Judging by the fact that they mentioned being in grad school at the time and that the laser was infrared, I imagine engraving steel isn't too far off from what they were using it for.
It's all about absorption spectrum. 40W at 10600nm will burn clean through a few mm of wood but could only maybe lightly etch brass and be utterly useless on aluminum. 40W of energy is being delivered, what happens to the thing it's being delivered to is entirely dependent on absorptive potential.
