Achim Breiling asked:

>I also have a few discs that show a clear bronzing, but acustically I can
>not detect any problem.  Is it just a question of time till I will find
>some noise? Thus is it adviseable to ask for a replacement also if there
>is no acustic defect?

This is an excellent question, and I should have at least touched on it
in my original post.  There's no reason for the acetate that is used as
a clear, protective coating to be transparent at all frequencies of light.
All that is required is that the acetate be transparent at the frequency of
light used by the laser during playback.  Therefore, it is quite possible
and permissible that the acetate have a tint of its own.  This is not a
defect.  In fact, many of the discs PDO pressed have a golden hue to the
acetate and there's nothing defective about these discs.  There's also no
need for the reflective layer to be silver (actually pure aluminum).  It
can, for instance, be gold (some premium discs are made with 24kt gold
substrates on the premise that gold is a better reflector, and will never
corrode - examples being releases from Mobile Fidelity Sound Labs, Adda,
and Dabringhaus & Grimm).  Sometimes, if the substrate is not silver, this
can imbue the lacquer with a hue as well.

Bronzing of the type we're referring in this context is not subtle, though
it can start out that way.  In general, the corrosion does not affect the
entire disc evenly.  In all cases I'm aware of with respect to the discs
from PDO the bronzing starts at the outer edge first, and by the time the
playability becomes affected the bronzing is very pronounced.  Because of
the optical properties of CDs, its difficult to get a realistic scan of
one, but I've scanned two discs, one from Hyperion and another from ASV,
and made the images available at the web site.  The URLs are:

    http://www.classical.net/asv1.jpg
    http://www.classical.net/hyp1.jpg

Both of these discs are severely bronzed.  I should note that for these
scans I used a color temperature and gamma correction appropriate to a
standard PC monitor, so viewers on UNIX or Mac systems may see a washed-out
image.

I should also caution collectors to check their boxed sets.  In the
list for Hyperion, I included all the individual disc numbers, even for
multi-disc sets (Hyperion's numbering system makes this easy).  But other
labels may be problematic.  For instance, I just pulled my boxed set of
Scottish Renaissance polyphony by Robert Carver, and all three discs are
badly bronzed.  The set has the catalog number CDGAX319 and is not listed,
but the individual discs CDGAU124, 126 & 127 are listed as possibly
defective (and they certainly are in my case).  Looks like another note
to PDO is in order as I missed this set in my original letter to them.
Off to double check all my boxed sets...

Dave
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