al 1997a, 1997b, 1997c).
Cleaning of paper is necessary not only for aesthetic reasons but also for conservation purposes. Given that conventional mechanical and wet cleaning methods have proved insufficient in numerous cases, contactless cleaning by means of the laser technique could offer an appropriate solution.
In 1997 the European Union announced the Eureka/Eurocare LACLEPA (LAser CLEaning of PAper and PArchment) project (EU 1681). The participating countries (Austria, Germany, Slovenia, and Vatican City) are developing a prototype laser-cleaning system particularly fit for flexible paper and parchment. The method will be based on the use of ultraviolet (UV) pulse lasers, which will ensure preservation of the delicate artifacts by minimizing the absorption volume, the heat-affected zone, and mechanical shock. To complement the laser system, a catalog of working parameters for typical artifact types will be defined.
Institutes in each of the participating countries contribute their own expertise:
Austria (project coordinator): Institut für Papierrestaurierung (paper restoration), Österreichisches Museum für Angewandte Kunst (paper restoration), Österreichisches Staatsarchiv (paper restoration) (Müller-Hess et al 1999);
Germany: BAM, Laboratorium für Dünnschichttechnologien (dry-laser cleaning by means of UV pulse lasers), Freie Universität Berlin, Kunsthistorisches Institut (historical and ethical context of antique paper artifacts), Staatsbibliothek zu Berlin-Preussischer Kulturbesitz (paper restoration), Bayerische Staatsbibliothek (paper restoration) (Kautek et al 1998; Rudolph et al 1998);
Slovenia: National and University Library, Conservation Department (testing of new conservation treatments); University of Ljubljana, Chemistry and Chemical Technology Faculty (evaluation of possible damage to cellulose); Fotona dd., Ljubljana (manufacturer of laser systems) (Kolar and Strlic 1998, 2000; Kolar et al 2000; Kolar et al, in press);
Vatican City: Biblioteca Apostolica Vaticana (paper restoration).
The first goal of the joint research project was to assess the immediate effects of lasers running at three different wavelengths (308 nm, 532 nm, and 1064 nm) on paper and to determine the long-term impact that the treatments may exert on the stability of cellulose. The paper samples were purified cotton linters cellulose (Whatman filter paper), elemental chlorine-free (ECF)-bleached sulfate pulp, gelatin-sized handmade paper from rags (from 1600), Fabriano Roma paper, coated paper, and modern book paper from bleached chemical pulp. One side of each sample was treated either with excimer pulse laser (running at 308 nm) or with Nd-YAG pulsed laser (running at 532 nm or 1064 nm). After microscopic examination, accelerated-aging tests were performed at 90°C and 65 percent RH for up to six days. The paper parameters tested included the DP and brightness.
On the basis of the Eureka/Eurocare LACLEPA project, a two-year follow-up study was initiated in Slovenia in 2000. In cooperation with the Slovenian manufacturer of laser systems Fotona dd. (Ljubljana), Jana Kolar and Matija Strlic are attempting to define optimum parameters for cleaning cellulose-based substrates using Nd-YAG laser. The immediate as well as the long-term effects of Nd-YAG laser irradiation on paper have been studied. Analysis by FTIR indicates that laser treatment induces the cross-linking of cellulose, resulting in an increased DP. Changes in content of acidic or carbonyl groups were below threshold sensitivity of the method (Kolar et al 2000).
Mass deacidification has become an integral part of mass conservation and preservation strategy in the United States and several European countries. Recognition of the benefits of deacidification has been accompanied by a diminished interest in research in this field. Since the joint publication of the European Commission on Preservation and Access and the Commission on Preservation and Access on the possibilities and limitations of the current mass-deacidification techniques (Porck 1996), deacidification has received relatively little attention in preservation research. Nonetheless, several developments are worth mentioning.
