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Magnetic Media

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  1. 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)

  2. 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

  3. 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

  4. 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)

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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 • Don’t squeeze reel flanges (could damage tape edges)

  12. 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)

  13. 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)

  14. 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

  15. 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

  16. Please Contact Us Weissman Preservation Center 617-495-8596 http://preserve.harvard.edu Jane Hedberg jane_hedberg@harvard.edu Elizabeth Walters elizabeth_walters@harvard.edu Liz Coffey coffey@fas.harvard.edu