Efficient Swedish Furnace

About: Teaching and making electronics since - well I don't even know

You know why is it called "swedish" - the buckets come from that famous furniture store. Now you know where to get some steel planters/buckets, the big one for structural reasons, and the small one hopefully can be used as a crucible. Easy to get, easy to replace!

The project goal is simple:

  • A simple furnace to build
  • For metal like aluminium, copper, brass
  • No special tools needed to keep costs at minimum (but at least a lathe would have been REALLY useful to make the rotating joint)
  • Raise incoming air temperature with the excess heat

I mostly want to melt brass. The fire cement I used is good up to 1100°C only, so no steel melting, also the steel tubing would melt at those really high temperatures - duh.

It's not too conspicuous at first sight, but it's not a huge monolithic "just fill it with cement" type of average DIY furnace. It consists of two halves - a "plug", the top part with the removable air inlet + heat exchanger and the place where the crucible goes, and the bottom part, the fire chamber. The plug is much narrower, so the bottom can have much more coal burning happily, the steel tube blows fresh and hot air to rise efficiency, so no drilling vents on the side this time.

Supplies:

  • Fire cement - I used 15 kg
  • Filler: tube, carbdoard, foam, whatever you can shape easily for the fire chamber and "chimney"
  • Flexible steel hose for air, something between 1/2" and 1" in diameter should be OK
  • A big steel bucket or planter and a small one
  • Plastic wrap

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Step 1: Forming the Fire Chamber

Just prepare the fire cement as the manual says. I used 5 kg for this part. Since it will be the bottom layer, mixing was done in the big bucket. No one likes to clean the tools and the mixing bucket anyway!

I used some scrap aluminium flexible ducting, about 20 cm in diameter. A plastic bucket would do too. Wrap it in plastic, so it will be easier (and cleaner) to remove. Now let the fun begin: use the steel hose to form its place by adding it to our sculpture.

Form the fire chamber - push it after mixing the cement or layering, you decide. Let it dry, mine needs 24-48 hours... When it's finished, you can easily pull the tube, it's clean, ready to reuse.

Step 2: Preparing for Round 2

The final version should look like this:

  • the steel tube is in place
  • the "chimney" can be formed
  • can pour the new mix of cement on the top
  • and this should be removed

So I used some cardboard inserts to fill the fire chamber (not just one, a lot...) - this will be the support for the next round. Leave some space for the steel tube!

Step 3: Preparing YourTubes

Now search for something narrower than the chamber for the chimney, but not too narrow - we have to leave some space for the tubing! This coffee pot is just not big enough! I had some 15 cm ducting too, easy.

Wrap the chimney in plastic, and put everything to their final place with the cardboards in place.

Step 4: The Plug

Get all the residue off, because it's disgusting! We are making a beautiful little furnace!

Now comes the most annoying part: making a little plastic layer between the chamber and the plug. We want it to be removable, and breaking from the metal can lead to a catastrophe. Be on the safe side. Lay a little layer on the bottom, on the inner wall, drop some F-bombs because the wrap will stick everywhere, so be very patient! Or use tinfoil? Use whatever you want!

And fill the remaining part with fire cement. Let it dry.

Step 5: Pop and Snake

If everything went fine and the drying time passed, the plug just pops out. The chimney - not so easy. But the inner surface looks good and only that matters.

We "forgot" to make a platform for the crucible, it will just fall in the fire!. Correct our mistake!

Form a holder for our cup. It's not that easy, I know. We use a steel tube here! It says "flexible" on the label, but it's pretty stiff! At least it will hold our steel cup. But how do we blow air THERE?

Step 6: The Crown

The swedish king has a crown, our swedish furnace can have one too!

This is where a custom steel rotating joint would come handy. We have to join the fixed tube with the removable/rotating heat exchanger. This type of tube can be joined with some creativity. No one said it's easy. I broke 2 pipe clamps when tried to shrink the crown's end. Loosening the other end was easier. Some pipe fittings would solve this problem, but I only found brass in the local stores, and brass would probably melt.

This is where a custom rotating steel joint would come handy, but no one will buy a lathe for this.

So, the furnace is finally ready to go! I had only so short steel tube, you can use a longer one. Blow cold air from this side, melt brass in the inside! But one last thing: my fire cement stated: it should be operated first only between 750-800°C first!

Thanks for reading!

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