International Advisory Committee

for the UNESCO

Memory of the World Programme

Sub-Committee on Technology

Contributors:

Astrid Brandt

Bibliothèque Nationale de France, Paris, France

Wolf Buchmann

Bundesarchiv, Koblenz, Germany

Helen Forde

Public Record Office, London, United Kingdom

Trudi Noordermeer

Koninklijke Bibliotheek, Den Haag, The Netherlands

Jonas Palm

Det Kongelige Bibliotek, København, Denmark

Dietrich Schüller

Österreichische Akademie der Wissenschaften, Wien, Austria

Editor:

George Boston

Milton Keynes, United Kingdom

Preface i1. Introduction 12. General Preservation Factors 43. Paper and Other Traditional Materials 64. Photographic Materials 95. Mechanical Carriers 166. Magnetic Materials 207. Optical Media 308. Electronic Publications, Electronic Documents and Virtual Information 349. Glossary 42

Preface

The most significant evidence of the intellectual and cultural endeavours of mankind is contained in documents. They take many forms, from papyri through parchment and paper to the modern electronic documents. As with many other man­made artefacts, however, documents are vulnerable and fragile. There is a significant danger that a substantial number, part of the collective memory of mankind, will be lost. Only a dramatic increase of effort will ensure the survival of the documents kept in the repositories worldwide. Preservation of the documents is not an aim in itself. It is, however, a pre­requisite for access to information, which is itself a fundamental democratic right. The Memory of the World­Programme is, therefore,working to promote the preservation of, and the access to, the documentary heritage of mankind.

The first topic examined by the Sub­Committee of Technology was how to facilitate access to documents. The Sub-Committee made several recommendations - especially for the consideration of the use of digital techniques. These recommendations are currently under review and will be re­issued shortly together with recommendations on the harmonization of access to electronic documents within the Memory of the World Programme.

With this guide, the Sub­Committee on Technology is addressing the other important element of the Memory of the World Programme; namely preservation. The guide gives an overview of the recommendations and measures related to the issue of safeguarding the documentary heritage. It is not attempting to deal in depth with all aspects of preservation, but to provide a guide or pointer to standards, recommended practices and reference literature related to the basic topic of preservation of documents. The guide covers the various information and data carriers in terms of their physical nature - occasionally bridging the boundaries of different traditional groupings of documents. An additional chapter deals with the strategic aspects of capturing and safeguarding electronic documents and publications ­ although their physical preservation problems are dealt with in the chapters for magnetic and optical media.

In order to keep pace with the technological development, especially in the field of audiovisual and electronic documents, this guide will be periodically updated.

The Sub­Committee on Technology welcomes any comments and suggestions.

Dietrich Schüller

Chairman

Sub­Committee on Technology

1. Introduction

This document offers a guide to the important standards, recommended practices and reference works in the field of the preservation and conservation of documents of all types. It does not attempt to include all the publications for a particular type of document - only the most important ones. The level of authority of the publications varies. They range from de-facto standards that are widely accepted by the practitioners in the field to formal International Standards produced by the International Standards Organisation (ISO). International co­operation in standardisation work is generally coordinated by ISO which defines itself as:

" - - a world­wide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organisations, governmental and non­governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (TEC) on all matters of electrotechnical standardisation."

A standard can be used either as a guideline or as a source of specifications. Standards by themselves, however, are only an introduction to the subject. In the use of a standard it is important that the user is familiar with the subject in question, thus being able to relate the information and the circumstances to each other. In particular, standards and other publications must not be used as the sole source of information about conservation techniques. Seek advice from experts and practitioners in the field before trying to conserve any document. Much protection can be offered by ensuring that documents are stored in appropriate containers, are not subject to fluctuations of temperature and relative humidity and are handled with care. Specialist treatment is available for all types of document, from the most ancient to the most recent, and professional advice should be sought from appropriate museums, libraries and archives before undertaking any work on them. Specific advice is also often available in the standards quoted.

Standards are very important, but unfortunately standards often become modified by users after some time which can make the standard out-dated and even impede the easy exchange of information. A good example of this are the Machine Readable Cataloguing Formats (MARC) which are all based on the ISO 2709-1981 standard. However, many countries have developed their own national version and this makes the exchange of bibliographic records more difficult. Other examples include the many Document Type Definitions (DTDs) which are based on the SGML standard.

The documents in the archives and libraries of the world are indispensable sources for many scholarly disciplines. They are also sources for more informal purposes: self-education, entertainment and general interest. No evaluation of politics, history, everyday life, music and performing arts would be possible without these documents.

Information should be available to all people as freely and as easily as possible. Preservation of that information in all formats ensures access and should be pursued actively for that reason.

The safeguarding of all these documents has until recently been primarily associated with the keeping of books and other written materials. This is partly, perhaps, because textual libraries have existed for more than 4000 years, while audio­visual archives have been in existence for only less than 100 years. The newest forms of document have been in common existence for less than a decade. There are, however, fundamental differences between the different types of documents.

Printed matter represents human thoughts by the use of a stock of symbols. A certain amount of redundancy is intrinsic in speech and writing. Letters, sometimes even words, may be omitted without any real detriment to communication. Good examples are the scripts of Semitic languages which generally do not represent all vowels which are spoken. But still, even complex texts like philosophical tracts can be communicated by these languages.

In contrast, the audiovisual document is an analogue representation of a physical status or event: every part of such a document is information. While a speck of mould in a book does not normally hamper the understanding of the text, comparable damage on a photograph would cover up information, and, on a magnetic tape, it could even render the tape unreadable. Seen, therefore, from the perspective of redundancy, audio­visual documents call for a higher degree of protection and security than written materials. Digital data can also be similarly endangered.

The modern electronic documents are to some extent insubstantial - many exist for part of the time only as a pulse of energy (for example, E-Mail messages passed over a telephone wire). They do, however, have certain safeguards built into them to help ensure the safe and complete arrival and storage of a message. They also have to be stored at some point on a physical carrier for later access.

One factor that most, if not all documents, have in common is their reliance on polymeric materials. The traditional materials of paper, parchment, leather, palm leaves etc. are all natural polymers. The newer media of tapes, discs and films rely on man-made polymers such as PVC and polyester. The rate of chemical de-composition of the various polymers varies greatly. Some will last - and have lasted - for millennia; others may struggle to survive for a decade.

All polymers decay. The decay cannot be stopped - but it can be slowed down by careful handling and favourable storage. It can also be greatly accelerated by careless handling and poor storage. All the storage conditions given in standards and other publications are for guidance. If the conditions are met, the decay does not stop. The figures quoted for temperature and humidity levels are a compromise between the rate of decay on the one hand and the costs of maintaining the conditions, of transfer and of conservation on the other. The conditions can be relaxed but at the expense of more rapid decay.

Format of the Guide

The various types of documents covered in this guide are divided into five groups:

Paper and Other Traditional Materials

Photographic and Micrographic Materials

Mechanical Carriers

Magnetic Materials

Optical Materials

There is an additional chapter covering the particular problems of preserving Electronic Publications, Electronic Documents and Virtual Information plus chapters giving some general preservation information and a glossary.

The Paper and Other Traditional Materials group include paper, parchment, leather and palm leaves. Seals are also included in this group. This is the oldest and largest group of documents.

The Photographic Materials group include all types of still photographic images - black and white and colour; negative and positive; transparency and print - on all types of carriers - paper, glass, cellulose and other materials and includes micrographs of all types.

The Mechanical Carriers group covers sound recordings on cylinders and discs.

The Magnetic Materials group include all forms of magnetic material - tapes, hard discs and floppy discs.

The Optical Materials group includes all laser read and written materials including CD-Audio, CD-ROM, CD-Recordable, magneto-optical disks and optical tape.

This guide is a compilation of contributions from a number of people, each expert in the preservation of a different type of document. Each field of expertise has developed its own terminology and, while there are many terms shared by all the disciplines, some are not. No attempt has been made to harmonise the terms used and so you will find different terms used in different chapters but meaning the same thing eg. user copy and access copy.

You will also find places where the topic has been covered in another chapter as well. Again, efforts have not been made to avoid this. Many readers will wish to read a chapter in isolation and this repetition will ensure that they receive all the information necessary for the understanding of the chapter.

2. General Preservation Factors

Preservation Masters and Access Copies

Many data carriers, especially modern high density formats, are, by their very nature, vulnerable. Additionally, there is always the risk of accidental damage through improper handling, malfunctioning equipment or disaster.

For the long term storage of many types of documents it is increasingly becoming necessary to review the strategies for preservation. One strategy that is widely used is the creation of access copies of documents. A poor quality copy can act as an adjunct to the catalogue to aid researchers to decide what documents they wish to study. A good quality copy may be acceptable for study in place of the original. The use of copies to reduce the frequency of access to the original document will reduce the stress on the original and help to preserve it. A clear policy about the classes of researchers allowed access to original documents - particularly fragile ones - will also help documents survive. It is clearly impossible to totally restrict access to originals but many users can perform their research using good quality access copies.

