PRACTICALGUIDE

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ecords deteriorate from many causes—use

and handling, exposure to light, contact with

injurious housing materials, gaseous and

particulate pollutants, oxygen, improper humidity, and

heat. While it may be true, as has been suggested with

tongue firmly in cheek, that records would last essentially

forever if sealed in an inert atmosphere in an opaque

container kept at near absolute zero, they would

obviously serve little purpose thus sequestered from

access.

The records of long-term or permanent value in archives, re-search libraries, and historical societies are diverse in format andcomposition: books, documents, photographs, maps, motion pic-tures, computer disks and tapes, posters, audio and video record-ings. They can range in age from a thousand or more years old toyesterday. Most are composed primarily of organic compounds,and as such are subject to continuous deterioration. Some inor-ganic materials—most notably the silver of photographic images,the iron of magnetic media, and the aluminum reflective layer ofoptical disks—can also deteriorate, with loss of information.

Older, “traditional,” records materials such as paper madefrom pure fibers, parchment, carbon-based inks, and some leathershave often lasted for many centuries. It is a considerable over-simplification, but nevertheless approximately true, that the morerecent the records, the more chemically unstable they are, espe-cially those from the 19th and 20th centuries.

Photography, invented in the first half of the 19th century,contributed many unstable (as well as some relatively perma-nent) records. The introduction of poorly processed wood fibersand acidic sizing materials in the middle of the past centuryproduced paper that becomes seriously embrittled in 50 years orso. This century has contributed the growing use of often fugi-

WHAT MAKES RECORDSDETERIORATE

By Paul N. Banks

tive color and the rapid growth of magnetic media, which havefinite life expectancies. The greater areal density for digital in-formation generally makes magnetic media highly vulnerable toall kinds of deterioration and damage.

The major chemical deterioration mechanisms of records areoxidation and hydrolysis. Many types of modern records are es-pecially subject to one or both of these types of deterioration.Probably the best known example of rapid deterioration of mod-ern records is the embrittlement of paper in libraries. While oxi-dation plays a role in this deterioration, the dominant mecha-nism is acid-catalyzed hydrolysis, which breaks up the cellulosemolecules that are the main structural component of paper. Acidhydrolysis is also the major culprit in two alarming manifesta-tions of the deterioration of modern record types:

1. The shrinking and embrittlement of cellulose acetate, whichis the base used for the vast majority of photographic nega-tives and motion pictures and some older audiotapes, and

2. The “sticky shed” of the binder that holds the magnetic par-ticles onto the base of audio, video, and computer and instru-ment data tapes.

Oxidative deterioration, chemical changes caused by oxygenor other oxidants such as ozone or oxides of nitrogen, can seri-ously degrade some kinds of modern records. The metallic silverimages of black-and-white photographs fade by oxidation. In theiroxidized state (and with high enough relative humidity) they canmigrate to the surface of the negative or print, causing “mirror-ing,” a silvery sheen of the surface. The metal particles or coat-ings of magnetic media rust, in simplest terms, in response tooxidation, thereby losing at least part of their useful properties.

As the need for higher data packing densities has acceler-ated, magnetic “pigments” such as metallic iron and thin, pureevaporated coatings have come into use. These thin coatings aremore vulnerable to oxidation than the older gamma ferric oxide(which is already in an oxidized state). Similarly, the thin reflec-tive layer upon which the usability of CDs, most CD-ROMsand similar media depends is most often aluminum. It is pro-

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The following article was published in ASHRAE Journal, April 1999. © Copyright 1999 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paperform without permission of ASHRAE.

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tected from the atmosphere only by a very thin lacquer. Thiscritical layer is also subject to oxidation. Oxidation is the domi-nant cause of fading of dyes in color photographs even whenprotected from light.

In many of the cases described, the reacting chemicals—asidefrom oxygen, which obviously cannot normally be kept fromrecords—originate in the records themselves. The aluminumsulfate used in paper sizings and the acetyl or nitrate radicals inphotographic film are common examples.

However, external contaminants are also of concern as causesof deterioration. Atmospheric pollutants—acidic and oxidizinggases, VOCs, etc.—contribute to oxidation and hydrolysis, butmore harm may be caused by pollutants generated indoors. Fa-miliar indoor sources are finishes, wallboards and unfinishedconcrete. Materials sometimes used for records containers havebeen implicated in the deterioration of photographic materials.1

These materials include chlorinated and heavily plasticized vi-nyl plastics and unpurified wood pulp, formerly used for examplefor microfilm cartons. Unpurified wood pulp liberates peroxidesas it ages. Probably the most dramatic examples of self-gener-ated pollutants are the oxides of nitrogen and acetic acid liber-ated as photographic film ages.

