The Preservation of Recorded
Sound Materials

The Degradation Mechanisms of Sound Recording

The lifespan of a plastic is largely determined at the manufacturing stage. Variables such as basic resin, the materials added to the basic resin to alter its properties, the lamination of materials with dissimilar properties, and the manufacturing process itself, all directly affect the lifespan of the plastic. Post-manufacture environmental factors such as storage conditions, temperature, humidity, and handling also contribute to the long-term stability of the plastics.

Plastics can be divided into two main classes, thermoplastic and thermosetting. Thermoplastics soften and flow when heated and are normally shaped by heat and pressure. They will soften and flow again when re-heated. Vinyl, used in the manufacturing of LP's, is a thermoplastic.

Thermosetting plastics are moulded under heat and pressure. A chemical reaction occurs so that once molded they do not soften when re-heated and will normally char before melting. Most 78s are made of thermosetting plastics.

Acetate Discs

Prior to the advent of magnetic tape, instantaneous recordings were made chiefly on acetate discs. The chemical makeup of these discs, therefore, had to be a compromise between ease of engraving and the quality of the recording that resulted.

Since the 1930s, most blank acetate discs have been manufactured with a base, usually aluminum (although glass was used during the war years and cardboard for inexpensive home recordings), that was coated with nitrocellulose lacquer plasticized with castor oil. Because of the lacquer's inherent properties, acetate discs are the least stable type of sound recording.

Continuous Shrinkage of the lacquer coating due to the loss of the castor oil plasticizer is the primary destructive force. The gradual loss of plasticizer causes progressive embrittlement and the irreversible loss of sound information. Because the coating is bonded to a core which cannot shrink, internal stresses result, which in turn cause cracking and peeling of the coating.

Nitrocellulose acetate decomposes continuously and over time reacts with water vapour or oxygen to produce acids that act as a catalyst for several other chemical reactions. As with most chemical reactions, these reactions are accelerated with elevated temperature and humidity levels.

Vulcanite (early Berliner records)

Vulcanite (hard rubber) was the first material used commercially by Berliner and provided the necessary basis for the exploitation of the flat disc.

In 1839 Hancock in England and Goodyear in the US independently discovered vulcanization. Vulcanization is the process of treating crude rubber with sulfur or sulfur compounds in varying proportions and at different temperatures. The result is an increase in the rubber's strength and elasticity, yielding either soft rubber or vulcanite. Vulcanite has been used to make combs, buttons, jewels, fountain pens, musical instruments, etc.

Vulcanite is stable in the dark and retains its appearance and properties very well. In response to light and/or heat the material loses sulfur then becomes brittle and loses its shine. Light induces oxidation of the rubber and forms oxides of sulfur and sulfuric acid in the presence of humidity. The acidity builds up to a level at which the degrading plastic is attacked and eventually decomposes¹. The degradation can be demonstrated when playing an afflicted Berliner. The surface of the disc is shaved off by the pressure of the stylus against the groove wall.

Vulcanite also posed problems in the production of discs. The uneven shrinkage during cooling caused severe warping; entrapped gas would produce blisters; hard particles create loud pops and clicks; and the coarseness of the Vulcanite structure produced terrible background noise.

Shellac Discs

The first shellac discs date form the early 1900s. Shellac is a composite word -- it's a combination of shell and lac. The word is a Hindus name of an insect that infests certain types of trees. The lac draws sap from these trees, processes it through its digestive system and secretes it so that it becomes an attached protective shell around its body. Thus the shell is generally smaller than a grain of rice. Harvest involved scraping off the encrusted shells from twigs and branches.

After WWII resins such as Vinsol, Valite, Vynil chloride acetate and other commercial resins replaced organic shellac as the main binder. These plastics are slightly more stable than organic type discs. It is often difficult to distinguish between shellac and shellac type discs by inspection.

Determining the causes of shellac disc degradation is rather difficult because a very wide range of qualities of shellac and "fillers" have been used by manufacturers. Therefore one cannot expect consistent behavior of all stored shellac discs. The disc properties are as much a function of the filler as they are of the cementing agent. The fillers used run the gamut of natural cellulosic materials as well as of minerals.

For example two separate chemical analyses of "typical" shellac discs showed the following:

Example 1²

Example 2³

The average shellac content in these "shellac" discs is approximately 15% shellac.