It is imperative, therefore, to have at least two copies of each document - one preservation master and one access copy. These should be stored in two different locations, ideally under different climatic conditions (see below). Several archives have established a policy to produce, in addition to the preservation master, an additional safety copy. Computer centres have established similar policies to safeguard the data in their care.

Archive quality microfilm, used by many institutions both as a preservation and a high quality access medium, is being supplemented by digital storage systems using optical discs and magnetic tapes as the carrier. A further advance being considered by a number of institutions is the self­controlling and self­regenerating digital mass storage system. This is currently being tested as a medium for the safeguarding of some collections of audiovisual documents. This kind of concept may also provide a solution for the major problem of the safeguarding of electronic documents. Such mass storage systems are, at the same time, an indispensable pre­requisite for the functioning of all kinds of services in the forthcoming information age, for example "digital libraries" and "video on demand".

The price of mass storage systems is relatively high at present but will soon come within the reach of average budgets. Contrary to fears expressed by some, this concept does not call exclusively for huge, centralized stores: it will also allow individually tailored solutions for smaller applications. Thus, such systems could also be a solution for the preservation of documents in countries with adverse climatic conditions.

While in hot and humid environments conventional preservation techniques may be inappropriate due to the notorious lack of funds for the proper air-conditioning of storage areas, mass storage systems, requiring relatively small floor space, can be effectively airconditioned at a lower cost. As an example, a cabinet available from one typical system, occupying one square metre of floor space, can hold the equivalent of over one million pages of A4 text.

Obsolescence of Hardware

With development of technology, recording systems, ie the carriers together with the necessary recording and replay equipment, have become increasingly sophisticated. Even for textual materials, the almost universal use of word processors can create unexpected difficulties. At the same time, the (commercial) lifetime of systems has become increasingly shorter. While for traditional audiovisual formats such as analogue tape the lifetime of the tape was of predominant interest, the future availability of suitable and functioning replay equipment is becoming the burning issue for several modern formats. In the computer world, the availability of dedicated drives has always been of greater concern than the stability of the respective carriers. The situation is additionally aggravated by the obsolescence of dedicated software and operating systems. Several audio archives are now systematically transferring their holdings into self-controlling and self-regenerating mass storage systems (see above) to escape the vicious circle of ever decreasing life spans of carriers and their dedicated hardware. It has become apparent that, in setting up strategies for the safeguarding of both audiovisual and electronic documents, the future migration has to be taken into account.

Maintenance of Equipment

With all machine readable documents, the performance of recording and replay equipment is a central factor in the safeguarding of data. Great efforts have to be made to keep equipment in the best possible condition. To this end, many audio-visual archives and computer centres have their own service departments employing well trained personnel. In a time of ever increasing sophistication of equipment, however, there is an increasing amount of work that has to be done by specialists from outside. When out sourcing such jobs to third parties, the essential role of machine maintenance for the safeguarding of the collection has to be kept in mind.

Climatic Conditions

To ensure a long life for the polymers carrying the information in storage, it is necessary to control the climatic conditions in the store. The basic requirement is for stable temperatures and stable humidity levels. Large variations in either parameter will accelerate the decay processes.

High temperatures will accelerate the decay processes; cooler temperatures will slow them. Similarly, high levels of humidity in the storage areas will encourage hydrolysis and, if above 65% RH, will encourage the growth of moulds and fungi. It is common knowledge that many polymers are best stored at low temperatures. It is less commonly known that humidity also must be controlled. The problem for many collections is that is relatively easy to keep the temperature in the storage areas stable but controlling the humidity level is more difficult and expensive.

As the temperature drops the air can hold less moisture and the relative humidity rises. If a store is cooled without simultaneously controlling humidity, there is a danger that moulds and fungi will be encouraged to grow. These growths will not only eat away at paper and other natural polymers but will also make machine readable carriers - magnetic tapes, optical discs etc - unplayable and possibly damaging the equipment. If it is not possible to keep humidity below 65%RH when the store is cooled, it would be better to set a higher, but stable, target temperature that will ensure that humidity is kept stable and below 65%RH. The penalty, however, will be a shorter life for the carriers.

3. Paper and Other Traditional Materials

This group, with the greatest diversity, contains the oldest materials. They are accessible with the naked eye and the majority are made of materials which have the capacity to last for a long time if cared for correctly. Many of the standards cover a range of materials, and include details of the best storage conditions.

The options for preservation range from appropriate stable environmental conditions, through boxing and packing, copying on to some other medium, to conservation. The latter is the most expensive solution and can only be afforded for a small number of items of particular importance. Each institution or organisation will have to make its own list of priorities and decide how best to afford appropriate protection to every category of material. Basic protection, such as good housekeeping, training staff to handle items with care, or storing them on shelving rather than the floor does not cost much money and should be part of every programme for preservation.

Microfilms can provide both an access copy of original documents and also a safeguard against loss through decay or disaster. If high fidelity microfilms are made, they can also be used as the source for making digital copies.

Paper

Both Oriental and Western papers are made using cellulose (vegetable fibre or materials made from such fibres such as rags) and water, the former being beaten into a pulp, dispersed in water and then drained through a tight mesh mould. Once the water has drained away, the film of pulp remaining on the mould is transferred to felts, dried and pressed to produce a sheet of paper which may require further treatment depending on the proposed use. While the original ingredients were relatively pure the paper maintained good strength and longevity. However, the introduction of wood pulp in the nineteenth century to satisfy the increasing demand for paper led to a deterioration in quality. Additives in the shape of alum, or alum rosin, chlorine, sulphate of soda or other chemicals designed to speed the production of the paper, or counteract the natural acidity of the wood, caused more damage by breaking down the fibres which in consequence embrittled the paper.

The reasons for the decay of nineteenth and twentieth century papers are well known, but until recently few solutions have been proffered. The adoption of rigorous standards in the manufacture of paper intended for long term use is the solution for the future survival of paper documents but does nothing for those which already exist. Good storage, which includes boxing archive and library material whenever possible, maintaining stable environmental conditions, excluding light, keeping the store rooms clean and dry and handling the material carefully contribute to the overall preservation of these materials. The range of temperature and relative humidity will vary according to the area of the world but it is particularly important not to exceed 65% relative humidity, the point at which mould growth is triggered. Specialist conservation work, which involves treatment of the material, should be left to trained conservators who will be able to assess the problems of individual items. Mass de-acidification is used in some archives and libraries but is not practised universally and is expensive. Some modern papers used for copying or faxing are coated and retain the text for a very short time only. These should be copied as soon as possible using good quality paper.

Parchment

Parchment is made from the skin of an animal, usually sheep but calf skin is also used. It consists of collagen fibres, arranged in three dimensional bundles, which are themselves proteins composed of long chain amino acids. The skin is turned into parchment during a long process which involves the removal of the fur and flesh with the use of lime, scraping the skin, stretching it on frames to dry under tension, polishing it and finally rubbing it with French chalk to provide a good writing surface. It was the most common writing medium of the Middle Ages in Europe, superseding papyrus by the third or fourth century AD, until the invention of printing in the fifteenth century increased the demand for a lighter, more flexible medium.

It is a very tough and versatile material which, due to its nature, is hygroscopic. It reacts to external temperature and humidity which must, therefore, be kept as stable as possible. Excess relative humidity endangers the parchment from both mould growth and deterioration but, by contrast, if the parchment is too dry, the ink is likely to flake off. The ink is also susceptible to being abraded by rough handling or tensions induced by unstable environmental conditions. The range of conditions for the survival of parchment are similar to those required for paper.

Palm Leaves and Birch Bark

These materials are cellulose based, being vegetable fibre. As natural materials, without the processing involved in paper making, they are quite hard wearing but are susceptible to cracking. The inks used are frequently carbon based and may be endangered by incorrect storage, environmental conditions or handling. The standards for these are similar to those for paper and parchment

Standards

ISO NP 11799	Storage requirements for archive and library materials
ISO WD 11108	Archival paper ­ requirements for permanence and durability
ISO/TC46 SC10	Storage requirements for library and archival materials
ISO/TC46 SC10	Requirements for binding of books, periodicals, serials and other paper documents for archive and library use. Methods and materials
ISO DIS 11800	Information and documentation - Requirements for binding materials and methods used in the manufacture of books.
ISO CD 14416	Information and documentation - Requirements for binding of books, periodicals, serials and other paper documents for archive and library use - Methods and materials

Reference Literature

Dureau, JN and Clements, DNG

Principles for the preservation and conservation of library materials

IFLA Professional report no 8 1986

Preservation of Historical Records

Committee on Preservation of Historical Records, of the National Research Council of the National Academy of Sciences. National Academy Press, 1985

Ellis, J

Keeping Archives

Thorpe and Australian Society of Archivists; 1993

Favier, J, Neirinck, D

La Pratique Archivistique Francaise

Paris, Archives Nationales, 1993

Ritzenthaler, ML

Archives and manuscripts

Society of American Archivists, 1993

UNESCO RAMP studies in general. Available in Arabic, Chinese, English, French, Russian and Spanish.

