Magnetic Media

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Magnetic Media

• Signal is recorded by moving some magnetic
  substance (steel wire, disc, or flexible tape
  coated with ferric oxide) over an electromagnet
  that captures the electrical output from a
  microphone playback reverses the process

• 1898 Valdemar Poulson builds the first working
  magnetic recorder (wire)
• 1929 Steel tape magnetic recorder developed
• 1935 BASF AEG unveil first coated magnetic
  tape recorder at Berlin Radio Show
• 1947 Ampex demos first US-made magnetic tape
  recorder

• 1963 Sony markets first home VTR
• 1965 Philips introduces compact (audio)cassette
• 1971 US launch of first home videocassette
  (U-Matic)
• 1981 Sony unveils Walkman portable cassette
  player
• 1987 Sony introduces Digital Audio Tape (DAT)

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Magnetic Media

• Hundreds of analog and digital formats include a
  wide variety of wire recordings, dictation belts
  and discs, open-reel audiotape and videotape,
  audio and video cassettes and cartridges,
  recordable MiniDiscs, and magnetic soundtracks
  for motion-picture film
• Formats most often found in special collections
  include open-reel audiotape and videotape,
  U-Matic ¾ videotape, VHS, compact cassettes,
  DAT, MiniDV, and Betacam SP
• Nearly all of these formats may be considered
  obsolete and therefore at-risk, even if the
  carrier itself is healthy
• Understanding a few basic facts about the nature
  of the constituent elements will go a long way
  toward helping you identify preservation issues
  affecting magnetic media

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Magnetic Tape Composition

• All magnetic tape has
• Base layer (support for magnetic layer)
• Paper, PVC, Acetate, Polyester
• Magnetic layer (takes and holds the data)
• Material may be magnetic particles or magnetic
  film
• Gamma ferric oxide, Chromium dioxide, Metal
  evaporate, Metal particulate, etc.
• Some tape has
• Binder layer (holds magnetic particles in place
  and adds lubricant to reduce playback friction)
• Usually some form of polyurethane sometimes PVC
• Weak link in polyester tape (very susceptible to
  hydrolysis)
• Backcoat layer (reduces static and tape slippage)
• Many tapes since the early 1980s have some type
  of backcoating

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Magnetic Tape Base Layer

• Paper
• Very rare produced 1940s-1950s
• Distinctive blotchy brown paper backing
• Polyvinyl Chloride (PVC)
• Rare produced 1943 to early 1970s, mainly in
  Germany
• Reasonably stable mechanically, but has
  plasticizer issues
• Stretches instead of breaking under stress
  resulting deformation is irreversible
• Cellulose Acetate
• Produced late 1940s to mid 1970s
• Dimensionally unstable prone to shrinkage and
  breakage
• Suffers from Vinegar Syndrome (acid
  hydrolysis), plasticizer issues
• Breaks easily under stress
• Polyester
• Produced from mid 1950s onward most common base
  since 1960s
• Mechanically robust, chemically stable
• No plasticizer issues, but binder is problematic
• Stretches instead of breaking cleanly (like PVC)

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Magnetic Tape Magnetic Layer

• Gamma Ferric Oxide
• Stable
• 2 Quad videotape, Type I Compact Cassette
• Chromium Dioxide
• Found to interact with polyester urethane to
  accelerate acid hydrolysis
• Type II Compact Cassettes, analog video formats
• Metal Evaporate
• Has no binder, but does have lubricant to reduce
  friction
• Type IV Compact Cassettes, DAT
• Metal Particulate
• Prone to oxidation
• Type IV Compact Cassettes, digital video formats,
  DAT, Hi-8

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Acetate or Polyester?

• Two easy ways to tell the difference
• Light Test Acetate tape appears translucent when
  held up to the light polyester (and PVC) tapes
  appear opaque
• Snap Test Acetate tape will break without
  stretching polyester tape will stretch and curl,
  but will not break

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Deterioration (1)

• Binder degradation
• Causes Poor storage conditions (high RH),
  natural aging
• Indicators Sticky Shed Syndrome (powder or
  gummy residue on tape surface, tape layers
  adhere, oxide flaking or shedding) strong smell
  of dirty socks
• Affects polyester base tapes, especially 1/2
  open-reel videotape
• Acetate base deterioration
• Causes Vinegar Syndrome (acid hydrolysis)
• Indicators Strong vinegar smell (acetic acid),
  tape shrinkage or breakage, flaking binder layer
• Most likely to occur in open-reel ¼ audiotapes
• Can measure severity with A-D Strips
• Contagious Segregate affected tapes, store in
  cool, dry environment