Lynn Kidder, Terry Boone, and Susan Russick (LC) have studied treatment of paper artifacts with Bookkeeper spray deacidification. They found that humidifying the objects after the spray treatment improved the effectiveness of the deacidification process (Kidder, Boone, and Russick 1998).
Since the end of the 1980s, the Bibliothèque nationale de France (Paris) has used a mass-deacidification system adapted from the Canadian Wei T’o process. Research into the effectiveness of this system has produced satisfactory results; however, questions remain about both the amount and the distribution of the alkaline reserve in the paper after treatment. The CRCDG investigated the different stages in the deacidification procedure to find ways to increase the final alkaline reserve in the deacidified paper and to improve the homogeneity of its distribution (Daniel et al 1999a).
Thi-Phuong Nguyen (Centre Technique de Bussy-Saint-Georges, France) described a new approach to mass deacidification that is being developed under the auspices of the Bibliothèque nationale de France. The procedure involves microencapsulation of the deacidification agents and the use of supercritical carbon dioxide as a carrier gas. Part of the work will focus on the possibility of combining deacidification with reinforcement of the paper. Detailed information on the progress of the studies is not yet available.
A Wei T’o mass-deacidification system has been used in Canada for many years. It was implemented by the Conservation Division of the National Archives, and, since October 1997, has been run by the National Library of Canada (Ottawa). One of the major challenges has been the replacement of the original chlorofluorocarbon (CFC) solvents, consequent to a ban on CFCs that became effective January 1, 1996, under the Montreal Protocol. Initially, CFCs were replaced by hydrochlorofluorocarbons (HCFCs). Although the use of the HCFCs could be continued until the year 2000, when the state of Ontario planned to ban its use, it was decided in 1997 to test a new chemical formula using hydrofluorocarbons (HFCs). The results of these tests have been fruitful. Inks that had been affected by the previous solvents remained stable in the new solution, which was named the “Good News Formula.” Work is in progress to improve the recovery of the solvent after treatment (Couture 1999).
At the 15th Annual Preservation Conference of the National Archives and Records Administration (NARA, Washington, D.C., USA), held in March 2000 under the title “Deacidification Reconsidered,” conservation scientists, preservation professionals, and conservators discussed technical issues related to deacidification. The following recent research results were reported:
- John Bogaard (Carnegie Mellon Research Institute, Pittsburgh, Pennsylvania, USA) presented the results of chemical studies of the beneficial effects of calcium-enriched wash water applied in the course of the conservation treatment of paper objects. The compounds used were calcium hydroxide, calcium bicarbonate, and calcium chloride. Chemical properties such as DP, pH, and carbonyl and carboxyl groups were followed to monitor the behavior during accelerated thermal aging and exposure of the treated papers to UV light.
- Chandru Shahani (LC) discussed new insights into the effects of deacidification on the life expectancy of paper-based collections. Recent research suggests that acidic paper ages considerably faster than has been indicated by currently accepted aging tests.
- Elissa O’Loughlin and Anne Witty (NARA Document Conservation Laboratory, Washington, D.C., USA) addressed the possible impact of previous deacidification on the conservation treatment and care of paper artifacts.
Insects can cause extensive, and often irreversible, damage to paper and other cellulose-containing materials. Although the use of insecticides is often successful, it has several drawbacks. These compounds are not only generally harmful to humans but also can produce damaging reactions with paper artifacts.
Attention has recently focused on the applicability of a natural insecticide extracted from seeds of the neem tree (Azadirachta indica), a tropical evergreen. Robert O. Larson of Vikwood Botanicals (Sheboygan, Wisconsin, USA) has developed the pesticide Margosan-O, a neem extract in ethanol. The unique qualities of the neem product have been investigated intensively and have yielded encouraging results. In particular, insecticides containing significant amounts of neem oil do not appear to be harmful to human health.
John Dean from Cornell University, Department of Preservation and Conservation (Ithaca, New York, USA) has designed a research project to study the effects of neem products on treated materials. Specifically, the project aims to
- determine the effectiveness of neem products as repellents when applied directly to paper
- test the effects of neem products on paper appearance and longevity
- evaluate the effects of the product on inks, dyes, and pigments
- identify the most appropriate methods of application.