Also see the publications of the Commission on Preservation and Access (Washington D.C. USA) and the European Commission on Preservation and Access (The Hague, The Netherlands).

4. Photographic Materials

Deterioration of Photographic Materials

Photography can be defined as any method producing a visible image by the inter-action of light with a layer of chemicals. Since the birth of still photography in 1839, photographs have been manufactured employing many different methods. About 40 of these methods have been used commercially and examples of the resulting images can now be found in great numbers and varieties in archives and library collections.

The development of a commercially successful system for recording and viewing moving images was the result of work by many people in the latter part of the 19th century. The first successful public demonstrations were given by Lumi_re in Paris in 1895. Since then many advances have been made including the introduction of sound and of colour. Many different frame rates and sizes of film were devised before the industry stabilised on to a few "standard" formats. A film collection still has to be able to handle films on many formats.

Microfilm was developed to secure original print and image material with special historical, commercial or scientific value. The use of microfilms can also improve the access to the information carried by the original documents. The use of microfilm for access will, as with other forms of access copy, help preserve the original by protecting it from wear and tear and from theft.

The most recent developments are as a result of the computer revolution. New techniques have been developed using equipment such as ink-jet and thermal sublimation printers to produce copies of digitised images. These should be considered as printing techniques and not as photographic materials although they can provide a good representation of the original photographic image. Because of the short life expectancy and the sensitivity to light and heat, these printing techniques cannot be considered a substitute for photographic materials.

The best practice for photographic materials is to have several sets of images:

The Original Image kept in ideal conditions and disturbed as infrequently as possible.

A Safety Master used as a reserve copy. It should be stored in a separate place to the original in case of the loss of the original in a fire or some other disaster and also kept in good storage conditions.

A User Copy Master made from the original or the safety master and used to make User Copies.

User Copies for routine access to the images.

Though photographic images have been made in a great number of different sizes - from microfilms to large posters - the deterioration and preservation principles are dependent upon the chemical process used to make the image and not the size or purpose of the image. As the production of photographs has included many different chemical processes in the capture of the image, photographs also have a wide variety of ageing properties. Some materials were made of extremely self­destructive components, others were very sensitive to physical contact and almost every photographic material is sensitive to the environment, not only temperature, relative humidity and air pollution but also oxidising substances found in emissions from some building materials, wall paints and wooden furnishing. The card­board and paper in boxes and envelopes used for protecting the items from physical damage may also contain harmful substances.

Deterioration Factors

Deterioration factors can be categorised in two ways ­ internal and external.

1. Internal Deterioration

Internal deterioration factors are dependent on the components of a photographic item and the residual chemicals from developing­ and post treatment processes. The speed of the decay processes is related to relative humidity, temperature and oxidising substances.

The most commonly known example of a photographic material deteriorating from internal processes is cellulose nitrate film, which during deterioration emits substances that both accelerate the deterioration process as well as attacking materials in the vicinity.

Another materials group exposed to self destruction is that of acetate film ­ the first safety film. Until recently, acetate film was considered as very stable but today the problem of the Vinegar Syndrome ­ the popular name for the deterioration of acetate film with the emission of acetic acid (vinegar) vapour as a by-product that acts to accelerate the rate of decay ­ is widely known. Still another example, although involving an old process, is the yellowing of albumen prints, where the egg white in the emulsion bleaches the silver image.

Colour photographs - negatives, prints and transparencies - generally have bad ageing properties as the colour­components are unstable unless kept below 0°C. Photographic colour materials are not only subject to light fading - fading of the colours and image in the presence of light - but also to dark fading - fading in the absence of light. Transparencies are commonly considered to have better colour stability than colour negatives and prints but ageing properties may differ greatly due to different chemical properties.

A Few Examples

Collodion, one of the earliest photographic emulsion materials, was used in several similar photographic techniques during the mid­l8th century, e.g. ambrotypes, collodion wet plates, pannotypes, ferrotypes and celloidin paper. The collodion emulsion contains cellulose nitrate (also used for the first "plastic­type" film base) and emits nitrous gases, though far less than cellulose nitrate film. These gases may attack other objects in the vicinity and, due to the loss of gas which leads to shrinkage of the emulsion, the emulsion may eventually crack.

Supports that are subject to self-deterioration include cellulose nitrate film, acetate film and some of the modern resin coated or so called plastic paper. The main ingredient of nitrate film is cellulose nitrate which emits nitrous gases. The gases are not only oxidative but also toxic and explosive. In a self­accelerating deterioration process, the support ­ the film base ­ and the emulsion are eventually completely destroyed. What is left is a sticky substance. Cellulose nitrate film is flammable at fairly low temperatures and rolls of film, like motion picture films, might even self ignite at a room temperature as low as 41°C when kept for an extended period of time in a badly ventilated environment, for example in the traditional metal film can. Cellulose nitrate film sheets do not self ignite in the same way because the mass per volume is much less and normally the emitted gases slowly evaporate away from the negatives when they are kept in envelopes and open boxes.

Acetate film was introduced in the l920s as a substitute for the flammable cellulose nitrate film. It was labelled "safety film" as it was less flammable than its predecessor. The early acetate film lacked dimensional stability which made it shrink and loosen the emulsion from the support. The acetate base was improved and was considered more or less stable until the vinegar syndrome was discovered during last decade.

PE or Resin Coated papers are made from paper fibres covered with polyethylene with the gelatine emulsion outside the polyethylene layer. Until about the mid 1980s this photographic print paper had bad ageing characteristics. The paper base contained optical whiteners which absorbed light energy. An oxidising substance was formed which attacked the resin coating resulting in cracking. The oxidant also attacked the silver image and bleached it. During the last decade an anti­oxidant has been introduced and thus the resin coated papers now have improved longevity.

Microfilms have been and are produced using a variety of processes but the silver-gelatine developing-out film is considered to have the best long-term stability. Diazo- and vesicular processes are commonly used for making access copies but they do not have long-term stability and are not recommended for preservation copies.

2. External Deterioration Factors

External deterioration factors are harmful substances in the preservation environment. Among the many contaminants, a few should be particularly mentioned. Lignin, alum rosin sizing and oxidative residual chemicals in paper and cardboard used for envelopes, boxes and mounting boards as well as plasticisers in PVC­folders and similar storage media are the most common together with air pollutants. Furbishing in repositories should not consist of materials emitting oxidising gases. Oxidising gases react with photographic materials in a similar way as common air pollutants. High temperature and relative humidity accelerates these processes.

Synergetic Effects of Internal and External Deterioration Factors

The external deterioration factors may co­operate with the internal factors to increase the reaction speed of the internal deterioration factors.

Materials with good initial ageing properties ­ i.e. with few internal deterioration factors ­ may last longer in a bad environment than an object with bad ageing properties ­ i.e. with many internal deterioration factors ­ kept in a good preservation environment.

Good storage conditions will counteract deterioration of materials with bad ageing properties to a certain point, while bad storage conditions will always accelerate deterioration processes.

Recommended Measures for Improving Preservation Conditions

The best way to preserve photographic materials is to emphasise measures on preventive care. The necessity of proper storage materials ­ envelopes, boxes, archive furbishing etc. ­ and storage climate cannot be over estimated.

If possible a photographic collection should be divided and stored in two archives; an active and a passive. The active archive is for frequently used material ­ mainly copies of originals ­ and the passive archive is for long term keeping of the originals. The passive archive should have a stable climate with low temperature and relative humidity. A number of recommendations exist but they do not differ significantly from the requirements listed in the following table. These are weighted for a good cost/effectiveness ratio. The requirements can be difficult to achieve but must always remain the target. The target temperature and humidity readings can be relaxed provided that the conditions are kept stable and with the proviso that the humidity level is kept above 25% and below about 65% - the level above which moulds are encouraged to grow. The penalty in most cases is, however, a shorter life expectancy for the carriers.

Basements and attics are usually not suitable for storing photographic materials. Basements are usually very humid and often accommodate plumbing which, if it starts to leak, may cause irreversible damages. Attics, if not properly insulated, will have an uncontrolled climate affected by the out­door conditions.

High temperature and high relative humidity (RH) accelerates most deterioration processes. The cooler the temperature the slower the deterioration rate. The control of relative humidity is even more important in an archive with photographic materials.

