Well, WB, I think the heat or thermal conductivity is what we are looking at here, coupled with the ignition temperature. Wood has a very low thermal conductivity but also a low ignition temperature (about 450 F). Ceramic and stone have about 10 times the conductivity of wood but an extremely high ignition temp. Steel has 260 times the conductivity of wood, a lower ignition temp than ceramic, but well above wood.I'd imagine the absorbtion to be negligible. Could be wrong.
BTW - what's a 'thet'? Normally I wouldn't say anything, but with you being the resident Conan the Gramarian...well I hold you to a higher standard then others. Thing with standards is once you loose your them, than it gets harder to get them back.
Did I get thet right?
Thus the ideal knob material for a firescreen would have about the same conductivity as wood but a higher ignition temperature to avoid the scorch that Ian and others are a bit concerned about. Frankly, I'm not sure what that common material is.