If  I recall correctly Jim or somebody wanted to know how CD Read Writers
work. There is a great article in June of PC Computing describing the process.

In conventional readers first there is the Amorphous Phase:
A high Power infared laser beam selectively melts areas on a discs' silvery
layer. these burned area called pits are less reflective then non burned
areas or lands when read, pits scatter light and are recognized as 1's:
lands are read as 0s.
CD-ROM CD-RW's laser beam is made of infared light, 780 nanometers wide.
the wavelength determines how small pits can be, which dictates how much
data the disc can hold. CD-RW stores 650MB.

With Read Write CDs there is a Annealing Phase....to erase or rewrite a
CD-RW or DVD disc the laser uses its high power beam. To convert a pit into
a land, the laser uses a lower energy beam that is only hot enough to
recrystallize the pit to it's original state.

Right now Read Writes are a 100+ dollars more then standard write once
burners but I think the price difference will soon be negligible eventually
replacing write once drives. The only really use I see for them is
repetitive backup and my guess is that  at present write only is a more
reliable medium. The read write disks are very expensive