These types of damage may occur when the RH is TOO HIGH:

• Mould and fungi start to grow when RH rises above 65%.
• The emulsion swells and get sticky.
• Residual chemicals will accelerate deterioration processes.
• Glass plates might start to deteriorate and the glass may turn foggy.
• Deterioration processes caused by air pollutants, paints etc. may accelerate.
• Photographs on metal support, Ferrotypes, may start to corrode.

The following damages may occur when RH is TOO LOW:

• The emulsion dries out and might flake.
• Dry emulsion may fall off the support.
• Film support may lose its flexibility

It may be difficult to keep the air in an archive clean since most major archives usually are situated in the centre of major cities. But it is nevertheless of the utmost importance to keep the areas free from air pollutants as possible. They are very reactive with substances in both b/ w and colour photographs. Listed in the following table are the requirements for clean air in photographic collections.

Other harmful substances exist in the air but good chemical filters customised for the substances listed in the table will control these as well.

If the collection includes any nitrate moving films, seek advice from the local fire authorities about the storage requirements, the maximum quantity of film that can be kept in one storage area and any other restrictions that they may require. This action is not merely good advice - it is essential. Nitrate movie film is considered to be an explosive by the fire authorities in many countries.

Conclusion

Photographic objects belong to a very delicate category of our cultural heritage which need special attention by trained personnel. Materials are susceptible to air pollutants, both fuel generated and emitted from furbishing and protective materials in repositories, as well as high humidity and temperature. It is important, therefore, to be in control of the preservation environment. It is also important to be able to identify the photographic methods represented in a collection and thus be aware of specific preservation problems.

Specifications, methods and measures for improving the preservation environment for photographic materials can be found in special literature and standards. Some of these are listed below.

Standards

ISO 417	Photography ­ Determination of residual thiosulfate and other related chemicals in processed photographic materials ­ Methods using iodine­amylose, methylene blue and silver sulfide.
ISO 543	Cinematography­ Motion picture safety film ­ Definition, testing and marking
ISO 3897	Photography ­ Processed photographic plates ­ Storage practices.
ISO 4331	Photography ­ Processed photographic black­and­white film for archival records ­ Silver­gelatin type on cellulose ester base ­ Specifications
ISO 4332	Photography ­ Processed photographic black­and­white film for archival records ­ Silver­gelatin type on poly(ethylene terephthalate) base ­Specifications
ISO 5466	Photography ­ Processed safety photographic films ­ Storage practices
ISO 6051	Photography ­ Processed reflection prints - Storage practices.
ISO 6200	Micrographics - First generation silver-gelatine microforms of source documents - Density specifications
ISO 8126	Micrographics - Diazo and vesicular films - Visual density - Specifications
ISO 9718	Photography ­ Processed versicular photographic film ­ Specifications for density
ISO 10214	Photography ­ Processed photo graphic materials ­ Filing enclosure for storage.
ISO 10602	Photography ­ Processed silver­gelatine type black­and­white film ­ Specifications for stability.
ISO 5­1	Photography Density measurements Part 1: Terms, symbols and notations
ISO 5­2	Photography ­ Density measurements ­ Part 2: Geometric conditions for transmission density
ISO 5­3	Photography Density measurements ­ Part 3: Special conditions
ISO 5­4	Photography ­ Density measurements ­ Part 4: Geometric conditions for reflection density

Reference Literature

Garry Thomson

The Museum Environment

Butterworth­Heinemann, Oxford 1986

ISBN 07506 2041 2

Preservation of Microfilming ­ does it have a future?

Proceedings of the First National Conference of the National Preservation Office, at the State Library of South Australia, 4­6 May 1994, Canberra 1995

ISBN 0 642 10639 8

Guidelines for Preservation Microfilming in Canadian Libraries

National Library of Canada for The Canadian Cooperative Preservation Project (In English and French) ISBN 0 660 57970 7

Henry Wilhelm & Carol Brower

The Permanence and Care of Colour Photographs: Traditional and Digital Colour Prints, Colour Negatives, Slides, and Motion Pictures.

Grinnell, Iowa, 1993, ISBN 0­911515­00­3 (hardcover)

ISBN 0 911515 01 1 (paperback)

Imaging Processes and Materials

Ed. by John M. Sturge, Vivian Walworth & Allan Shepp, New York 1989

ISBN 0 442 28042 6

James M. Reilly

Care and Identification of 19th­Century Photographic Prints

KODAK Publication No. C­25, CAT 160 7787

ISBN 0 87985 365A

Schrock, Nancy Carlson

Preservation and storage

In Picture Librarianship ed. By Helen P Harrison, Library Association, London 1985

The Conservation of Photographs

Eastman Kodak, Rochester, New York, 1985

Brown, Harold Godard

Basic Film Handling

FIAF Preservation Commission, Brussels

Brown, Harold Godard

Problems of Storing Film for Archive Purposes

British Kinematography No. 20, 1952

The Book of Film Care. Publication F-30

Eastman Kodak Ltd, Rochester, New York, 1983

Handling, Preservation and Storage of Nitrate Film

FIAF, Brussels, 1987

5. Mechanical Carriers

Within the group of documents commonly labelled audiovisual (photographic still and moving images, audio and video recordings) are sound recordings on cylinders and discs. The common factor with this group of documents is the method of recording the information. This is by means of a groove cut into the surface by a cutting stylus and which is modulated by the sounds, either directly in the case of acoustic recordings or by electronic amplifiers.

There are no official standards for the preservation of these materials but there are, however, a number of standard reference publications available. These are listed in the Bibliography that follows. In addition, the Proceedings of the series of Technical Symposia organised by the international archive federations (FIAF, FIAT, IASA and ICA) on the preservation of sounds and moving images contain many papers of interest.

Phonograph Cylinders

Cylinders, originally developed for use as dictating devices, have been used since around 1889 for original recordings in the academic world and later also as mass produced recordings for the entertainment industry in competition with early gramophone (shellac) discs. While industrial production ceased in the late twenties, they continued to be used for field recording until the

1950s (!). Most cylinders are made of wax; some of the mass replicated cylinders are made from celluloid. There are about 300,000 cylinders in the custody of recorded sound collections world­wide. They are extremely brittle and fragile and if they have been stored under conditions which are too humid, they suffer from mould. Fortunately, most of these holdings have already been transferred onto modern media and thus their contents, which are frequently of unique historical value, are already safeguarded .

Shellac Discs

Coarse groove gramophone discs, commonly called shellacs or 78s, were the main mass produced audio format of the first half of our century. It is estimated that the world­wide stocks of this format amount to 10 million discs. They were produced from 1898 until the mid­fifties. The discs consist of various mineral substances bound together by organic substances like shellac or similar binding materials. Although breakable if dropped, these gramophone records are fairly stable and there are no reports of a systemic instability problem.

Instantaneous Discs

Prior to the introduction of magnetic tape, which occurred in the late 1940s and early 1950s, the "instantaneous discs" - so called because they can be played immediately after recording the sounds without the need for the lengthy processes required for mass produced discs - were the only medium for audio recordings that could be played back immediately. The total number in existence amounts to about three million. Practically all of these discs are irreplaceable originals, many of them of great cultural, historical and scholarly importance.

Unfortunately, the largest group of these instantaneous discs, the "acetate discs", are at the greatest risk. These discs are laminates consisting of a core plate, usually of aluminium but plates of glass, steel and card are also known, with a lacquer coating of nitrate or acetate cellulose which is soft enough to be cut by a recording machine, but hard enough to withstand several replays. With age, the coating shrinks and becomes brittle by a hydrolytic process: the stresses between the shrinking lacquer and the stable core increase until, suddenly, the lacquer breaks apart, and flakes off. By this means a considerable portion of the holdings world­wide have already been lost. Even if programmes to transfer the sounds were hastily established, further losses of irreplaceable information cannot be prevented. Every day, apparently intact records are being affected by this phenomenon.

Microgroove Discs

From the late 1940s onward microgroove discs (vinyl or LP records) replaced shellac discs and only relatively recently (since about 1990) has this format been replaced by the compact disc (CD). The total number of microgroove discs in sound archives world­wide is estimated to be more than 30 million. They are made mainly of polyvinyl chloride. No systematic stability problems on a great scale have arisen so far, but their stability in the long term, thinking in centuries, is unknown.

The Stability of Mechanical Carriers

The main factors related to instability of mechanical carriers and retrievability of information can be summarized as:

• Humidity and temperature
• Mechanical deformation
• Dust and dirt of all kinds.

Humidity, as with all other data carriers, is a most dangerous factor. While shellac and vinyl discs are less prone to hydrolytic instability, most kinds of instantaneous discs are extremely endangered by hydrolysis. Additionally, all mechanical carriers may be affected by fungus growth which occurs at humidity levels above 65% RH.

