Compact Disc Bronzing

Compact Disc Bronzing

CD bronzing is a specific variant of disc rot, a type of corrosion that affects the reflective layer of CDs and renders them unreadable over time. The phenomenon was first reported by John McKelvey in the September/October 1994 issue of American Record Guide.

Affected discs show an uneven brownish discolouring that usually starts at the edge of the disc and slowly works its way towards the center. The top or label layer is affected before the bottom layer. The disc becomes progressively darker over time; tracks at the end of the disc show an increasing number of disc read errors before becoming unplayable.

CD bronzing seems to occur mostly with audio CDs manufactured by Philips and Dupont Optical (PDO) at their plant in Blackburn, Lancashire, UK, between the years 1988 and 1993. Most, but not all, of these discs have "Made in U.K. by PDO" etched into them (see image). Discs manufactured by PDO in other countries do not seem to be affected. A similar, if considerably less widespread problem occurred with discs manufactured by Optical Media Storage (Opti.Me.S) in Italy.

PDO acknowledged that the problem was due to a manufacturing error on its part, but gave different explanations for the problem. The most widely acknowledged explanation is that the lacquer used to coat the discs was not resistant to the sulphur content of the paper in the booklets, which led to the corrosion of the aluminium layer of the disc, even though PDO later said it was because "a silver coating had been used on its discs instead of the standard gold." Peter Copeland of the British Library Sound Archive confirmed that silver instead of aluminium in the reflective layer of the CD would react with sulphuriferous sleeves, forming silver sulphate, which has a bronze colour. A combination of the two factors seems likely because. As Barbara Hirsch of the University of California points out, the oxidation could only have occurred if the protective lacquer did not seal the metal film and substrate well enough.

There are also isolated reports of CD discolouring with discs from other pressing plants, but these do not seem to be as widespread and may be due to other reasons than the manufacturing error that occurred at PDO. In particular, colour changes that occur along with the visible disintegration of the data layer (i.e. holes) are not typical of CD bronzing, but should be considered CD rot.

PDO also manufactured CDs that have an even yellowish-golden tint. This is due to a yellow dye in the disc and is unrelated to the bronzing effect.

As bronzing is a progressive effect that cannot be stopped, any PDO-manufactured CDs that are not yet showing any signs of bronzing by now are very likely safe.