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Magnetic and Digital Materials

By Stephen Ball

Magnetic storage techniques progressed hand in hand with the many electronic developments of the twentieth century, and eventually became a principal means of storing sound, video and digital information. Museum collections now find themselves responsible for an increasing range of audiovisual and computing material, from historically interesting wire recordings to present day audio recordings, videotapes and computer CDs.

The Anatomy of Magnetic Media

Most magnetic media are constructed from a base material (also known as a substrate) and a binder layer or topcoat. The binder layer is analogous to a film emulsion, and carries the magnetic particles whose magnetisation and orientation determine the stored information.

From the 1930s until the 1960s, magnetic tapes had a cellulose acetate base similar to that used in cine films, with added plasticisers that can be a source of eventual deterioration and brittleness. More recent tapes and computer floppy disks use a polyester base. Polyester tapes are less prone to brittle breakages but do stretch, thus altering the rate and conformation of the magnetic material passing playback or recording heads.

Computer hard disks and 'Winchester' drives use rigid base materials such as glass and aluminium. These rotate inside sealed housings at extremely high speeds, the read/write head literally floating over the 'platter' on a minute cushion of air at operating speeds, but settling back on to the surface when the disk is stationary.

The binder layer of a tape or disk is a complex, usually secret, mix of materials held in some kind of resin - the binder itself - which is nowadays usually composed of polyester. This layer must be flexible, but at the same time durable, resistant to scratches, stable, and resistant to static build up. It is the deterioration of the hard-working binder that is most likely to shorten the life of magnetic media.

Computer Media

Since the beginning of electronic computing there has been a heavy reliance on magnetic media for both long and short term data storage. In most cases, the software and hardware required to read and write the stored information has a short practical life, given the rapid turnover and obsolescence of computer technologies. This means that you should copy any still-usable data from the obsolete media to current media as soon as possible - do not wait until the old technology fails.

Hard disks, whether the bulky Winchester drives of the late 1960s or the more modern PC hard drive, are working objects in the museum sense. They are electro-mechanical devices that suffer from two main defects: the progressive deterioration of the recording surface, and eventual mechanical failure. The moving parts operate inside a sealed case that excludes dust, which is why hard drives are essentially unrepairable.

Floppy disks come in a huge variety of sizes, types and formats. Your first task is to identify which types you have - it's the disk itself that's flexible, not necessarily the outer sleeve - and whether you have working equipment that can read them. Disks have reduced in size over the years, with the 3.5 inch disk being the most common format today. As well as obvious size and material differences, there are various access, indexing and identification slots and holes, and different approaches to formatting. Floppy disks also use different 'densities', which refers to the coercivity of the magnetic medium. Modern high density disks cram more information on to the surface by placing more and narrower radial 'tracks' on to the disk, which in turn places greater demands on the magnetic material.

Large mainframes used large format open reel tape as a storage medium. These machines were designed to run in a climate-controlled environment, so it is unlikely that you will be able to present them in a working form. There are also a number of tape back up formats for more modern computers, most based on cassettes or cartridges - even ordinary audio cassettes were used in the early phase of home computing and for the first generation of IBM PCs.

Non-magnetic computer media

Not all computer storage is based on magnetic media. There are newer storage devices that employ laser or other optical methods, and the CD-ROM (see below) is fast becoming a standard accessory on desktop PCs. These devices are unlikely to be of interest to most small museums, but the rapid pace of technological development in computing means that they will soon find a place in information technology collections.

One potentially vulnerable type of storage device that may already feature in displays of computers or their circuit boards is the erasable programmable read-only memory chip, or EPROM. These chips store 'permanent' information such as BIOS code or factory-preset configuration data, and are written, rewritten and erased using ultraviolet (UV) light. Prolonged exposure to daylight or other UV-rich sources will eventually corrupt or erase the stored information, so unless a computer board is a 'dead', purely static exhibit, protect it from UV sources. Seek advice if you are unable to recognise an EPROM.