Elevated temperatures beyond 60°C are dangerous, especially for vinyl discs and wax cylinders. Otherwise, as with other carriers, the temperature determines the speed of chemical reactions like hydrolysis and should, therefore, be kept reasonably low and, most importantly, stable to avoid unnecessary dimensional changes.

Mechanical integrity is of the greatest importance for mechanical carriers. It is imperative that scratches and other deformation caused by careless operation of replay equipment are avoided. The groove that carries the recorded information must be kept in an undistorted condition. Only specialist personnel should, therefore, be allowed to handle and replay mechanical carriers.

While shellac discs are very fragile, instantaneous and vinyl discs are more likely to be bent by improper storage. Generally, all mechanical discs should be shelved vertically. The only exceptions are some soft variants of instantaneous discs.

Dust and dirt of all kind will deviate the pick-up stylus from its proper path causing audible cracks and clicks. Fingerprints are an ideal adhesive for foreign matter. A dust-free environment and cleanliness is, therefore, essential.

Obsolescence of hardware is not yet a major issue for mechanical carriers. Replay equipment for microgroove and 78rpm discs is still available and several sound archives have constructed cylinder replay machines which offer excellent performance for cylinders of all formats. With the exception of instantaneous discs and cylinders, mechanical carriers are not generally at risk. Because the discs wear when played, migration to a modern digital format will be necessary for items in frequent demand.

Because of the extreme risk to the future survival of instantaneous discs, all existing holdings must be transferred with highest priority.

Operation areas (studios) should have the same climatic conditions as access storage areas.

It is of utmost importance to control both temperature and humidity simultaneously. Archivists are explicitly warned to not to cool the storage environment without dehumidification because such action will normally lead to an unacceptable rise of relative humidity and may encourage the growth of moulds and fungi.

Standards and Recommended Practices

IASA TC­03

The Safeguarding of the Audio Heritage: Ethics, Principles and Preservation Strategy. 1997

Reference Literature

Burt, LS

Chemical Technology in the Edison Recording Industry.

In: Journal of the Audio Engineering Society 10­11/1977

Calas, Marie­France, et Fontaine, Jean­Marc

La conservation des documents sonores.

CNRS­Editions, Paras 1996

Khanna, SK

Vinyl Compound for the Phonographic Industry.

In: Journal of the Audio Engineering Society 10­11/1977

Pickett, AG and Lemcoe, MM

Preservation and Storage of Sound Recordings.

Washington 1959. Reprint by ARSC, 1991

Schüller, Dietrich

Behandlung, Lagerung und Konservierung von Audio­ und Videoträgern.

In: Das Audiovisuelle Archiv, Das Audiovisuelle Archiv 31/32, 1992

(1993)

Schüller, Dietrich

Preservation of Audio and Video Materials in Tropical Countries.

In: IASA Journal 7/1996

St.Laurent, Gilles

The Care of Cylinders and Discs.

Technical Coordination Committee (Ed), Milton Keynes 1997

(available in English, French, German and Spanish)

6. Magnetic Materials

Magnetic media are essential to modern life. They are used in the form of tape to record sounds and images and to record digital data. In the form of hard and floppy discs they are used to store computer data. When applied in the form of a magnetic strip to a card, magnetic media control our access to money from cash dispensers, entry to doors and to many other things.

The basic principles for recording signals on a magnetic medium were set out in a paper by Oberlin Smith in the 1880's. The idea was not taken any further until Valdemar Poulsen developed his wire recording system in 1898. Magnetic tape was developed in Germany in the mid 1930's to record and store sounds. The use of tape for sound recording did not become widespread, however, until the 1950's. The BBC, for example, was still using disc recorders until around 1965.

The recording of images on magnetic tape came later. As with sound recording, there were several systems before tape recording came into common use. The first known recordings of images by a non-photographic method were made by John Logie-Baird in 1924. The images were recorded on to 78 rpm discs which are now in the National Sound Archive in London. The first practical recordings of television programmes were made using special film cameras filming video screens. The first video recording machine using tape was made by the BBC in 1955. It used a half inch tape running at 120 inches per second - just over 3 metres per second. This was swiftly superseded by the introduction of the Ampex Corporation's 2 inch video tape system. The arrival of new formats for recording video pictures has steadily increased since then. It has been calculated that, taking the different broadcast standards and electricity supplies into account, images have been recorded on over 100 different formats in the 40 years since video-tape recording started.

Although some dictating machines using a disc coated with a magnetic pigment were in use from the 1950's, disc based media did not develop until computers became widespread. The steady increase in the storage capacity and decrease in physical size of both hard and floppy disks has paralleled the developments in the tapes used for sound and image recording.

Magnetic Tapes

Magnetic media in the form of tapes on open reel or housed in cassettes and cartridges are the most widespread carriers for audio and video data and are widely used for the storage of large quantities of computer data. They are a reliable, low­risk and economical storage medium. Archivists can rely on a long period of experience in the care and handling of magnetic tapes in archives. If free from production faults, they can be preserved for many years. The oldest audio tapes are now over 50 years old and still perfectly readable.

Types of Magnetic Tape Construction

Early audio tapes used cellulose acetate as the support film material, which is also used for safety film. Cellulose acetate has a tendency to become brittle through hydrolysis caused by the moisture contained in the atmosphere. This brittleness generally causes serious problems when playing old audio tapes. Tapes with severe cases of hydrolysis can suffer from the so­called "Vinegar Syndrome", an auto-catalytic process whereby acetic acid is set free in ever increasing quantities and thus creates an accelerating effect on the decay process. This has been particularly experienced in film archives, especially in hot and humid climatic areas. Affected films become soft and limp, ending up as powder or slime. While, in theory, this may also happen to acetate audio tapes, no disastrous losses similar to those in the film world have been reported. Still, acetate tapes, which were produced until the mid 1960s, are at risk and transfer onto other carriers must be envisaged.

Another group of historical audio tapes used polyvinyl chloride (PVC) as the base film material. As with vinyl discs, these tapes have not exhibited any systematic instability; the long term prospects are, however, unknown.

Polyester is the base film material which is used for all modern audio and all video and computer tapes. It has the greatest resistance of all base materials to mechanical stress and the influence of humidity. No systematic stability problems have occurred so far but, again, its stability over very long periods (centuries) is unknown.

Many varieties of magnetic materials have been used for the pigment layer, for example the various oxides of iron used from the very first tapes until today and chromium dioxide. Only metal powder, as used in more recent high density tape formats, has given cause for serious concern. Early tapes metal powder tapes suffered from corrosion but this problem now seems to be under control. There is, again, no precise answer to the question of how long metal particle tapes will keep their information undistorted and readable. It must be emphasized, however, that, contrary to layman's expectations, the magnetic information on properly stored and handled tape does not fade away.

The greatest problem related to magnetic tape is the stability of the pigment binder - the glue that holds the magnetic particles together and to the base film. A considerable number of audio and video tapes, especially amongst those produced during the seventies and eighties, are suffering from pigment binder hydrolysis. The atmospheric moisture is absorbed by the pigment binder causing the polymer to hydrolyse and lose its binding property. Tapes of this kind deposit a smear of magnetic particles onto the replay heads. This clogs the heads and swiftly makes the tape unreadable. In extreme cases, the oxide layer the completely delaminates from base in large segments when the tape is played. Processes to render such tapes playable again are available, but the restoration process is cumbersome, time consuming and cannot restore the most severely affected tapes. This problem has been found especially in hot and humid areas where many tapes do not last longer than a few years.

Types of Magnetic Tape Housing

There are three basic methods for the immediate storage of tape: open spool, cassette and cartridge. The tape on an open spool has to be threaded on to the machine and the free end secured on a second spool by hand - a time consuming task and easily performed incorrectly. Tape in a cassette is enclosed in a shell and the two ends of the tape are securely fixed to captive spools. A cartridge is also fully enclosed but the tape is in the form of a continuous loop. Cassettes and cartridges are easier to load on to a machine than open spool tapes and are also suitable for use in robotic storage systems. Cassettes are common for modern video and computers but relatively few are used for professional audio. Cartridges are most commonly encountered with data but some are used for audio - particularly for short items such as station idents and commercials.

Open Spool Tapes

Open spools were until recently the main form of tape used for professional audio recordings. Continental European tradition generally uses professional tape on flangeless hubs only, a practice that requires additional care when handling the tapes. Some expensive professional digital audio formats like DASH and PD (ProDigi) use reel-to-reel tape and stationary head technology. Early video and many data tape formats also used tape in open spool form.

Cassette Tapes

Cassettes are used for many purposes. They range from the Compact Audio Cassette or Musicassette through the many types of video cassette to the latest Digital Audio Tapes with rotary heads (R-DAT). They are probably the most widely used form of tape used in modern systems.

