Propane Metal Melting Furnace

I’ve been interested in building a furnace ever since I got hooked on metal casting videos on YouTube. Casting metal was something that always felt out of reach. Most because of the equipment needed, but also the leap in knowledge as well as temperatures needed compared to something like making tin soldiers.

The metals I want to try casting are primarily aluminium and brass, and if things work out, maybe copper too. These don’t need crazy temperatures like steel. Once you venture into steel melting temperatures everything gets way harder and more expensive. To melt brass, copper and aluminium you can pretty much use tools and materials used for pottery.

Furnace Material Selection

After watching a few hours of YouTube videos I found that the most common beginner furnace is the sand+plaster of Paris type. This is really simple to make and uses readily available materials. The drawback seemed to be that it crumbles after just a few uses.

While looking for suitable plaster of Paris here in Sweden, I quickly found out that it would cost about as much as using fire bricks. These were about 30SEK (USD 3,5) per piece at the local hardware store and are rated up to 1270°C. That seemed much better than replacing USD 20 worth of plaster of Paris every now and then.

Furnace Construction

After some napkin sketches and calculations, I made up a simple plan of a octagonal furnace design that needed about 12-15 bricks depending on the lid and bottom construction. From here on, the plans pretty much ended and I decided to go one a “just build it” basis, so unfortunately, there are no sketches or drawings of anything.

I decided to go for a propane to power the furnace since it’s easy to get hold of and I could use a “weed burner” as the burner. The plan was to make the octagonal shape with an angled inlet to make the fire/hot air circulate around the crucible and then a smaller hole in the lid as the exhaust.

Shaping the Firebricks

For an octagonal chamber, I needed to miter the firebricks using 22.5° (360/16) sides. First thought was to use a diamond cutting wheel but I quickly found out that the diamond grinding cup was much faster. I calculated how far in I needed to go to get the 22.5° and drew a line to aim for. An appropriate breathing mask, as well as eye and ear protection were mandatory doing this.

Soaking the bricks made a great improvement in decreasing the dust.

Inlet

For the inlet I measured the nozzle diameter of my weed burner and drew a circle about 10 mm larger. Mine, as most furnace designs, do angle the inlet so that the hot gasses circle around the crucible for an even temperature. I have not seen any measurements or tests of this hypothesis but it makes sense.

After cutting the brick in half, I drew an projected angle and then used the grinding cup until the burner nozzle fit into it.

Box Construction

Construction of the box was pretty straightforward using angle iron for corners and sheet metal for the sides. I tried finding a suitable metal bucket or something similar that most people seem to use but didn’t find any suitable ones close by.

My welding skills have been slightly improved but still far from where I would like to be, I still use my MIG/MAG welder as a hot glue gun for metal. To quote the YouTuber and mining vernacularist AvE, “a grinder and paint makes me the welder I ain’t”.

What isn’t visible here is the inlet that was cut as well as an exhaust pipe from a car that serves as a holder and guide for the weed burner.

Lid Construction

The lid was made by enclosing three bricks in a frame out of angle iron. I cut a hole in the center brick using the same method as when cutting the inlet. The hole in the lid serves two purposes, the first and most important is to act as an exhaust, the second is as a simple way to add material to the crucible without removing the lid.

I also attached two handles made out of rebar which you can see in the photo of the finished furnace.

The angle iron were recessed into the bricks using the grinding cup. I also had to chamfer the edges of the bricks so that it matched the radius of the angle iron.

Insulation and Assembly

Ready for expanded clay/mortar mix infill.

The original plan was to use mineral wool or glass wool as an insulator but since I didn’t have a good (and not time-consuming) idea how to stabilize the bricks within the box I though of using expanded clay pebbles mixed with mortar. They insulate pretty well and seems to withstand the temperatures I will be using without issues.

Testing

This was my setup, weedburner and a PC10 propane bottle.

After letting the mortar and pebble mix fully cure for a few days I set up the propane tank and weed burner in the backyard. I got a low flame going for maybe 30 minutes to be sure there wasn’t any moisture trapped somewhere.

I put some aluminium scrap into an old steel canister and turned up the gas flow. After about 15-20 minutes the aluminium started to melt and I added some more.

First successful aluminium melting.

Closing Notes

I had fun making this project and as usual learned a few things. This being a bit off-the-cuff-building resulted in a few improvised solutions which in hindsight were not the best. If I were to make it again and allowing a higher budget I would have gone for insulating fire bricks instead, these tend to suck away some of the heat and it takes a good while before it gets hot. I have not yet tried to melt metals with a higher melting point such as brass or copper. I would also try to make it more modular so bricks and parts can be replaced, but this of course would add time and planning.

If anyone have any questions drop a comment below or contact me.

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