The project is currently seeking funding.
Long before the invention of photocopying, other methods were used to duplicate documents. Knowledge of these early techniques is rapidly vanishing. Because of the need to preserve these materials, there is renewed interest in these non-photographic methods.
Sebastian Dobrusskin started a project at the Conservation Program of the Berner Fachhochschule (Bern, Switzerland) to study the history, technology, identification, and conservation of early, non-photographic copying and duplicating techniques. In addition, this project examines the effects of mass deacidification on such early copies. The goal is to develop recommendations for the preservation of collections of non-photographic copies.
The individual techniques have been systematically ordered. The technology of the direct dye-transfer copying techniques has been described in depth with all its variations, coloring agents, and support materials. The study showed that several of the coloring agents used for these techniques are sensitive to pH, humidity, organic solvents, and light. In the next stage of the project, additional copying and duplicating techniques will be studied. The materials will be tested to understand their response to conservation treatment (Dobrusskin 1999).
Reinforcing deteriorated paper objects on a large scale has proved to be problematic, although many attempts have been made to combine mass deacidification with paper strengthening. The mechanization of conservation procedures using paper splitting has made much progress recently and offers good prospects.
The Zentrum für Bucherhaltung (Leipzig, Germany) is exploiting a mass-conservation system for loose sheets of paper. The system uses several consecutive processes, including aqueous washing and deacidification, leaf casting and mechanized paper splitting, and insertion of a thin layer of paper that forms the new core of the original sheets (Wächter et al 1996). Results of independent research into the effectiveness and possible negative side effects of this technique are not yet available; nonetheless, there is a growing worldwide interest in the paper-splitting system. The Bibliothèque nationale de France supported a study on mechanical reinforcement methods for paper that compared thermal gluing with splitting. The investigation, carried out on different types of printed paper, demonstrated that splitting resulted in a greater improvement of the mechanical properties of papers, combined with an unaltered readability of the text, than did gluing. On the basis of the results, the reversibility of the splitting process was also considered satisfactory (Vilmont et al 1996).
Plasma, defined as “almost completely ionized gas, containing equal numbers of free electrons and positive ions . . . formed by heating low-pressure gases until the atoms have sufficient energy to ionize each other,” has been used in the restoration of metal objects.
Little has been published on attempts to use plasma treatment in the conservation of paper. What has been published includes initial results of efforts to remove mold spores and other stains on paper, suggestions to use plasma in paper deacidification and strengthening, and indications that a low-temperature plasma treatment by glow discharge of hydrogen can improve the strength of aged papers (Anders et al 1996; Vohrer et al 1996). In 1995, John Havermans organized an expert meeting to discuss the potential of plasma treatment for strengthening brittle paper and to establish a joint research program (Havermans 1996). Definite plans for this program have not yet been worked out.
In the development of devices that exploit the benefits of suction and airflow in the conservation treatment of paper artifacts, attention is being focused on designing suction tables with a safe, built-in light source. There is also growing interest in using small suction devices in the treatment of paper and other materials for local conservation treatments, including the removal of non-aqueous solvents.
Paul Heinrichs and Stefan Michalski from the CCI are attempting to make several of the desired improvements. With respect to the addition of a light source, the prototypes have proved the utility of the concept and shown that a commercial fiberoptic delivery system is most effective. With respect to the selection and testing of solvent-capture devices that can be inserted between the vacuum table and the vacuum source, work is still in progress.
Archival storage materials have received much attention lately, particularly the product MicroChamber; however, independent research into this archival paper product is scarce. First marketed in 1992, MicroChamber is a lignin-free, sulfur-free, alkaline-pulped, alkaline-reserve paperboard with an additional elementmolecular traps or sieves. At the CRCDG, Floréal Daniel, Vassiliki Hatzigeorgiou, Serge Copy, and Françoise Flieder compared the protective quality of MicroChamber with that of other archival papers. These researchers concentrated on two of the most widely used MicroChamber products: MicroWrap 155 g/m2 and End Leaf 130 g/m2. The papers contained 10 to 15 percent mineral absorbents (zeolites, calcium carbonate).