The Compact Audio Cassette was originally designed for use with dictating machines. Its convenient size led to it becoming used for the issue of commercial music recordings and for home recording. Except as an access tape, it was not normally used for professional work. In addition to the Compact Cassette, there have been several other cassette tape formats used for dictating machines.

Many types of cassette have been used for analogue video recordings both professionally and in the home. The commonest is the ubiquitous VHS cassette. Other formats include the ¾ inch

U-Matic - a semi-professional format - and the ½ inch BetaCam used by many broadcasters around the world. All video tape formats, analogue and digital, use rotary head technology. Some of these digital video formats were also adapted for the storage of general computer data.

Rotary head technology is used for the digital audio format R-DAT, while stationary head technology is employed for the DCC (Digital Compact Cassette), a data-reduced digital consumer audio format designed to replace the Compact Audio Cassette.

A variety of cassette formats are used in the computer world as back-up tapes for the information held on hard discs - the so-called streamer tape formats. These include the QIC- 80, Exabyte and a derivate of the R-DAT audio format.

The R-DAT format potentially makes an ideal data backup media. However, there is little experience of their long­term storage qualities. Opinions are divided. Some experts say that a five year re-copying term is appropriate, others claim that DAT are not suitable for long­term storage. For safety reasons, a two year recopying term is advisable until more is known of the long-term performance of these formats.

Tape Cartridges

The primary use for cartridges is for storing computer data but a variant was extensively used to record short sound sequences for commercials, station identifications and the like. These audio cartridges were either monophonic or stereophonic (two tracks). The cartridges used for computer data, however, use 24 tracks which permits a storage capacity of 12700 bpi. Due to the sequential recording, the average access­time is relatively long.

Magnetic Disks

There are two types of magnetic disk - the hard disks and the floppy disks. While reading and writing, the disk is rotating around its centre. The data are recorded in circular tracks, sector by sector. Because of the sectorial access to the data, the average access­ time is relatively short .

Floppy disks are thin, flexible plastic plates covered with a magnetic oxide layer and protected by a firmly fastened square plastic jacket. At present, the common format is the 3×5 inch­disk. The older 3×0, 5×25 and 8×0 inch disks are no longer in use and it is difficult to find drives for them. The storage capacity of a 3×5 inch disk is 1×4 MB. 3×5 inch disks with a capacity of 2×88 MB have been developed but are not very common yet.

Data interchange on floppy disks usually causes no media problems provided that a drive for the physical format of the disk is available. Disks are not suitable for long­term storage. They can deform because of the instability of the plastic material and damage the drive. They should, therefore, only be used for a limited period of time.

Hard disks are usually found installed permanently in computer systems and used for very fast access, short term storage. Removable hard disks exist but they are not common. Although hard disks are reliable, it is advisable to make back-up copies of data stored on them. Storage capacities in excess of 2 GB are now common and, when hard disks are used in an array (RAIDs), very large storage capacities can be achieved - albeit at great cost compared with other storage formats. Hard disks in RAIDs will be in continuous use and have a life expectancy of several years

Data Density versus Data Security

The history of magnetic storage media is the history of ever increasing data density. This has been achieved by the steady decrease in size of the elementary magnetic structure - from iron oxide via chromium dioxide to "pure metal" as used in metal particle tape and hard disks. In parallel with this, has come the development of ever smaller gaps in the reading heads, very thin base films (some R-DAT tapes are only 9mm "thick") and very narrow tracks widths (13mm on R-DAT). By use of these developments, ever increasing quantities of information can be recorded on ever decreasing sizes of carriers.

The danger is, however, that the recorded information becomes increasingly vulnerable. It is generally true to state that, because of their increased data density, modern formats are less reliable than older formats with their lesser storage capacities. Correct recording and reading of information onto or from modern magnetic formats is highly dependent on the physical and chemical condition of the recording medium being in pristine condition, the replay equipment functioning perfectly and an environment free from disturbing factors such as smoke, dust, and other pollutants.

The Stability of Magnetic Carriers

The main factors that affect the stability of carriers and the retrieval of information can be summarised as:

• Humidity and temperature.
• Mechanical deformation.
• Dust and dirt of all kinds.
• Magnetic stray fields.

Humidity is the most dangerous environmental factor. Water is the agent of the main chemical deterioration process of polymers: hydrolysis. Additionally, high humidity values (above 65% RH) encourage fungus growth, which literally eats up the pigment layer of magnetic tapes and floppy disks and also disturbs, if not prevents, proper reading of information.

Temperature is responsible for dimensional changes of carriers, which is a particular problem for high density tape formats. Temperature also determines the speed of chemical processes: the higher the temperature, the faster a chemical reaction (eg hydrolysis) takes place; the lower the temperature, the slower the chemical reaction.

Mechanical integrity is a much underrated factor in the retrievability of data recorded on magnetic media: even slight deformations may cause severe deficiencies in the play back process. Most careful handling has, therefore, to be exercised, along with regular professional maintenance of replay equipment, which, if it malfunctions, can destroy delicate carriers such as R-DAT very quickly. With all tape formats, it is most important to obtain an absolutely flat surface of the tape wind to prevent damage to the tape edges which serve as mechanical references in the replay of many high density formats. All forms of tape - open reel, cassettes and cartridges - and floppy disks should be stored upright.

Dust and dirt prevents the intimate contact of replay heads to media which is essential for the retrieval of information, especially with high density carriers. The higher the data density, the more cleanliness has to be observed. Even particles of cigarette smoke are big enough to hide information on modern magnetic formats. Dust may also be responsible for "head crashes" of computer hard disks and of rotary head formats, which inevitably leads to irretrievable loss of data. It goes without saying that in addition to the mechanical problems caused by dust, fingerprints and smoke, chemical pollution caused by industrial smog can accelerate chemical deterioration. The effective prevention of dust and other kinds of dirt and pollution is, therefore, an indispensable measure for the proper preservation of magnetic media.

Stray magnetic fields, finally, are the natural enemy of magnetically recorded information. Sources of dangerous fields are dynamic microphones, loudspeakers and head sets. Also magnets used for magnetic notice boards etc, possess magnetic fields of dangerous magnitudes. By their nature, analogue audio recordings, including audio tracks on video tapes, are the most sensitive to magnetic stray fields. Analogue video and all digital recordings are less sensitive. For the safeguarding of analogue audio recordings is necessary to keep to the following maximum magnetic stray fields:

• AC fields: 5 Oe (Oersted) = 400 A/m (Ampere per metre)
• DC fields 25 Oe = 2000 A/m.

It should be noted that normally a distance of 10-15cm is enough to diminish the field strength of even strong magnets to acceptably low values.

Fluctuations of target temperature and humidity values should be kept to a minimum. Operational areas (studios) should, therefore, have the same climatic condition as the access storage areas.Tapes must be allowed to slowly acclimatise to the change in conditions when brought out of or returned to the preservation storage.

It is of utmost importance that both temperature and humidity are controlled simultaneously. Damage to tapes can occur if attempts are made to cool the storage environment without dehumidification. Such action will normally lead to an excessive rise in relative humidity.

Summary

Although magnetic media are generally quite stable and analogue magnetic tape has been in existence for over 60 years, none of the magnetic media are designed with longevity in mind. Even if all recommended standards and practices for the preservation of original magnetic carriers are carefully observed, it not possible to preserve them for long periods. Sooner or later copies will have to be made. Because of the increasing deterioration of audiovisual information when copying in the analogue domain, a preservation policy based on copying can only be successful in the digital domain. For digital documents, audiovisual as well as computer data, the problems related to the preservation of the carriers are increasingly overshadowed by the problems of the obsolescence of hardware and associated software. The management of future migration is, therefore, becoming the central issue of audiovisual as well as of general data preservation. Self-checking and self-regenerating digital mass storage systems are likely to become a powerful tool in future preservation policy.

The digitization of analogue carriers and the migration of information already in the digital domain are gigantic tasks. Because these will take time to complete, digitization/migration projects must be arranged in a hierarchical order. Priority must be given to those documents which are in frequent demand and to those which are at immediate risk. Meanwhile, those documents which are in good condition can wait. They must, however, be stored with all possible care in order to keep them in the best possible physical condition until their turn for digitization or migration comes.

Standards

There are few relevant international standards at present. The American National Standards Institute (ANSI) and the British Standards Institute (BS) have issued some useful national standards. The Audio Engineering Society (AES) and ANSI are jointly working on a series of new standards dealing specifically with the storage of magnetic materials.

