Building a Furnace

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The following article was posted on Home Foundry Yahoo Group

Sun Apr 28, 2013 3:09 pm (PDT

Every furnace is a trade off of parameters.

High temp refractory holds up well against breakdown at high temperatures, but is a poor(er) insulator. It takes longer to heat, given the same heat input, and the outside gets hotter faster. Your 3,000 degree specs require a less insulative, higher temp refractory. Still, the high temp refractory won't make as much difference in melt times as other choices will.

The "make your own or buy commercial refractory" debate isn't ever won by either side. People have successfully made their own, others like me choose to buy the commercial product. A higher temperature refractory may be harder to DIY than a lower temp version.

A furnace and it's lining are consumables, it will wear out in time, and you shouldn't try to make your first furnace perfect, because your needs will change - if you are deeply invested in your furnace, you may never admit to yourself that a different size/shape/lining/burner would be better. I've hobbled along with an over-thick walled, high temp refractory, too small furnace for years, so I speak with authority.

Almost any containment vehicle will allow you to melt aluminum. My friend Joe hot-shorts and inadvertently melts aluminum wheels in an uninsulated large diameter steel drum set on it's side and open to the atmosphere. If you add enough more heat than you lose to the air around it, you can melt metal in open air (see welding).

How to make one particular kind of furnace.

  1. Decide on a fuel: propane, natural gas, coal, coke, charcoal, wood, used motor oil, vegetable oil, electricity, etc
  2. If natural gas or propane, decide on a burner style: blown or naturally aspirated
  3. If propane, research burners, then build a naturally aspirated burner from Mike Porter's book "Gas Burners for Forges, Furnaces, and Kilns"
  4. Choose and buy a size of crucible, unless you will melt directly in the containment vessel.
  5. Design and build tongs to handle the crucible with, either side grab tongs-shank combo for a top hat furnace, or standard grip-and-raise-out-of-the-furnace-tongs if building a fixed base-wall furnace.
  6. If making a fixed shell furnace, measure the clearance needed to get the tongs into and around the crucible, this is usually the determining internal size for a fixed base-wall furnace. It should allow 1" to 2" of flame flow around the crucible.
  7. Find a sacrificial form to use for the inside diameter.
  8. Find a form for the shell that will hold up to moving the furnace from place to place. High temp refractory is heavy, so over-build. Allow for 2" or more of refractory.
  9. Make a form for the lid, again with a sacrificial form for the hole in the lid (3" - 5" for your stated size, larger lets bigger metal in for filling the pot but loses more heat.)
  10. Cast the bottom, with provisions for a drain hole with a removable plug if so desired (I don't want one, my friend does). Include a replaceable plinth to set the crucible on. You may want a shallow depression to hold the plinth in place. Cast plinths from the extra refractory you mixed up but can't use. Have molds pre-made for plinths.
  11. Make holes in the shell and inside form for a wooden plug that is a CLOSE fit for your burner nozzle - you have already made a burner, haven't you?
  12. If the bottom you previously cast is firm, set the inside form and wood plug in place and mix and ram the refractory between the inside form and the shell of the furnace, being careful to align the wooden plug so the flame front is tangential to the plinth-crucible, i.e., the flame is not impinging on the plinth or crucible. Pay special attention to forming refractory around the wooden plug.
  13. Cast the lid.
  14. Dry everything, air dry to start for a few days, stick an incandescent light bulb in for a couple of days, remove the wood plug and use the burner to heat it up a little bit, when it stops steaming heat a little more, repeat.
  15. When fully dry, _ No More Steam _ run at full temperature to red heat to vitrify the refractory.

When done, you will have a standard propane fueled fixed shell furnace with a removeable lid and replaceable plinth blocks. You'll need propane, a hose, fittings, and a higher pressure gas regulator than you can buy at a barbecue store. You already have a crucible and tongs, so build a pouring shank, get some greensand, make flasks, and create castings!!!

Note that every possible choice I glossed over allows multiple variations, and I described ONLY ONE possibility.

I have prejudices:

  1. Mike Porter is a personal friend, with too much knowledge in his brain! Just listening to him stirs the gray matter - and he likes to inspire people to try new stuff.
  2. We tried oil fired. It gets really hot, and is smelly and messy, and I don't like it at all.
  3. I have a heavy, high temp blown furnace - sounds like a loud jet engine and is so noisy you can't converse anywhere around it. I would never build another like it! It is the major reason I like naturally aspirated burners, which sound like a jet engine, and are somewhat difficult to converse around... ;-))
  4. A top hat furnace is great for a crucible one man can lift - 10 pounds of metal in a six pound crucible requires good arm strength when it's hanging out there on the end of a three foot long shank/tong device, with it's own inherent weight. There are ways around this that add complexity.
  5. ANY refractory, even rammed earth, especially clay soil, is satisfactory if it doesn't collapse and ruin the melt.

In Portland, you can buy refractory at La Grande Industrial Supply.


Rex Bosse

Portland, Oregon