Compact disk (CD)

The now familiar CD is not yet a common museum object but it will feature more frequently in the years ahead. At first presented as an 'indestructible' medium that was perfect for 'flawless' sound quality, the CD has spread to computing where, as the CD-ROM, it is increasingly becoming the most common means of software distribution packaging. New, rewritable formats have now joined the original read-only forms.

Accelerated ageing tests by major manufacturers suggest that well-kept CDs will remain in usable condition for between 50 and 100 years, and controlled temperature and humidity could prolong a disk's life to as much as 200 years. The 'business' side of a CD is the underside, which is silver for CD-ROM, green or greenish-gold for writable CDs. The information is stored in minute 'pits' in spiral grooves in the metallic layer that is sandwiched inside the durable polycarbonate outer coating and read by a laser beam during playback. This is why there is no label on the underside of the disk, and why careless handling that deposits grease and dirt may interfere with accurate information retrieval.

Storage, Access and Display

The temperature and humidity requirements of most magnetic media are within the same range as a large number of other museum objects. Keep the items in a cool, dry and stable environment - temperature 9-15ºC and RH 25-40% - with adequate ventilation to ensure that there are no stagnant air pockets. Ultraviolet (UV) radiation affects plastics, so keep light to a minimum. Storage areas should be kept dark except for access or safety purposes.

Never touch the playing surface of magnetic tapes or disks - something that is easy to do when the surface is exposed, as in open reel tapes and 5.25 inch floppy disks. If you must handle the medium directly, wear lint-free cotton gloves.

Store all tapes vertically in their sleeves or boxes to prevent the winds of tape creeping over one another under gravity, but avoid tight packing. Ideally, sleeves and boxes should be of archive quality card or plastic, but the original packing is the next best option. Provide regular vertical supports, and do not mix tall and short items side by side on the shelf. Support heavy reels by rods through their hubs to avoid the weight settling on the reel edges.

If the sides of the tape on a reel look ridged or irregular, carefully rewind the tape. The winding tension is an important factor in tape life: in general, do not rewind tapes after recording or playback. Rewinding normally sets up a higher tension in the tape than the slower and more even playback process. By storing tapes unwound - the so-called archival wind - and rewinding immediately before playback, the rewind tension is dissipated during play rather than held during long periods of storage.

Store floppy disks vertically, in the same way as tapes and for similar reasons. If a disk displays any errors in use, immediately copy all important data from it and then reject the disk for future data storage. Ideally, write on labels before sticking them to the disk to prevent pressure on the medium. Do not allow smoking in the vicinity of stored disks or operating drives. Back up important data stored on a hard disk. And do not be tempted to open the sealed casing of a hard disk for any reason: you will immediately destroy the drive.

Look for stray magnetic fields that could corrupt or destroy the data stored by magnetic media. This is probably a bigger danger during use than during storage - people are apt to rest floppy disks on top of VDUs, for example - but check stores for hidden dangers. Is there an electric bell immediately behind a row of tapes, or an electric motor next to a partition wall by the shelving? Even a single mains cable to a light switch generates a weak magnetic field.

Although there are as yet no published standards for the care of CDs, the best storage conditions are similar to those for other audio-visual media - dark, relatively dry, and relatively cool. Aim at temperatures below 25ºC (but avoid freezing), and keep RH in the 20-50% band. As always, stable environmental conditions are the key - rapidly varying conditions could cause warping and distortion.

The 'Working Object' Problem

Magnetic and other media only come to life when run on suitable machines (working objects such as computers, tape players, video recorders, etc.) - machines that may be important museum objects in their own right. And in many ways, tapes and disks are themselves working objects. As always, the museum presumption from the conservation standpoint is not to operate working objects, but this is something you must decide for each object against a background of museum policy, the status of the object, and no doubt expert advice.

There may be several options for a given object. For example, a 1980s video arcade game machine can be left running with all its original components until it finally fails, or kept as a static exhibit, or run indefinitely with regular maintenance and repair using modern substitutes when the original components are no longer available. This third option clearly compromises the integrity of the object.

You should keep all necessary equipment clean and in good working order - faulty or poorly maintained equipment can irrevocably damage magnetic media. Review these objects at intervals and plan for their inevitable obsolescence or final breakdown.

