Vinyl Record

Shellac Discs

Although commonly called shellac some discs of this type contain no shellac at all.  However shellac is normally present as a binding agent and a typical disc may contain 12% shellac by weight but 15% other gums and resins.  The main ingredients by weight are normally fillers such as whiting (CaCo3) and aluminium silicate.  Below are two chemical analyses of shellac discs showing the chemical make-up of the discs.

Shellac 13.5%
White filler (powdered Indiana limestone) 37.5%
Red filler (powdered Pennsylvania slate) 37.5%
Vinsol (a plastic with a low melting point) 8.5%
Congo Gum (a flexible binder) 1%
Carbon Black (colorant) 1.5%
Zinc stearate (lubricant for mould release) 0.5%

Flake shellac 15.63%
Congo Gum 6.51%
Vinsol Resin 5.86%
Carbon Black 2.61%
Zinc stearate 0.32%
Whiting (CaCO3) 52.13%)
Aluminium Silicate 13.03%
Flock (long fibre) 3.91%

Shellac gradually 'cures' becoming more brittle and therefore the newer the disc the less fragile it tends to be.  Eventually the shellac becomes fully cured.  In some cases this leads to the break up of the grooved surface even when the disc is played with a correct diameter stylus and lightweight pickup.  The curing process varies greatly but under correct storage conditions it can take up to 100 years.  Curing is accelerated by moisture.

Dry storage conditions are essential for shellacs as, besides curing, moisture attracts mould growth causing irreversible damage to the grooved surface.

Shellac discs are easily damaged by rough handling and in some cases can crack or even shatter.  Never touch the playing surface as the grooves are the most vulnerable part of the disc.

Before playback discs should be cleaned with a soft, clean, lint free cloth working round the disc in the direction of the grooves.  If the disc has attracted a lot of dirt particles then the disc may be, with care, washed in water (containing a small amount of mild detergent where applicable) and then rinsed in demineralised water.  Extreme care should however be taken as some shellacs may contain fillers and compounds which may absorb water.  Also some shellacs have a cardboard core which can often be identified through the centre hole.  On no account should such discs be cleaned in water.  Chemical cleaners should be avoided. 

Store the discs in acid-free sleeves out of direct sunlight and away from direct heat sources.  The discs should be stored vertically with dividers every 25 discs or so to defuse any lateral pressure.

RISKS  The discs are often embrittled and break easily.  They are prone to some mould growth and warping.  Scratches across the audio surface are commonplace as is groove deterioration caused by the use of steel & fibre needles.

CONSERVATION PRACTICE  The surface can be cleaned by hand using distilled water and a natural sponge.  A 5% solution of Synperonic-n may be added to the distilled water.  Avoid wetting the centre label.  Dry naturally or with a clean air jet.  Do not attempt to clean laminate discs (wool, cardboard etc.) using this method.  For playback use conical truncated or elliptical styli of the correct diameter to fit the groove width.  Hill and Dale discs, as opposed to lateral groove discs, need an appropriate stylus and pick-up arm for correct sound reproduction.

HANDLING  Fingers should be kept off the playing surface.  Use the fingers to support the centre label  and the thumb on the outside rim.  Cotton or latex gloves are recommended.

STORAGE  Store each disc vertically, ideally in an acid-free cover with dividers roughly every ten to fifteen discs to avoid lateral pressure.  As powdered or ground limestone is extensively used in manufacture floor and shelf loading must be carefully assessed.  Avoid temperatures above 15C and relative humidity in excess of 55%RH.


Groove of a shellac disc showing damage caused by using the wrong diameter stylus

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