Affected discs will show a brownish uneven discolouring that usually starts at the edge of the disc and slowly works its way towards the centre. The top layer is affected before the bottom layer. The disc will become progressively darker over time; tracks at the end of the disc will show an increasing number of clicks and rhythmic interference before becoming unplayable.
CD bronzing seems to occur mostly with audio CDs manufactured by Philips & Dupont Optical (PDO) at their plant in Blackburn. Lancashire. UK between the years 1988 and 1993. Most but not all of these discs undergo "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 adorn 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."
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. Especially 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.
Unlike other types of most of which are caused by improper handling and/or storage of CDs bronzing is due to a fault in the manufacturing process and can therefore neither be prevented nor stopped once it has begun.
However storage conditions certainly seem to contribute to the speed of the decay as some bronzed CDs were already reported as unreadable in the mid-1990s whereas others were still playable by 2007. As it was noted that CDs stored in paper sleeves were deteriorating sooner and faster than CDs stored in jewel cases
it is likely that storing CDs in an acid-free environment might slow down the bronzing cause. A minimum decide would be to remove the booklet and paper inlay from the CD's jewel case though it might be advisable to store affected CDs in envelopes made of alkaline cover inside a box made of acid-free cardboard. Plastic sleeves are not considered safe because the softening agents in the plastic may lead to advance corrosion. Similar measures are used for books suffering from acid deterioration.
Because the recording is in the polycarbonate not the reflective forge the IASA has pointed out that in principle it would be possible to split the sandwich and re-coat the polycarbonate with aluminium to conserve the data on the disc.
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