Handling of sound carriers - Practical suggestions
Remarks on discs
This type of disc was commonly used for instant recordings before being replaced by the magnetic wire or tape. It was conceived as a compromise offering facility of recording and reproduction, but this advantage was offset by negative effects on the long-term preservation of the carrier. The composition of these discs has changed considerably over the years, progressing from wax to ethylcellulose, to acetylcellulose and, lastly, to nitrocellulose.
The shellac disc
Shellac, or an equivalent material, was used for pressing 78rpm discs with a normal groove (lateral or vertical cutting). In this case too, there were different construction processes, but these carriers also were characterized by their variable duration over time.
The plastic disc
As long-playing discs, or microgroove discs, are composed almost exclusively of synthetic material, they cannot be considered as a mere improvement of shellac discs. They are made of polyvinylchloride (PVC) or polystyrene. The most important agents causing chemical degradation of PVC discs are exposure to ultraviolet rays (UV) and heat. For polystyrene discs, instead, there is a risk of oxidation.
The optical disc
The optical disc is decidedly the most stable of all the discs considered so far. Normally, it consists of a base of transparent polycarbonate. The information is recorded on the upper surface of this material. This face is then covered with a thin reflecting metallic layer and, in turn, this is covered with a layer of protective lacquer on which the label is printed. The most interesting element of this construction technique is that the surface of the disc is physically separated from the surface on which the information is recorded, so the precise focusing of the latter makes the presence of any anomalies (dirt, scratches, ...) on the disc irrelevant.
The magneto-optical disc
In this context, it is necessary to mention also magneto-optical discs. Used originally in the computer world to store data, they have increasingly lost importance with the dramatic increase of capacity of the hard disc (HDD). They continue to survive in the commercial world in the (re-writable) MiniDisc format.
Remarks on tapes
The magnetic tape was introduced immediately after the Second World War. By 1950, its degree of reliability was such that it completely replaced the acetate disc for direct recordings.
Each magnetic tape (reel or cassette, audio or video) consists of a carriering base (paper, acetylcellulose, PVC, PET, ...) onto which is fixed a layer of magnetic particles (Fe3O4, Fe2O3, CrO2, ...). From the structural standpoint, it is therefore exposed to the same risks as other sound carriers, with some additions characterized by the shape and the particular system of recording and reproduction.
Causes of deterioration
The most important alterations in the structure of an audio carrier are the internal reactions normally caused by the prevailing environmental conditions. The physical and chemical properties of a resin may be altered by various factors. These structural changes are caused by:
- Heat: Thermal energy is responsible in plastics for physical and chemical alterations such as: permanent deformation or warping, changes of viscosity, delamination. A compromise must therefore be reached as regards operational temperature, also keeping in mind human needs, to avoid any sudden violent changes.
- Light: The radiant energy of ultraviolet rays, or in any case from the highest band of spectrum frequencies, is often a cause of degradation. Direct exposure to sunlight or to other similar sources should therefore be avoided.
- Water: Damp, or humidity, also contributes to the physical and chemical degradation of sound carriers. It causes variations of dimensions in some resins and fillers modifying their resistance to impacts. It can act as a solvent or produce hydrolysis or catalysis. Humidity is present in different forms including water vapour, or steam.
- Oxygen: This can be an important element, as it favours oxidation. This process is particularly influential during the construction of the carrier.
- Atmospheric contaminants: The main atmospheric contaminants to be considered are carbon monoxide, sulfur dioxide and nitrogen. Fortunately, they are normally present only in very modest quantities, so their action is limited to areas with a particularly high concentration of poisonous gas.
- Dust and sand: Every audio carrier is subject to abrasion.
- Static electricity: Thermoplastic compounds are, unfortunately, bad conductors of electricity and are charged with static electricity during pressing. This charge remains active for a long time and has the property of being regenerated with the handling and reading of the carriers, attracting dust onto their surfaces.
Further causes of deterioration, specific to tapes, are:
- Winding tension, or traction, between the coils and between the tape and the core. Some types of tape, for instance those with a shiny bottom layer, tend to unravel causing notable differences of stress in the structure of the tape. These differences normally lead to the breaking or stretching of the tape itself.
- Print through effect. This effect, which is well known, is more or less evident in all magnetic tapes. In digital recordings, it is less noticeable, but this effect does exist nonetheless! It is due to the magnetizing of one or more adjacent coils throughout the entire structure of the tape.
Mycosis, formation of mold
If we go back in time, we observe a more intensive use of organic materials, especially in the field of additives, which also offered a high potential nutrient capacity for fungi. Nowadays, a possible source of nourishment for these organisms is the fat deposited by the hands or other parts of the human body. The damage caused by fungi, contrary to what is generally thought, also extends to materials that are not in themselves nutrients. In fact, the digestion of their food takes place externally with the secretion of enzymes and acids that can have negative effects on any material.
In this case, reference is made to daily use, storage, transport, etc.
Important points to be remembered
For mechanical cut discs
- New discs, just pressed, are more likely to become undulated than discs that have already been in store for a long time.
- During aging, it is probable that changes of dimensions and shape may occur.
- discs made of shellac are more exposed to the damaging effects of damp than those made of semi-synthetic materials.
- The worst form of chemical degradation is the decomposition of the material to the extent that the mere playing or reproduction of the disc causes wear of its surface and leaves behind a dark dust.
For optical discs
- The protective lacquer applied to the reflecting metallic surface is so thin that it can be damaged very easily. Any damage caused to this lacquer influences the adjacent metallic layer causing a partial or total loss of information.
- This type of carrier has been available for some time, but it is still evolving (CD, DVD, etc.). In some cases, above all for recordable carriers, problems arise deriving from: oxidation, exposure to heat, damp and humidity, wear, and incompatibility between the materials.
- The increasing density of data, with the introduction also of techniques of multi-layer cutting, only makes the situation worse, by dangerously lowering the threshold of non-return (impossibility of reading the whole carrier).
- The temperature, in the room where the sound carriers are stored, should never exceed 25°C. The relative humidity should never exceed 55%. The optimal values are 19°C and 40% RH.
- Do not stack the sound carriers horizontally or in contact with materials that have an irregular surface. Physical deformations can be limited by placing the discs vertically and slightly compressed between each other.
- Do not touch the grooves of the discs, the recorded surface of tapes, and the reflecting surface of optical discs. After listening, always put the sound carriers back into their protective covers. Avoid dropping them.
- Avoid exposure of the sound carriers to direct sunlight, to other UV rays and to magnetic fields, such as: household appliances, electrical motors, loudspeakers, etc.
- Store all the tapes (including cassettes) wound to the end. In this way: 1) to reproduce a tape you are forced to rewind it completely, thus partly attenuating the print through effect; 2) a tape wound to the beginning tends to produce a pre-echo that, at times, can be annoying (for analog recordings). If the tape is wound to the end, this pre-echo will become instead a post-echo, with a much more acceptable result.
- Wind and rewind the tapes at least once a year to eliminate the forces created during archiving and to maintain the print through effect within acceptable levels.
- In case of doubt or for any additional information, or for simple curiosity, please do not hesitate to contact the Swiss National Sound Archives.
The Swiss National Sound Archives is part of the Swiss National Library