Carefully follow any instructions and manuals, train operators, and allow only these trained people to run the machines. Keep records of working sessions in both the machine's and the media's documentation. Cleaning equipment means more than just dusting or wiping it: somebody must regularly clean and demagnetise read and write heads, tape transports, disk drives and other crucial components.

Only run copies of the tapes or disks and not the originals, which you should keep in controlled storage. Analogue recordings - which probably make up most of your audio and video collection - suffer information quality degradation through repeated copying. Digital copying may be the answer, provided the digitisation of the original analogue material is acceptable (see below). This is because subsequent generations copied from a good source should be of equal quality to the digital master.

Early reel-to-reel systems have given way to cassette tape for all but specialised audio and video uses. Cassettes protect the tape from fingers and dust, but also incorporate an internal mechanism that is not always in good condition. You may think of this mechanism as an extension of the tape player; it too needs cleaning and maintenance, and sometimes replacement.

Magnetic media slowly deteriorate with each playing or read operation, even when machines are kept in good condition. This cannot be prevented, though with care it can be minimised. Friction causes surface wear, and the lubricants in the binder slowly disappear, either by transfer to the heads and mechanism or through evaporation. At the same time as the material wears, the magnetically stored information degrades.

Copying

The progressive obsolescence of media formats and technologies, together with the inevitable slow deterioration of all magnetic media, makes copying a necessity if date is to be preserved. One great advantage of copying is that the original object can be put into optimal storage. And continuing technological development now permits copying across media - as when a mass of data is transferred from hundreds of computer tapes and disks to a single CD-ROM. However, copying isn't a panacea. It is a form of use, with all the attendant risks associated with the use of objects.

Copying brings copyright and ethical problems in its wake, especially when aided by new technology. For example, you must make reasonable efforts to discover the copyright owner before making copies to show to the public.

Integrity problems arise from the ability to enhance and 'improve' an original in countless ways, especially with the aid of digital techniques - but should you? The question is beyond the scope of this fact sheet. It relates to larger issues that continually confront museums, archives, libraries, galleries, and the whole 'heritage' industry, as well as the IT and contemporary entertainment businesses.

Digital as opposed to analogue copying brings the great advantage that a copy of a good digital original on to a good quality medium will be 'perfect'. The drawback is that few if any items in a collection, apart from IT objects themselves, will be in digital form. The digitisation of analogue sounds and images necessarily involves an approximation involving various software manipulations that subtly change the form of the original. This is because digitisation turns the continuously varying analogue information into discrete steps that can be manipulated by computers and digital playback equipment.

Copying methods

Good tape copies demand top quality tape and the best available recording equipment. For example, standard, mini- and micro- audio cassettes use thin, narrow tape, which is thinner still in extended play versions. This tape is prone to 'print through' - a pre-echo effect - and is totally unsuitable for archiving. Use archival grade Mylar tape for all masters and copies (you may need to take advice on suitable audio and videotape).

Recording and playback equipment must be of the best available quality and in good repair. Whoever looks after the equipment should be aware of the importance of tape path and head cleanliness, head demagnetisation routines, azimuth settings, and the need to set the correct bias and equalisation when recording. If you don't understand these terms, talk to somebody who does.

Copy important computer data to new disks and/or tapes every 5 to 7 years, even if it never leaves the store, to keep the medium fresh and the digital information in perfect condition. Remember - and plan for - the associated problems of hardware and software obsolescence.

Finally, care for the copy as you would the original - the sooner you wear out the copy, the sooner you must return to the original. One sensible solution is to create new 'masters' that obviate the need to re-use the original, which can be put into permanent storage.

Sources of Information and Advice

The Kodak website contains a number of useful information sheets on the care and preservation of photographic, magnetic and digital products. You can specify a search from the following web page:

http://www.kodak.com/cgi-bin/searchKodak.cgi

For more information about private conservation work please contact:

Conservation Register
Tel: 020 7721 8246

Copies of this fact sheet can be provided in alternative formats. Please contact Viola Lewis, Information Officer at MLA for further information.

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