Effects of Temperature and HumidityTemperature and relative humidity affect most deteriorative

reactions dramatically. The effect of temperature on the rate ofchemical reactions has been understood at least since the 19th

century Swedish chemist Arrhenius published the method usedin the age testing of materials. In that test, a material is heated todifferent temperatures and the resultant deterioration points ex-trapolated to room temperature. Except for the extreme case ofhumidity so high as to foster biological damage, the influence ofhumidity on the deterioration of records has been less under-stood—or indeed largely misunderstood.

Figure 1: Isoperms. Linesof constant relative life atvarying temperature andrelative humidity.

Notions of “correct” temperature and relative humidity forrecords were based, consciously or unconsciously, on human com-fort conditions and the limitations of control technology, and insome cases (for example, in the British Isles) by the realities ofthe climate. Research at the U.S. National Bureau of Stan-dards in the 1970s on the effects of environment on paper wasapparently the first to make a significant impact on the libraryand archives conservation community. However, the broad out-lines of the effects of temperature and humidity on records hadin fact been published by scientists at the Bureau nearly 50 yearsearlier.2

The effect of temperature on the deterioration of records issimple: the higher the temperature, the faster records deteriorate.3

The effects of relative humidity are more specific to particularmaterials, although in general, the higher the humidity, the fasterrecords will deteriorate. Humidity, or, more accurately, the mois-ture content of materials as it is affected by ambient relative hu-midity, is (perhaps self-evidently) most important in hydrolyticdeterioration. Examples include the deterioration of acidic pa-per and leather and the “sticky shed” of magnetic media, whichis a hydrolysis reaction.

Two examples of the effects of temperature and relative hu-midity on records will demonstrate the point. The useful life ofthe acidic paper on which most books were printed during thepast century or so can be doubled by lowering the temperaturefrom 68°F (20°C) to 60°F (16°C), or lowering the RH from50% to about 25%. Lowering both the temperature and RH bythese amounts increases the paper’s life span by more than fourtimes.4 Increasing poor paper’s useful life from 50 to 100 yearsis not an insignificant improvement for preservation managers,and quadrupling it buys collecting institutions a great deal ofleeway to find alternatives for embrittled books. Even more dra-matically, the usefulness of color photographs, most of whichshow significant fading in an exceptionally short time, can be

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Practical Guide

increased from 25 years at 75°F (24°C) and 50% RH to 1,000years at 35°F (2°C) and 40% RH.5

Temperature and relative humidity also play a role in the physi-cal or mechanical deterioration of records, although in most casesperhaps somewhat less so than many of us have supposed. Intruth, the mechanical properties of most records materials arerelatively unaffected by temperature that is within shooting rangeof room temperature, with one important exception. Magnetictape has a high enough coefficient of thermal expansion thattemperature fluctuations have serious preservation implications.

For example, a 2400-ft (730 m) reel of data tape can expandor contract by 1 ft (0.3 m) for each 10°F (5°C) change in tem-perature (as well as by a 10% change in RH). Such expansionand contraction can lead in extreme cases to cinching of the tapeon its reel with consequent loss data or even breaking of thetape.6

The mechanical properties of the organic materials that con-stitute most recordsare significantly af-fected by their equi-librium moisturecontent (EMC).However, it is noteasy to say with cer-tainty what the op-timum RH is, norwhat allowable tol-erances around thechosen setpointshould be. The ab-solute upper limit ofRH is that at whichrisk of fungal attackbecomes great, usu-ally given as 60% to 70%. Fifty percent is now widely recom-mended as the maximum RH to limit chemical deterioration.For many organic materials, an absolute lower limit of around20% or 25% can be set, the level at which chemically-boundwater begins to be lost and irreversible changes such as cross-linking may take place.

Also, some laminar materials—early glass photographic nega-tives, for example, or magnetic tape on polyester base—may tendto delaminate at low relative humidities. Between these two ex-tremes—say 25% and 50%—the curve of mechanical proper-ties versus RH (or EMC) for most records materials is not ex-tremely steep. In the case of paper, some mechanical propertiespeak at about 32%, while others show a rather steep slope withincrease in humidity.7

Allowable fluctuation is one of the most vexed questions inthe library, archives, and museum conservation worlds today (seeMecklenburg elsewhere in this supplement). It is probably truethat “flat lining” RH—attempting to keep it within 2% or 3%of the chosen setpoint—is overkill, even if it is really achievable.8

However, the dimensional and flexibility properties of a few ma-terials, most notably vellum and parchment (which are animal

skins having a relatively low level of processing) are dramaticallyaffected by small differences in EMC. The dimensional changescaused by changing EMC can cause flaking of the ink or paintthat is less flexible than the skin substrate. The effect of RH onmagnetic tape has already been mentioned.

