Electric Zinc Melter

I was looking for a simple way to melt small quantities of zinc for casting, and after much puzzling about how to do this in a cheap, safe, low maintenance way came up with the design described herein.

Basically it's a steel walled, insulated, resistance furnace. It has quite a modest operating temperature range, but I only need 4-600C to melt the zinc at a reasonable rate.

What's good about the design:

The basic construction technique is this:

  1. Get an old propane tank (free for the asking if it's past its 7 year life). Remove the valve, and wrench off the carrying ring at the top.
  2. Cut the top out pumpkin style (there are safe techniques for cutting old propane tanks, and then there are suicidally stupid ones. Pick one of the safe ones.)
  3. Find an old stove on it's way to the junk heap and take out the large elements, the pans they sit in, the high temperature wiring that supplies them, the connectors that hold them, and the control knob (with timer if possible) that controls them. Make sure the element and its ring/pan fit through the hole in the top of the tank. Hint - A notch in the edge of the hole let's you fit a bigger ring without making the hole itself bigger.
  4. Drill three holes around the lower periphery, and install jam nutted bolts so that their heads support the element ring at a heigh you like.
  5. Cut a square hole in the side of the tank near the bottom so that it lines up with the hole in the element pan. This is where the wiring will run.
  6. Weld four tabs of metal into the sides of the square hole, long enougth to touch the element pan. These tabs form the walls of the wiring chanel through the insulation, and provide a grounded shell so that if something goes wrong with the element wiring there's less of a chance that the electricity will come out and bite you.
  7. Weld up the hinge for the lid. I suggest you balance things so that the lid wants to stay open in its full up position, or put in a place to stick a pin to hold it open.
  8. Bolt an electrical junction box to the outside of the tank, at a convenient position. Make the box big enough to hold the controller from the stove.
  9. Bolt or weld a piece of steel channel onto the side of the tank to form a conduit from the junction box to the square hole. The high temperature wiring will run through this channel. The idea, again, is to keep the wiring protected inside a solid, conductive envelope.
  10. Make up a shield that protects the controller box from impacts/spills of molten metal from above. This should be solidly welded to the tank.
  11. Mix up some plaster/vermiculite insulation. A method that worked for me was to add water to a bucket of vermiculite until it was thorougly soaked (but no standing water), then pour on and knead in plaster powder until the whole mess was sticky enough to pack together, but still noticably granular.
  12. Pack this mixture onto the bottom of the tank, around the tabs and bolts, and press the element ring an pan into it so that it rests on the bolt heads.
  13. Clean up the wiring channel and the pan while the plaster is still setting, as this is much easier before the stuff fully hardens.
  14. Mix up more plaster/vermiculite, and pack a nice even coating all the way around, and up the walls of the tank. Make it a bit smaller than you want the final insulation thickness to be. Again, carve to shape while it's still only partially set.
  15. Drill a pattern of holes in the lid, and loop steel wire though them to create a support web for the lid insulation (which would otherwise fall off)
  16. Mix up more plaster/vermiculite and form a nice pie of insulation on the inside of the lid. Pay attention to the swing path of the lid. At this stage leave a nice big gap (4mm or so) between the lid insulation and the tank insulation.
  17. Mix up some pure plaster, nice an wet, and use it to "wash" the inside of the tank and lid insulation with a thin (3mm), smooth layer of plaster. This helps to protect the vermiculite mix, which is otherwise a bit crumbly, and let's you fill in some of the rough texture.
  18. You'll need to do some finish carving on the lid/cast insulation junction to get it to seat nicely. The smaller the gap the better at this stage.
  19. Drill a drain hole (13mm or so) at the bottom out through the hole in the element pan. This way if your crucible springs a leak it won't bugger up the element and wiring.
  20. Now install the element, and wire it up with some of the high temperature wiring you took out of the stove.
  21. Wire up the controller, watching your voltages.
  22. Gingerly power the thing up, checking the shell and element for voltages to make sure no power is going where you don't want it to.
  23. Starting at low power (1) let the thing dry out for a day. Don't go higher than 3 until the insulation is dry to the touch. I suggest moving the power setting up one notch every 12 hours or so until the element is glowing a nice deep red.

For a crucible I use a cheap stainless steel pot that nicely fits the chamber with about 30mm to spare on each side. So far the zinc shows no sign of interacting with the metal.

Here's a labelled cross section view:

I've used mine a couple of times now. Once to make sure it runs, and once to cast a wiggly shaped blob. I'll post pics at some point.

Boing