Also, record manufacturers would introduce scrap as filler into new mixtures. The manufacturers would recycle returned, unsold shellac discs. It was not uncommon for the scrap to included soft drink bottles litter, pieces of masonry or other unwanted material, all of which were ground up together and mixed in with the next batch of compound.⁴

Columbia discs (laminated disc)

In 1906 Columbia introduced the Marconi Velvet Tone developed by Giulemino Marconi. The manufacturing technique involved using a craft paper core cut to approximate record size. After the core was carefully flattened and dried, it was covered with a powdered shellac compound of a thin uniform thickness. The dust-coated core was put in an oven and the dust fused to the core. For two-sided records, the operation was repeated for the other side.⁵

The advantage of this construction was that the amount of surface material needed to carry the music grooves could be kept very small. This economy allowed the use the best plastic available at that time. Edison was to use this idea in 1912/13, in the manufacturing of his Diamond Disc.

In 1922 Columbia returned to the laminated record, this time with a coarser compound for the powder core that was bonded between two discs of craft paper.

In general shellac discs are relatively stable. The curing process of shellac during disc manufacturing generates a chemical reaction where certain simple molecules such as water or ammonia molecules are eliminated. Curing causes shellac to shrink, increasing its density and its brittleness. This condensation continues at a much slower rate after disc manufacturing. The speed at which condensation occurred is a function of storage temperature, storage humidity and completeness of cure. (The condensation reaction reduces the potential concentration of reacting elements. A semi-quantitative measure of the cure of shellac is its solubility in alcohol. Raw shellac is totally soluble in alcohol and completely cured shellac is insoluble, and the extent to which condensation has proceeded determines the degree of solubility of a shellac.⁶) Thus the condensation becomes the primary degenerative force. The internal reaction of the material and the rate at which the reaction occurs are related to storage temperature, storage humidity (moisture increases the condensation reaction rate) and completeness of the cured shellac.

Storage stability of these fillers vary widely. Organic materials in the aggregates are susceptible to fungus attack, while shellac itself is resistant to fungus attack.

In a proper storage environment, these discs suffer a slow, progressive embrittlement of the shellac. This embrittlement causes a fine powder to be shed from the disc after each playback. The behaviour of the other aggregate components is unpredictable, due to the wide combinations and variety of materials (and of material quality) that were used.

Edison Diamond disc

The Edison Diamond disc has the distinction of having been made of the first completely synthetic plastic, a material called phenol (phenol was also used in the manufacture of Bakelite).

The Edison diamond disc is a laminated disc made up of a thick core and a thin varnish layers covering each of its sides.

The ¼" core, which is also known as a powder blank, was manufactured by compressing the following ingredients in the following proportions:⁷

The varnish, named Edison condensite varnish was made-up of

The varnish would be applied to the blank by a brush as the blank rotated slowly. Four applications or coats were given each blank face with a drying period between. After the last coat the varnished blank would be placed in a steam-heated oven. This completed the drying and also effected a partial reaction of the varnish ingredients.

Prior to pressing, the blanks would be heated before applying pressure to soften. After the pressure was applied the heat was left on to complete the curing or reaction of the varnish. Then the moulds were cooled and the pressure released.

Prolonged contact with moisture or severe changes in humidity may cause damage to the surface through moisture absorption. In general Phenol is very stable and presents no serious degradation problems, neither is it prone to attack by bacteria, fungi or insects although, occasionally, under humid conditions moulds may grow and cause some surface attack on a nutrient filler such as wood or cotton, or be supported by a nutrient contaminant on the surface.

Vinyl Discs

Thus far, vinyl has proven to be the most stable of the materials that have been used in the manufacture of sound recordings.⁸ However, although stable, its life is not indefinite. Pickett and Lemcoe, in Preservation and Storage of Sound Recordings, states that "failure by chemical degradation of a vinyl disc in ordinary library environments should not occur in less than a century".⁹

Vinyl discs are made of polyvinyl chloride (PVC) and a small percentage (usually less than 25 percent) of "fillers", stabilizer, pigment, anti-static substances, etc. Internal plasticization, through a copolymerizing of vinyl acetate with vinyl chloride, is needed to achieve the required properties for the desired application.

Polyvinyl chloride degrades chemically when exposed to ultraviolet light or to heat. Phonograph discs are exposed to high temperatures during moulding and pressing. Unless stopped, this heat would be a catalyst for ongoing dehydrochlorination, which is the release of hydrochloric acid (HCl) from the PVC as a result of thermo-degradation. Stabilization is therefore achieved by adding a chemical to the resin during manufacture. This does not prevent the degradation but controls it, mainly by consuming the free HCl. Sufficient effective stabilizer remains in a plastic phonograph disc to protect it for several decades after pressing.

Magnetic Tape

Magnetic tape first appeared in North America just after World War II.