In some cases trade-offs in establishing RH setpoints mustbe made by conservation personnel between better mechanicalproperties (e.g., flexibility), which would improve resistance tohandling and use, and faster rates of chemical deterioration, es-pecially hydrolysis.

Improved Methods of Visualizingthe Effects of Environment

In 1989, Donald K. Sebera, then of the Preservation Researchand Testing Office of the Library of Congress, published a methodfor visualizing the effects of temperature and relative humidity onthe rate of deterioration of modern paper.4, 9 His concept of isopermsis based on assigning room conditions (20°C/68°F, 50%) as an

isoperm of 1, andrepresenting graphi-cally the rate atwhich longevity willbe increased or de-creased by varyingthe temperature orrelative humidity orboth (see Figure 1).As an alternative torigid standardsetpoints, Sebera’sapproach permitssome tinkering withtemperature andRH in response to

local conditions in order to achieve a level of longevity established asa matter of policy by the client.

Meanwhile, the Image Permanence Institute (IPI), affili-ated with Rochester Institute of Technology, expanded onthe isoperm concept. As a product of their research on thedeterioration of cellulose acetate, IPI first published a method(including graphs, tables and a calculator wheel) for estimat-ing the life expectancy of acetate materials using the isopermprinciple of integrating temperature and RH onto single linesrepresenting a resultant life expectancy.10 More recently, theyhave undertaken similar research and produced a similar guideto environmental effects on the longevity of color photographicmaterials.5

The isoperm method predicts life expectancy of a material atconstant temperature and RH but does not indicate the cumula-tive effect of varying conditions. Because deterioration, once ithas occurred, cannot reverse itself, the life expectancy of a mate-rial is necessarily reduced by any period of poorer conditionseven though it may have spent much of its life in “good” condi-tions. By considering the isoperm for a given combination of Tand RH as a preservation index (PI), IPI devised a system for

Figure 2: Preservation Index (PI) for an article stored in a basement and its Time WeightedPermanence Index (TWPI), which is the PI integrated over time.

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integrating varying conditions into a time-weighted permanenceindex (TWPI) (see Figure 2).

The TWPI can serve several practical uses, especially in situ-ations in which there is limited environmental control. An ex-ample given by the authors is choosing among a hypotheticalattic, closet and basement for storage of records. (The longerperiod of lower temperature of the basement wins—not neces-sarily the intuitive choice). The system can also be used by pres-ervation managers to support arguments to resource allocatorsfor improved environment, and to visualize the effects of fluctua-tions in a controlled system on rates of deterioration.

IPI will supply the formulas needed for computing the TWPIfrom accumulated environmental monitoring data using a spread-sheet. They have promised a data logger that will accumulate T,RH, PI and TWPI data for downloading into a computer, aswell as providing a direct readout on the instrument.11

When an end point can be established, the costs of alterna-tives can be estimated, and the isoperm and TWPI models canbe the basis for cost-benefit studies. End points might beembrittlement of paper to the point that a book must be micro-filmed (or otherwise copied) to preserve its text, or when theleast-stable dye in a color photograph has faded by 30%. Such astudy was undertaken at the U.S. National Archives to deter-mine whether providing optimum environment or copying is themost cost-effective approach for cellulose acetate based photo-graphic materials.12

The increased understanding of the effects of temperatureand relative humidity on the deterioration of records, based bothon recent research on newer record materials such as photographsand magnetic media and on better tools for visualizing the ef-fects of environment on a wide range of media, have led to animportant body of formal environmental standards for variousrecords. (Ironically, there is not yet one for paper-based records.)These standards are listed at the end of this article.

ConclusionThe key words for preservation of records are clean, cool,

dry, and stable. Specific numbers must be determined by thenature of the materials in specific collections, with due regard tothe most vulnerable materials in the collection.

Maintaining temperatures and humidities that are lowerand more stable than usual comfort conditions carry obviouscost penalties, especially in hot humid regions. (In addition,they may present system design challenges to engineers whosemain experience is with providing comfort conditions.) Thecost penalty can be at least partially contained by consolidat-ing and segregating those parts of collections that are espe-cially vulnerable to hygrothermal degradation, particularlyphotographic and magnetic media.

