This article is from the CD-Recordable FAQ, by Andy McFadden (firstname.lastname@example.org) with numerous contributions by others.
Disclaimer: I'm not recommending you put a CD into a microwave. CDs may
contain metals that will cause your microwave to arc, destroying the
microwave emitter (see cautions about metal objects in the manual for your
microwave). Don't try this at home. Better yet, don't try this at all.
The basic process is, take a disc that you don't want anymore, and put
it shiny-side-up on something like a coffee mug so it's nowhere near the
top, bottom, or sides of the microwave. (Actually, you may want to leave
it right-side-up if the disc doesn't have a label, because the foil is
closest to the top of the CD.) I'm told it is important to put something
in the cup to be on the safe side, so fill it most of the way with water.
Try to center it in the microwave. Turn off the lights. Program the
microwave for a 5-second burst on "high", and watch the fireworks.
Performing this operation on replicated CDs results in blue sparks that
dance along the CD, leaving fractal-ish patterns etched into the reflective
aluminum. For those of you not with the program, this also renders the CD
Trying this with a green/gold CD-R gives you a similar light show, but the
destruction patterns are different. While pressed CDs and CD-RWs don't
develop consistent patterns of destruction, CD-Rs tend to form circular
patterns, possibly because of the pre-formed spiral groove.
On a different note, CD-Rs seem to smell worse, or at least they start to
smell earlier, than pressed CDs. The materials used are non-toxic
("cyanine" comes from the color cyan, not from cyanide), but breathing the
fumes is something best avoided.
For the curious, here's a note about why they behave like they do:
"The aluminum layer in a CD-ROM is very thin. The microwave oven induces
large currents in the aluminum. This makes enough heat to vaporize the
aluminum. You then see a very small lightning storm as electric arcs go
through the vaporized aluminum. Within a few seconds there will be many
paths etched through the aluminum, leaving behind little metalic islands.
Some of the islands will be shaped so that they make very good microwave
antennas. These spots will focus the microwave energy, and get very hot.
Now you will see just a few bright spots spewing a lot of smoke. The good
part of the light show is over, turn off the oven.
I suspect that if you leave the oven going much longer, the CD-ROM will
burst into flame. This will smell very bad and may do bad things to your
oven and house. Don't do it."
-- Paul Haas (email@example.com), on http://hamjudo.com/notes/cdrom.html
Dreamcast GD-R discs come out just like CD-R, but DVD-R is a whole
Combining a microwaved CD-R with a tesla coil produces interesting results.