Experiment 51: Electric Resistance Furnace

Electric resistance furnaces differ from electric arc furnaces (like the one I made in Experiment 32) in that they use resistive heating (like a toaster) instead of giant lightning bolts to melt metal.  I had a hotplate that died (because a container holding hot sodium hydroxide solution broke on it), so I decided to use its heating elements to make a small furnace.

First, I played with the resistance wire to see what sort of current made it glow.  I found that 12 volts made a 7" piece glow red hot.  However, it didn't melt the wire, which was good.  I calculated the current using Ohm's law and determined how much wire I would need to use to get the same current at 120 volts.  Actually, the math is pretty simple--I needed to use ten times as much, or 70".

I used a hollowed-out low density soft alumina-silica firebrick as the furnace body.  It had seen previous service as an arc furnace, so it was fairly beat up and I didn't care if it broke.  I also used another piece of firebrick as an insulating lid.  Fitting 70" of resistance wire in a 2" diameter hole was difficult, but coiling the wire seemed to work well.  For connecting the mains electricity to the ends of the wire, I made clamps using small nuts and bolts that sandwhiched the wire snugly.

My crucible was a cut-off soup can bottom, and my metal of choice was aluminum.  Plugging the furnace into the outlet made the heating elements glow, but the really neat thing about this furnace was that it was silent.  I even read a book while I waited for my metal to melt!  An hour later, I checked on the melt, and the aluminum was undoubtedly molten!  I wanted to grow aluminum crystals inside a blob, so I tried pouring the molten aluminum onto some glass to insulate it better.  That was a rather bad idea, because the glass exploded, and molten aluminum was dispersed about the room.  Oops!

Even though the crystal growing didn't work out, I was quite impressed that aluminum could be melted silently with just some electricity!  Of all the metal-melting methods I have tried (there are at least 14 of them), this is one of the quietest.