Magnetic tape is made up of two layers: a "base" layer, and a thin "binder" layer which is bonded onto the base. The binder contains ferromagnetic particles whose permanent alignment within the binder produce the copy of sound waves.

Magnetic Tape Binder

Manufacturers are very secretive about the specific chemical makeup of their products. Binder chemical composition, uniformity and smoothness of application all affect audio quality, noise level, tape-to-head contact, and friction. These factors also affect the tape's aging properties.

The most common binder resin used today is polyester polyurethane. The most common ferromagnetic particle used is gamma ferric oxide (Fe3O2). Numerous additives may be used during the various manufacturing stages, including: solvents, used to obtain a suitable viscosity of emulsion and to improve the mixing and bonding operations; wetting agents, used to break binder/particle mixing tension to produce a more even ferromagnetic particle dispersion within the binder; plasticizers, used to add suppleness to plastic; stabilizers, used mostly as antioxidants to avoid chemical degradation that could lead to physical breakdown; lubricants, used to reduce drag so that speed deviation problems such as "wow" and "flutter" are diminished, and to minimize wear damage to heads; fine mineral powders, used to make polymers harder and more resistant to abrasion; conduction discharge (material such as carbon black), used to discharge electrical charges; and fungicides.

The most common and serious magnetic tape degradation occurs through hydrolysis, the chemical reaction wherein an ester such as the binder resin "consumes" water drawn from humidity in the air to liberate carboxylic acid and alcohol. Hydrolysis in magnetic tape results in the binder shedding a gummy and tacky material which causes tape layers to stick together and inhibits playback when it is deposited onto the tape recorder heads. The added friction increases tape stress and can cause machines to stop. Hydrolysis also causes a weakening in the bond holding the binder to the backing, which results in shedding or possible detachment.

Chromium dioxide (CrO2) is used extensively as the ferromagnetic particles in cassette magnetic tape. It has been found that CrO2 particles interact with the polyester polyurethane to accelerate hydrolytic degradation. Additives are now added to retard this degradation.

Other problems associated with binder manufacturing and deterioration are: incomplete dispersion of the ferromagnetic particles, causing momentary loss of signal ("dropout"); a weak bond that causes the binder to separate from the backing; lubricants that evaporate to the point where tapes are unplayable; fine oxide powders that shed from tapes and deposit onto heads, inhibiting playback.

Magnetic Tape Backing

The backing, which is the structural support of the tape, must resist stresses imposed by playback and storage without becoming permanently deformed (e.g., stretching), or losing dimensional stability (e.g., expanding through absorption of moisture or heat). Most magnetic tape backing has been made of either cellulose acetate or polyester, materials that have dissimilar physical and aging properties.

Cellulose acetate-backed tapes were manufactured from about 1935 until the early 1960s. These tapes rely heavily on plasticizer additives for suppleness, and these plasticizers are liable, over time, to evaporate and crystallize. These tapes have extremely low tensile strength and are easily broken. Cellulose acetate tapes are very susceptible to linear expansion in humid and/or warm conditions. Because of the different properties of the binder and the base, the absorption of humidity and heat result in tape curling and edge fluttering. These distortions greatly affect the tape-to-head contact, which in turn directly affects audio quality. Repeated dimensional changes due to environmental fluctuations grossly affect winding tension and can promote binder fatigue, cracking, and finally, catastrophic failure (i.e., the irreversible loss of data).

A serious problem affecting acetate tape is that of the "Vinegar Syndrome". Vinegar Syndrome is exhibited by the release an acetic acid (vinegar) odour from the tape and is a by-product of acetate tape breaking-down. The process is accelerated by the presence of moisture and iron ferromagnetic particles in the tapes.¹⁰ When the acetate is degrading --giving off acetic acid-- it will start to take up more moisture. The process of self-destruction is auto catalytic, once it has started it will continue with ever increasing speed; no solution for its interruption has yet been found. Tapes afflicted with the vinegar syndrome will infect healthy tapes.

Polyester ("mylar") came into use in the early 1960s, and quickly replaced cellulose acetate for magnetic tape backing. Accelerated aging tests have found polyester to be a stable material which in fact undergoes hydrolysis degradation at a much slower rate than does the binder, polyester polyurethane, with which it is combined. However, polyester-based tape has a high tensile strength that can cause it to stretch irreparably (instead of breaking cleanly and reparably as does acetate-backed tape).

A third coating is now added to professional, modern tape on the opposite side of the binder. Made of carbon black particles held in a thin binder, it protects the backing from scratches, minimizes static electricity, and provides a more even wind.

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