I don't know why, but suddenly I thought "Pyrography", (must be a Freudian thing), anyway I thought I'd have a look on YouTube, and of course someone else has. Take a look. Definately a path to go down at some time. Something else to use a CNC for.
I have just found this on the net. CNC Wood Burner Pyroprinter | CNC wood burner PyroPrinter Wow!! That's a lot of money for such a basic piece of equipment! I'm sure we could do this for a fraction of the price. However, I would be keen to see the program they use to control the speed as it burns, varying the shade of burn. Okay found this now: Interesting