I tried to burn the Slack 13.0 iso with K3B.
It appeared to go as usual, but at the end confirmation failed.

The media is unable to boot.
When placed on the DVD-drive, it is not recognized by the system.

Is there a way I can rescue and use the messed-up media? (after all, isn't it rewriteable).
Thanks for reading this.

I'm no expert but I'm pretty sure the DVD will be messed up. You should always check the integrity of your media after writing. In K3B you can check the "Verify written data" setting on the writing tab of the project page. Alternatively you could just run :
- or /dev/cdrom depending.

As it's not rewriteable you've lost it and might as well throw it away !

Edit; I've just noticed you headlined this " my DVD-RW media" - are you aware a DVD-RW is rewriteable ?

If you have the iso file in the same PC, you can create a bootable thumbdrive/stick and proceed with the installation.

mount the iso
locate folder 'usb-and-pxe-installers'
read file 'README_USB.TXT'

Once you've created the bootable usb stick, use it to proceed with the install.

When you reach the dvd source stage, press CTRL-ALT-F2 or ALT-F2 and mount the partition & the iso. The path for the source files is 'iso.mount.point/slackware'. Don't use /mnt to mount your partition/iso.

It's faster to install this way and I don't need any dvds at all.

Thanks for the info.
I also apologize for the writing of my initial post.
I meant:
because I try to use RW media whenever I can, for environmental reasons. That's why it bothers me having to throw away media.

All in all, the flashdrive method seems to be more efficient and sensible. I'll definitely try doing it that way.

Had this issue as well, the media is probably not recoverable (since it's not a dvd-rw) - if you had your disk(s) waiting at 99% to get ready for ages (and nothing happening beyond that)

What you can do instead is burn your iso from the command-line, like this:

growisofs -dvd-compat -Z /dev/dvd=Slackware13.0.iso -speed=2

Mind that /dev/dvd should be replaced to your actual dvd writer, probably /dev/writer
This will result in a bootable dvd, given that the iso image is correct.

K3b can also erase a rw disc before writing to it.
Sometimes it works for me and sometimes it doesn't.