Furnace Design Considerations

A foundry is an operation that does two things. It first melts metal in a furnace and second casts the molten metal in a mold, forming some useful shape as it solidifies. There are a number of design factors when making a backyard foundry:

What kinds of metal are to be cast? 

The kinds of metals you can cast is primarily a function of the heat of the furnace. Melting points of some familiar metals are:

  • Pewter (an alloy of tin and other stuff): about 400°F
  • Tin: 450°F
  • Lead: 621°F
  • Zinc: 787°F
  • Aluminum: 1220°F
  • Brass (an alloy of copper and zinc): about 1700°F
  • Bronze (an alloy of copper and tin): 1742°F
  • Silver: 1763°F
  • Gold: 1984°F
  • Copper: 1984°F
  • Iron: 2800°F

So as you can see, pewter, tin and lead are pretty easy to melt — heck, you could melt them in your kitchen stove — but also tend to make things that are pretty soft. Zinc is mostly used in alloys, and as we will see in the next episode, is insanely dangerous. Gold and silver are precious; I’m not interested in jewelry making here. Copper needs to get very hot, and iron is a whole other ballpark. That’s out of my league.

But melting aluminum, brass and bronze seems to be within reach, and these metals are hard enough to build tools yet soft enough to machine with easily-available steel bits. We can melt aluminum with ordinary charcoal or propane.

How heavy is the furnace?

I intend this furnace to be portable; I’ll store it in my detached garage when not in use, and run it in the back yard. This means that I’ll need to be able to move it, either by lifting it, or with a hand truck.

Let’s suppose that a furnace is a cylinder that is partially empty and partially full of cement. And let’s suppose, to make estimation easier, that the cylinder is as tall as it is wide. About two thirds of the volume of the cylinder will be full of cement, and about one third will be empty. Cement is approximately three times as dense as water. So the weight of a cylindrical furnace is twice the weight of the same cylinder full of water. Water weighs one tonnes per cubic metre, and the volume of a cylinder is pi times the height times the radius squared; the height, we’ve already said, is equal to the diameter.

So roughly we can expect that a furnace weighs:

  • 12kg if it is 20 cm in diameter – a paint can
  • 24kg if it is 25 cm in diameter – a normal bucket
  • 41kg if it is 30 cm in diameter – large bucket
  • 66kg if it is 35 cm in diameter – a small trash can
  • 100kg if it is 40 cm in diameter – wait, 100 kg is way too heavy

(American readers who do not yet know the metric system: a kg is about 2 lbs. 30 cm is about a foot.)

These numbers are rough, but they seem plausible.

How much metal can we melt at a time?

Molten metal resolidifies surprisingly quickly, so you want to make more than you need for a given casting. You don’t ever want to make a partial casting, then go back and melt more metal. So how much we make at a time is important.

We said that the interior of the furnace is about a third of its total volume. Let’s suppose that the size of the crucible – the removable container that holds the molten metal – is about half the remaining volume, and that we’re going to fill it two-thirds full of molten metal. So that’s one-ninth the total volume of the furnace devoted to the actual metal, which will be slightly less dense than the concrete.  

Heck, let’s just make the math easy. The maximum mass of metal we can melt will be on the order of a tenth the mass of the furnace. So maybe one kilogram for the smallest furnace and maybe four kilograms for the “large bucket” furnace.

What’s the fuel?

The two basic fuels for backyard metalcasting are charcoal and propane. Charcoal has, of course, been used as metal casting fuel for six thousand years; propane, somewhat less. Both are inexpensive and widely available. Propane furnaces apparently require a custom-built burner apparatus and pressure regulator. (The valve on a barbecue tank regulates rate of flow, not pressure.)

There are of course safety issues to consider: propane is a highly flammable gas delivered at liquification pressure which will explode in a huge fireball of vaporized accelerant if heated excessively. Bags of charcoal, by contrast, seldom explode.

Charcoal does present two inconveniences: it produces ash, which could get impurities into the molten metal (where they will float) and is generaly irritating to clean up, and second, the combustion chamber must be rather larger to accomodate the bulkiness of the fuel.

However, choice of fuel actually is one of the criteria that impacts the design the least, because both fuels require basically the same furnace design. That is, there must be a pipe at the bottom of the furnace that admits either blown air, in the case of a charcoal burner, or the propane. It should therefore be relatively easy to change fuels if necessary.

Summing up

A 30 cm diameter cylindrical furnace will weigh about 40 kg, be easily moved with a hand truck, take up as much space as a large bucket, and enable melting of several kg of aluminum at a time. The interior chamber will be plenty large enough for charcoal and a good sized crucible.

Next time: some thoughts on furnace materials.

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