Segregation of book collections from user areas, except forexplicitly rare books, is unfortunately contrary to current no-tions of making libraries attractive to users, even in researchlibraries for which long-term preservation is a goal. The stacksof archives and rare book and special collections departments,in contrast, are almost always closed to the public, so that

preservation conditions can more easily be maintained. Or-ganizations whose collections are primarily photographic ormagnetic tend to assume that they need to be segregated inorder to provide optimum environment at reasonable cost.

Engineers can assist their clients (both architects and owners)in providing better pre-servation for collections by alerting themto the studies and standards cited at the end of this article, byencouraging them to segregate especially environmentally-sensitivemedia, and by encouraging (or undertaking) cost-benefit studiesto demonstrate the cost-effectiveness of appropriate climatecontrol.

Notes1. Baer, Norbert S., and P. N. Banks. 1985. “Indoor air

pollution: Effects on cultural and historic materials.” TheInternational Journal of Museum Management andCuratorship 4:9–20.

2. Graminski, E. L., Parks, E. J., and Toth, E. E. “The effectsof temperature and moisture on the accelerated aging ofpaper.” R.K. Eby, ed., Durability of MacromolecularMaterials. Washington, D.C.: American Chemical Society,1979, pp. 341–55.

3. The exact rate of the accelerating effect of temperature de-pends on the activation energy of the specific chemical reac-tions involved. The Image Permanence Institute has foundthat the activation energy of the deterioration reactions ofmany of the most vulnerable materials in archives and li-brary collections—acidic paper, cellulose acetate, the colordyes used in photographs—centers around 25 Kcal, and afigure in that range is used in this paper.11

4. Sebera, Donald K. Isoperms: An environmental manage-ment tool. Washington, D.C.: The Commission on Preser-vation and Access, 1994.

5. Reilly, James M. Storage guide for color photographic ma-terials. Albany, New York: New York State Education De-partment, 1998.

6. Geller, Sidney B. 1983. Care and Handling of ComputerMagnetic Storage Media. Washington, D.C.: National Bu-reau of Standards. (“Computer Science and Technology;”NBS SP500-101)

7. Caulfield, D. F. and D. E. Gunderson. 1990. “Paper test-ing and strength characteristics.” Philip Luner, ed., PaperPreservation: Current Issues and Recent Developments. At-lanta: TAPPI Press.

8. Michalski, Stefan. “Relative humidity in museums, galleries,and archives: Specification and control.” William B. Roseand Anton TenWolde, eds., Bugs, Mold & Rot II. Work-shop Proceedings, November 16-17, 1993. Washington,D.C.: National Institute of Building Sciences, [1994] pp.51–62.

9. Sebera, Donald K. “A graphical representation of the rela-tionship of environmental conditions to the permanence of

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Practical Guide

hygroscopic materials and compos-ites.” International Council on Ar-chives and National Archives ofCanada. Proceedings of Conservationin Archives. Paris: ICA, 1989, pp.51–75.

10. Reilly, James M. IPI storage guidefor acetate film. Rochester, New York:Image Permanence Institute, 1993.

11. Reilly, James M., Nishimura, Dou-glas W., and Zinn, Edward. Newtools for preservation: Assessing long-term environmental effects on libraryand archives collections. Washington,D.C.: The Commission on Preserva-tion and Access, 1995.

12. Puglia, Steven. “Cost-benefit analy-sis for b/w acetate: cool/cold storagevs. duplication.” The Abbey News-letter 19:71–2 (1995).

Major U.S. Standards forStorage of Records1. American National Standards Insti-

tute. American National Standard forImaging Media—Reflection Prints—Storage Practices. New York:ANSI. 1996. (ANSI/NAPMIT9.20-1996).

(A) Imaging Media—ProcessedSafety Photographic Films—Storage. New York: ANSI,1996. (ANSI/NAPM IT9.11-1993).

(B) Imaging Materials—PolyesterBase Magnetic Tape—Storage.New York: ANSI, 1998.(ANSI/PIMA IT9.23-1998).

(C) Imaging Materials—ProcessedPhotographic Plates—StoragePractices. New York: ANSI,1996. (“ANSI/NAPM IT9.18-1996”).

2. Society of Motion Picture and Tele-vision Engineers. “Storage of motion-picture films.” White Plains, NewYork: SMPTE, 1994. (RP 131-1994).

Paul Banks is a library and archives con-sultant in New York City. =

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