International

AES22­xxxx	Draft AES Recommended Practice for Audio Preservation and Restoration ­ Storage of Polyester­base Magnetic Tape
AES/EBU	Recommended practice for digital audio engineering - serial transmission format for linearly represented digital audio data
CCIR 601	Digital video - composite format
ISO/IEC 11172	MPEG-1 video compression system
ISO/IEC 13818	MPEG-2 video compression system
IASA TC­03:	The Safeguarding of the Audio Heritage: Ethics, Principles and Preservation Strategy. 1997
SMPTE RP 190­1996	Recommended Practice: Care and Preservation of Audio Magnetic Tape

National

ANSI X3.14-1983	American national standard for information systems: recorded magnetic tape for information exchange
BS 4783:1972	Recommendations for the care and transportation of magnetic tape
BS 4783:1988	Recommendations for storage, transportation and maintenance of magnetic media for use in data processing and information storage. Part 2: Recommendations for magnetic tape on open spools
BS 4783:1988	Recommendations for storage, transportation and maintenance of magnetic media for use in data processing and information storage. Part 3: Recommendations for flexible disk cartridges
BS 4783:1988	Recommendations for storage, transportation and maintenance of magnetic media for use in data processing and information storage. Part 4: Recommendations for magnetic tape cartridges and cassettes

Reference Literature

Allen, Norman et.al.

Factors Influencing the Degradation of Polyester Based Cinematographic Film and Audio­Visual Tapes. In: Boston, George (Ed.), Archiving the Audio­visual Heritage. Proceedings of the Third Joint Technical Syposium, Ottawa 1990. 1992

Bertram, HN, and Cuddihy, EF

Kinetics of the Humic Aging of Magnetic Recording Tape.

In: IEEE Transactions on Magnetics 27:4388­43955, 1982

Bradley, Kevin

Restoration of Tapes with a Polyester Urethane Binder.

In: Phonographic Bulletin 61/1992

Calas, Marie­France, et Fontaine, Jean­Marc

La conservation des documents sonores. C

NRS­Editions, Paris 1996

Dumont, J, , Johansen, J, and Kihlander, G

Handling and Storage of Recorded Video­tape.

European Broadcasting Union, Technical Centre, Lausanne 1989

Edge, Michelle

The Deterioration of Polymers in Audio­visual Materials.

In: Boston, George (Ed.), Archiving the Audio­visual Heritage. Proceedings of the Third Joint Technical Symposium, Ottawa 1990. 1992

Gibson, Gerald D.

Magnetic Tape Deterioration: Recognition, Recovery and Prevention.

In: IASA Journal 8/1996

Gilmour, Ian, and Fumic, Victor

Recent Developments in Decomposition and Preservation of Magnetic Tape.

In: Phonographic Bulletin 61/1992

Haefner, Albrecht

The Introduction of Digital Mass Storage Systems in Radio Broadcasting: A Report on the Progress within the ARD.

In: IASA Journal 3/1994

Hayama, F. et al.

Study of Corrosion Stability on DAT Metal Tape.

AES Preprint 3237

Heitmann, J

Zukuenftige Archivierungssysteme.

In: Fernseh­ und Kinotechnik 50,7/1996

Jenkinsen, Brian

Long­term Storage of Video­Tape.

BKSTS Journal, March 1982

Knight, GA

Factors Relating to Long Term Storage of Magnetic Tape.

In: Phonographic Bulletin 18/1977

Mathur, MCA, Hudson, GF, and Hackett, LD

A Detailled Study of the Environmental Stability of Metal Particle Tapes.

In: IEEE Transactions on Magnetics 28:2362­2364, 1992

Pickett, AG and Lemcoe, MM

Preservation and Storage of Sound Recordings.

Washington 1959. Reprinted by ARSC, 1991

Rothenberg, Jeff

Ensuring the Longevity of Digital Documents.

In: Scientific American 272 (January)/1995

Schüller, Dietrich

Auf dem Weg zum "ewigen", vollautomatischen Schallarchiv.

In: 17.Tonmeistertagung Karlsruhe 1992, Bericht. Muenchen, etc. 1993

Schüller, Dietrich

Behandlung, Lagerung und Konservierung von Audio­ und Videotraegern.

In: Das audiovisuelle Archiv 31/32, 1992 (1993)

Schüller, Dietrich

Preservation of Audio and Video Materials in Tropical Countries.

In: IASA Journal 7/1996

Schüller, Dietrich

Safeguarding Audio and Video Materials in the Long­term.

In: Stephen Foster (Ed), Proceedings of the 1st International Memory of the World Conference, Oslo, 3­5 June 1996. UNESCO (Paris) 1996

Smith, LE

Factors Governing the Long Term Stability of Polyester­Based Recording Media.

National Institute of Standards and Technology (NIST), Washington 1989

Van Bogart, John

Magnetic Tape Storage and Handling. A Guide for Libraries and Archives.

Commission on Preservation and Access, Washington DC, 1995

Welz, G.

On the Problem of Storing Videotapes.

In: Orbanz, Eva (Ed), Archiving the Audio­visual Heritage. Proceedings of the (Second) Joint Technical Symposium, Berlin 1987. Berlin 1988

7. Optical Media

Optical media are used for storing digital sounds, images and data. There are three main families:

• The commercially issued, mass produced, CD family including the digital audio CD- both 12cm and the "single" 8cm disc - CD-ROM, CD-I and CD-V and the analogue Video Disc.
• Optical disks and tapes that can be recorded on once.
• Re-recordable disks.

Jukeboxes are available for most types of disc allowing automated access to a number of discs.

Mass Produced Discs

The mass-produced discs of the CD family have the digital information in the form of microscopic pits pressed into a polycarbonate base which is coated with a light reflective layer. This reflective layer is usually of aluminium, but gold and silver are also used. A transparent lacquer is then placed over the reflective surface to protect it. This surface also carries any label information. As the data on members of the are impressed, they cannot be altered or rewritten.

Because of the high costs to setup the production of a pressed disc, the discs are only used when large numbers of copies are required (over about 100), for example, encyclopaedia or sound recordings. The higher the number of discs issued, the lower is the unit price. The storage capacity of a 12cm CD is about 650 MB or one hour of audio. The average access time is about 300 ms with a double speed player, 250 ms with quadruple speed and 130 ms with sextuple speed.

The first disc in the family to be developed was the 30cm analogue LV (Laser Vision) Disc for video. This usually consisted of two discs stuck back-to-back to form a double sided disc with one hour of video per side. A sub-format was developed which could store up to 54000 still video images per side. The LV disc was the most successful of several attempts to generate market acceptance but is expected to be superseded by the DVD (Digital Versatile Disc or Digital Video Disc) that is being launched in 1997.

The DVD is the same diameter as the CD (12cm) but, by using a laser with a shorter wave length, the storage capacity of one layer is increased by a factor of seven to 4×7 GB. Additionally, a dual layer structure will be possible, read by two different laser wave lengths, thus doubling the capacity to 9 GB. In principle, by glueing two such double layer disks together like the LV video disks, a total capacity of 18 GB can be achieved. The disk is intended for the storage of data-reduced video-films or, like CD-ROMs, texts and multimedia data with, however, considerably higher storage capacities.

Write-Once Recordable Media

There are several types of write-once recordable disks. The format that is becoming the most widely used is the recordable CD (CD-R or CD-WO) which has been available since 1993. Having the same format and storage capacity as the audio CD and the CD­ROM, the CD­R can be played on the appropriate standard CD drives. The polycarbonate body of the disk has a dye layer placed on it which is then coated with a metallic reflective layer. The dye layer carries the data in place of the pits of pressed discs. When recording, high­intensity laser pulses change the dye from opaque to transparent. The low-intensity read laser reads the changes in reflected light as a digital bit stream. Once written, the data cannot be altered. CD writing drives are already available on different speed levels. The CD­R is a well established and standardized format. Different standardized software protocols are available for recording Audio CDS and CD-ROMs. The Photo-CD is a CD-R with a proprietary software protocol to record photographs as electronic still images.

A recordable version of the DVD in not yet available, but may be expected in the near future.

CD-Rs are but the latest and most prominent examples of so-called WORM (Write Once, Read Many) disks which have been in use as computer storage media for quite some time. The biggest problem with WORMs is the great variety of systems and formats. A number of producers offer WORMs with a continuous helical recording format similar to a sound LP disk; others offer disks with ring-shaped tracks as on computer floppy and hard disks. Some can use both formats. The proprietary software of WORMs poses a problem, too. Not even the physical dimensions are standardized.

One writing method used by a number of manufacturers including LMS, Toshiba and Sony burns pits in the metallic surface of the disc with a laser beam. Another system supported by ATG and Optimen creates bubbles by the heat of the laser beam. In both cases the reflectance of the metallic layer is changed and the data can be read by a low power laser beam.

Optical Tape

Optical tape is made by ICI and packaged in a cassette for use as a WORM format data storage tape. The tape drives are made by EMASS in the USA and supplied in Europe by GRAU Storage Systems. Kodak are about to launch a competing system.

The tape contains a dye layer which changes its state when a high power laser beam is applied and can be read by a lower power laser - the same basic method as for CD-Rs. Because the tape is a sequential carrier, the access time can be quite long. In compensation, the storage capacity of one tape is considerably greater than a disc (up to 100GB).