Interestingly, the verso and the recto sides of each of the MicroChamber products showed different results. The MicroChamber papers absorbed much more sulfur dioxide than did the permanent papers. This difference appeared to be connected to the weight and sizing of the papers, rather than to the presence of absorbents. In general, MicroWrap performed much better than did End Leaf (Daniel et al 1999c).
Nitrogen dioxide pollution
The air-pollutant nitrogen dioxide is considered a growing threat for repositories of records of cultural heritage. One way of dealing with this problem is to protect archival materials by storing them in boxes. C. M. Guttman and W. R. Blair from NARA studied the effect of nitrogen dioxide on archival boxboards and model papers. They focused on determining the absorption coefficients of nitrogen dioxide in low-lignin and acid-free buffered boxboards that are used for storage containers. An earlier report indicated that few data existed on these coefficients for sulfur dioxide and nitrogen dioxide (Guttman and Blair 1996).
Polyester film encapsulation
Polyester film encapsulation is used to protect paper from harmful environmental factors such as air pollutants, dust, and microorganisms. The benefit of this preventive measure has often been discussed, and contradicting experimental results have been reported.
The system Archipress 1000 of the Dutch firm Multipak (Putten, the Netherlands) offers a technique by which an object can be encapsulated under low pressure. The CRCDG has studied the effects of this kind of storage, taking into account both the external and internal factors that can contribute to deterioration.
After analyzing the effects of several accelerated-aging tests on the DP of different kinds of paper, the authors concluded that encapsulation enhances the deterioration of acid paper. The rate of degradation of nonacid paper appeared to increase significantly only when such paper aged together with acid paper, especially when the mixed stack had been encapsulated. Additional experiments have shown that interleavage with alkaline or MicroChamber paper could partly circumvent this influence. Depending on the situation, this kind of interleavage should be weighed against a deacidification treatment before encapsulation (Daniel et al 1998, 1999b).
The choice of a range of relative humidity and temperature for storage depends on a number of factors. Relative humidity affects the preservation of objects in many ways. It influences the physical, chemical, and structural properties of the materials. It is a factor in many chemical reactions and determines whether biological attacks might occur. Changes in RH can produce dimensional changes that can result in strains, stresses, deformation, or fracture. Because each material is affected differently, research into the effects and optimum value or range of RH leads to overlapping, or even conflicting, recommendations.
The Smithsonian Institution’s Center for Materials Research and Education has investigated suitable conditions of RH in a general museum environment, with an emphasis on hygroscopic organic materials.
Measurements of the elastic modulus (stiffness), strain-to-yield (deformation required to cause permanent distortion), and strain-to-failure (deformation required to cause fracture or breakage) of cellulose-containing materials contradict the general assumption that these materials are necessarily brittle or stiff at all low RH values. In fact, if very low RH (less than 30 percent) is avoided, important physical properties, as well as chemical reactivity (rate of hydrolysis and cross-linking reactions) are relatively insensitive to RH over a wide range. Similar results have been found with aged paper, indicating that while paper may become weaker as it ages, its stiffness and response to RH do not change significantly (Erhardt and Mecklenburg 1995; Erhardt et al 1997).
By means of stress-strain studies, it could be shown in cellulose and other hygroscopic materials that changes caused by environmental fluctuations are generally reversible (non-damaging) within a relatively wide (10 to 15 percent) range in the moderate RH region (30 to 60 percent). This represents a much wider range than is generally supposed (Erhardt et al 1996, 1997).
An approach similar to that used in determining the mechanical and physical effects of RH has been used to evaluate the effects of temperature (Mecklenburg and Tumosa 1996).