HW: "the book" from Voiceprint

[K Henderson]

A quick tech question...

I've long wondered why some CDs have a strange two-tone hue to the playing
surface...you know, it looks like in the manufacturing process there's some
form of lacquer or something going down onto the Aluminum surface before
encasement in the acrylic (or whatever plastic it is).  And then for some
reason, the 'lacquer' doesn't coat evenly over the surface and forms an
irregular boundary somewhere in the middle of the disc (not to be confused
with the very regular boundary that is often visible which represents the
point at which the music 'runs out' from inside-to-outside).  I've got a lot
of discs with this strange two-toned hue, but none of them ever have any
problems being played or read.  But what I wonder is what causes it, and is
it really some sort of defect?  If so, then why does it happen so often?

Just curious on a cold January day...Grakkl (FAA)