Homebrew PCBs

There is a special type of material that is used on PCB's for membrane-switch 
contacts.  I believe it's carbon, and usually laid out in some "interlocking" 
fashion to ensure contact/bridging when the membrane touches it.  Seems to be 
made directly on the PCB.  Anyone know what this is, and how it's made?

No, I don't need any, but would like to know what it is exactly and how it's 
done.

I'm sure I have something with it here, and will take a pic when I find it.

Cheers,
-Neil.