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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.
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