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Deterioration (2)

• Tape Deformation
• Causes Poor handling and storage conditions,
  poor tape wind, misaligned playback equipment
• Indicators Cinching, stretching, edge damage
• Degradation of magnetic particles
• Causes Poor storage conditions (moisture and
  pollution), corrosion
• Particularly affects early versions of MP and ME
  tapes (cracking or delamination of thin magnetic
  layer)
• Mold

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Deterioration (3)

• Tape pack problems (loose or uneven wind)
• Causes misaligned playback equipment, poor
  storage conditions, exposure to excessive heat or
  cold
• Exacerbated by temperature fluctuations that
  produce uneven tension across tape pack
• Indicators Tape pack slippage (popped strands),
  flange pack, spoking, windowing
• Leads to tape deformation, base stretching, edge
  damage

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Care and Handling (1)

• Never touch the surface of a magnetic recording
• Do not drop or subject to sudden shock (prevents
  breakage and other problems with reels and
  cassette shells, container breakage, and tape
  pack distortion)
• Never leave media in playback equipment (prevents
  excessive heat exposure and mechanical damage)
• Keep media away from stray magnetic fields (most
  important for wire recordings)
• Labelling should not hinder tape transport in any
  way
• Inspect media periodically for signs of
  deterioration

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Care Handling (2)

• Cassettes and Cartridges
• Do not attempt to open tape cartridges
• Engage write-protect mechanism (prevents
  accidental re-recording)
• Open-Reel Tape
• Handle only by hub never touch tape surface
• Never pull the loose end of an open-reel tape to
  tighten the wind
• Dont squeeze reel flanges (could damage tape
  edges)

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Environment and Storage

• Store in a clean, cool, dry environment where
  temperature and RH are constant
• Avoid rapid or dramatic temperature or humidity
  changes (cycling stresses the layered structure
  of magnetic tape)
• Minimize exposure to extreme heat (polyester
  melts and deforms at higher temperatures) or high
  humidity (high temperatures and RH accelerate
  binder hydrolysis)
• Store all magnetic media vertically do not stack
  horizontally or allow to lean (upright storage
  helps maintain good tape pack stacking can warp
  cassette shells or tape reels)
• If possible, segregate acetate materials to
  prevent contamination of other materials by
  acetic acid
• Do not interfile recordings of difference sizes
  (small items get lost, larger items are exposed
  to uneven pressure)
• Return media to storage containers when not in
  use (to avoid surface damage and exposure to
  light)

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Preferred Containers General

• Rigid, vented, impact-resistant containers made
  of inert materials
• Should protect contents from dust, dirt, and
  moisture
• Should close and latch securely
• Avoid paper or plastic sleeves, slipcases, and
  other flexible containers (which provide little
  physical protection and may interact chemically
  with the media)

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Preferred Containers By Format

• Open-Reel Tape
• Replace original paper boxes (generally too
  acidic and do not provide adequate support and
  protection for the carrier
• Container should support reel by the hub
• Use unslotted reels if possible
• Videocassettes
• Original containers generally acceptable if they
  are rigid, lock securely, and stabilize the
  carrier do not use original paper or plastic
  slipcases
• Compact Cassettes
• Store in original Norelco boxes or hinged
  polyboxes
• Small Cassette Formats (e.g., Minicassette, DAT,
  MiniDV)
• Original Norelco type containers are acceptable
  (no good substitute)
• Should be grouped in larger boxes to prevent loss
  and damage

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Requires Immediate Attention

• Vinegar smell (or any strong smell)
• Likely indication of advanced deterioration
• Other obvious signs of media deterioration (oxide
  shedding, mold)
• Visible tape pack issues (loose wind, windowing,
  spoking, curling)
• Cracked or broken shells or reels
• Renders carrier unplayable, could damage tape

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Please Contact Us

• Weissman Preservation Center
• 617-495-8596
• http//preserve.harvard.edu
• Jane Hedberg jane_hedberg_at_harvard.edu
• Elizabeth Walters elizabeth_walters_at_harvard.edu
• Liz Coffey coffey_at_fas.harvard.edu