Laserdiscs are subject to what is commonly called laser rot, the deterioration of the aluminum layer due to oxidation or other chemical changes. This often results from the use of insufficiently pure metal for the reflective coating created during replication, but it can be exacerbated by mechanical shear stress due to bending, warping, or thermal cycles.

The large size of laserdiscs makes them flexible, so that movement along the bond between layers can break the seal, which is called delamination. Deterioration of the data layer can be caused by chemical contaminants or gasses in the glue, or by moisture that penetrates the plastic substrate.

Like laserdiscs, DVDs are made of two platters glued together, but DVDs are more rigid and use newer adhesives. DVDs are molded from polycarbonate, which absorbs about 10 times less moisture than the slightly hygroscopic acrylic (PMMA) used for laserdiscs. DVDs can have delamination problems, partly because some cases or players hold too tightly to the inner hub of the disc.

Delamination by itself can cause problems (because the data layer is no longer at the correct distance from the surface) and can also lead to oxidation. So far DVDs have had few “DVD rot” problems. Reports have been made of a few discs going bad, possibly due to delamination, contaminated adhesive, chemical reactions, or oxidation of the reflective layer (see www. mindspring.com/~yerington/ and www.andraste.org/discfault/discfault. htm).

Occurrences of “cloudiness” or “milkiness” in DVDs have been reported, a possible cause being improper replication. An example of this would be when the molten plastic cools off too fast or isn’t under enough pressure to completely fill all the bumps in the mold (see www. tapediscbusiness.com/issues/1998/0998/cloud.htm).

Minimal clouding doesn’t hurt playback and doesn’t seem to deteriorate. If you can see something with your naked eye, it is probably not oxidation or other deterioration. The result of deterioration is that a disc that played perfectly when it was new develops problems later, such as skipping, freezing, or picture breakup.

If a disc seems to go bad, make sure it’s not dirty, scratched, or warped. Try cleaning it and try playing it in other players. If the disc consistently has problems, it may have deteriorated. If so, you can’t do anything to fix it. Request a replacement from the supplier.