Rewritable Optical Media

In contrast to the preceding optical media, data on rewritable optical disks ("Erasable"), Magneto­Optical (M/O) and Phase­change, can be altered or deleted many times. There are rewritable optical disks in the 5×25 inch format and, more recently, in the 3×5 inch format. The most common still are the magneto­optical discs, where a laser beam in the write mode heats the inner layer of the optical disk and thus changes the polarity of a magnetic coating. The resulting microscopic magnetic marks of different polarity can be read as a bit stream by a low­energy laser beam in the read mode. A more recent recording technology is the Phase­change where the carrier layer is coated with a thin semi­metal film, which can be both in an amorphous and in a crystalline state. A laser beam in the write mode can change single spots to either an amorphous or a crystalline state so that, again, a digital bit stream is created. The Phase­change may replace M/O in the future.

Rewritable optical disks have a short access­time (600 milliseconds). The storage capacity has steadily increased up to the current 2×6 GB.

The Stability of Optical Carriers

The main factors that affect the stability of carriers and the retrieval of information can be summarised as:

• Humidity and temperature.
• Mechanical deformation.
• Dust and dirt of all kinds.

For some carriers there are additional factors:

• Light
• Stray magnetic fields.

Humidity is, as with other data carriers, a most dangerous factor. In the case of optical media it has a hydrolytic action on components such as the protection layer of CDs and a corrosive influence on all metal components including metallic reflective layers. As a secondary effect, high humidity levels (above 65% RH) encourages the growth of moulds and fungi which can obstruct the reading of optical information.

Temperature, as with all other data carriers, determines the speed of (deteriorating) chemical reactions. More importantly, it is responsible for dimensional changes which may be of concern, especially in the case of multi-layer media.

Mechanical integrity is of utmost, and underrated, importance. Even microscopic scratches can hinder the reading laser beam, as do fingerprints and other foreign matter. Mechanical bending of discs cause microscopic cracks which again divert the laser. While the WORM and MO-disks developed as computer storage media are housed in cartridges which only open when inserted into the respective players, the representatives of the CD-family must be handled with utmost care, keeping mechanical integrity in mind.

Dust and dirt prevents the proper reading of the recorded information. Cigarette smoke will accumulate on the disk surfaces and may hide information. The CD-family is again more exposed to this danger than those disks that are protected by cartridges.

Light may affect the dye layers used in recordable and erasable disks.

Stray magnetic fields must be kept away from magneto-optical disks.

Fluctuations of chosen parameters should be kept to a minimum. Operation areas (studios) should, therefore, have the same climatic conditions as storage areas. As with magnetic carriers, tighter parameters would be favourable for long term preservation. Such suggestions have, however, to be offset against the availability of hard- and software, which seems to be of greater concern than the stability of the carriers themselves.

Standards

AES28­xxxx	Draft AES Standard for Audio Preservation and Restoration ­ Method for Estimating Life Expectancy of Compact Discs (CD­ROM), Based on Effects of Temperature and Relative Humidity.
AESxy­xxxx	Draft AES Standard for Audio Preservation and Restoration ­ Method for Estimating Life Expectancy of Magneto­optical Disks, Based on Effects of Temperature and Relative Humidity.
ISO/DIS 9171­1.2. ISO/IEC 9171­1:1989	Information Processing ­ Information Interchange on 130 mm Optical Disk Cartridge ­ Write Once (5×25 inch­WORM, 297­327 MB on each Side), Teil 1: Unrecorded Optical Disk Cartridge (Technical concept, conditions for handling and storing, measures, mechanical and physical properties, optical properties or information, physical interchangeability between systems)
IASA TC­03	The Safeguarding of the Audio Heritage: Ethics, Principles and Preservation Strategy. 1997
ISO DP 10090­ Draft Proposal	Standards for Information Interchange on 86 mm Optical Disk Cartridges (3×5 inch Rewritable M/O, 120 MB on each Side) are still under preparation

Reference Literature

Calas, Marie­France, et Fontaine, Jean­Marc

La conservation des documents sonores.

CNRS­Editions, Paris 1996

Fontaine, Jean­Marc

The Preservation of Compact Discs ­ Principles of Analysis.

In: Boston, George (Ed.), Archiving the Audio­visual Heritage. Proceedings of the Third Joint Technical Syposium, Ottawa 1990. 1992

Herla, Siegbert, und Muecke, Herbert

CD­R(ecordable) ­ Sprengsatz in unseren Schallarchiven.

In: 19. Tonmeistertagung Karlsruhe 1996, Bericht. Muenchen 1997

Nugent, WR

Issues in Optical Disc Longevity.

In: Boston, George (Ed.), Archiving the Audio­visual Heritage. Proceedings of the Third Joint Technical Syposium, Ottawa 1990. 1992

Pohlmann, Ken

The Compact Disc ­ A Handbook of Theory and Use. 1989

Williams, EW

The CD­ROM and Optical Disc Recording System.

Oxford Science Publications. 1996

8. Electronic Publications, Electronic Documents and Virtual Information

This chapter reviews the special problems created by these new methods of creating and supplying information. Many of the documents rely on storage provided by physical media that have been discussed in the preceding chapters.

Electronic Publications

Electronic publications cover the rapidly increasing area of publications that require a computer to be used to access the information that they contain. They can be documents distributed free of charge or obtained by purchase. They are supplied in two forms - Off-line publications and On-line publications. Some electronic publications are not supplied on physical carriers and need to be copied into the libraries' access system and be stored on hard disc stacks, tape streamers or other data storage systems; others are supplied on physical carriers and can be stored on shelves. This chapter will, therefore, be looking not at the physical carriers - they have been covered in the preceding chapters - but at the specific problems of acquiring, selecting, storing and accessing this group of documents.

Definition and Typology of Electronic Publications

Off-line Publications

An off-line publication is an electronic document which is bibliographically identifiable, which is stored in machine readable form on an electronic storage medium. CD-ROM, diskettes or floppy discs and magnetic tapes are examples.

• Off-line monograph eg a CD-ROM encyclopaedia.
• Off-line serial eg a CD-ROM journal.

On-line Publications

An on-line publication (or resource) is an electronic document which is bibliographically identifiable, which is stored in machine readable form on an electronic storage medium and which is available on-line. For example - an electronic journal, a World Wide Web page or an on-line database.

• On-line monograph eg. a dictionary on the Web.
• On-line serial eg. an electronic journal on the Web
• On-line resource eg. an organisation's home page.

Electronic publications can be original electronic publications, but they can also be the digitised version of a written or printed document. For many collections, most of the electronic publications will be the digitised version of a written or printed document in their possession. Examples include the CD-ROM of the National Library in Prague which contains several manuscripts and other documents, the Saint Sophia Project from Bulgaria, the Radziwill Chronicle, the Sana'a Manuscripts and the Memoria de Iberoamerica.

The producers and publishers of electronic publications can be traditional publishers who expand into new areas of publishing. It can also be newly established content providers, especially in case of the new publications on the World Wide Web, who only offer on-line electronic publishing. In addition, some companies specialize in CD-ROM publishing.

Nowadays, most publications are written, edited and formatted using word processors and desk-top-publishing software. The printed version of the journal or the monograph is derived from the electronic form.

Distinction Between Audiovisual Material and an Electronic Publication

Multimedia publications are now produced which contain a mixture of material e.g. a biography, a bibliography, stills (photos), animation, video and sound. It sometimes becomes difficult to distinguish between an audiovisual document and an electronic publication related to text. For example, a movie with subtitling is audiovisual - a CD of Michael Jackson with a video clip consisting of moving images is considered to be an audio CD. A CD-ROM which contains a biography, a bibliography, texts of the songs, some sound, video and photos is considered to be a multimedia CD-ROM publication.

In short, an electronic publication must contain a considerable amount of text before a library will take it on deposit. Some libraries also take audiovisual publications into deposit. e.g. Die Deutsche Bibliothek in Frankfurt am Main in Germany.

Electronic Documents or Virtual Information

The term Electronic Documents or, as they are sometimes called, Virtual Information refers to the modern methods of transmitting documents between individuals, primarily text-based documents - the equivalents of letters and memoranda - by electronic means ie. without the use of paper. Many of the actual and potential problems created by electronic documents are similar to those created by electronic publications.

The documents, while stored on a physical carrier somewhere and easily accessible to a small group of people including the author, are, nethertheless, difficult for an archivist to obtain access to and preserve. The documents include E-Mail messages and computer files held on personal computers. When electronic documents are stored, it is on physical carriers used by other types of documents. The main factor that differentiates electronic documents from other documents is the method of transmission.

The first, and major, problem in the preservation of electronic documents is to gain access to them and discover what exists. Thi