FYI this is why its not a good idea to have large extension cords coiled up while in use, not so bad for a work light, but high current here can burn the cord, which in turn burns the shop. As has done in the past to too many unwary people.

I'm curious now, I figured that it was some kind of field, and obviously running a high current in loops will create a magnetic field. But how does this boil water or melt copper or have an effect on other non-ferrous material?

the high magnetic field induces and electric current in a metallic object whether ferrous or not. it is really an AC magnetic field and it switches back and forth in polarity
it wont work with DC except for one initial pulse

Is it possible to forge weld with induction heat without creating an inert atmosphere around the workpiece, or is the environment too oxygen-rich? I've seen lots of videos about heating/melting metal but not one example of forge welding.  I'm interested to know whether anybody here has done it, and how they go about it!

Yep,  not only is it possible its much simpler than welding any other way.   There are size constraints depending on machine size.  I weld bits in tomahawks in the induction forge and its painless and simple.    The parts get hot so fast there is no time for scale to form (if you are on it)    

 

Grant used to do a demo where he would just stick two half inch round bars in the coil and when they got to temp just twist them together in a butt weld.   

Awesome.  I'm psyched about this.  

Give me a few years, and I'll be looking into buying one!

in a well ventilated shop it would be possible to get a breeze of CO2 or similar, should not need a great deal and could be done through a solenoid valve like on a mig welder so it only operates when the heat is on and then only if wanted.

you could use an old mig torch just connected to the gas line in order to direct the gas where wanted and have a relay on your foot switch or whatever you are using to turn the heat on to operate the solenoid valve at the same time.

 

dont know if it is needed