The world is always evolving fast and new technologies emerging, like metal 3D printing, but sometimes it just feels good to do things the “old” way. Don’t get me wrong, I love technology but still is great to learn old techniques specially when they tend to disappear when less and less people actually care to learn them and the craft becomes closer to extinction. Not that this is the case, although a few foundries have been closing over the last years. Be that an excuse or not, Gonçalo and me decided to build a simple DIY furnace, not only for the reasons above but we’re crazy about forging and metal working, and we would love to create some amazing metal pieces. Besides this is a dangerous project, what more do I need to say?!!
First you need an empty gas cylinder, that will be the main structure of the furnace. Now, probably the most important part: before working on it, you must prepare it, so it is safe to use. How? Well first make sure it is really empty, next take it outdoors and remove the top connector, you might need a big wrench or special tool for it. Then be careful because even though it is empty, there is still quite a bit of gas trapped inside the tank. To get rid of it, you must fill the tank with water and then pour the water out of it. It will wash the gas off the tank. Then you should do it again just to be sure, fill it with water and pour it away. Now the gas cylinder should be safe to use.
We started by cutting off the top handle of the tank, we don’t want it there, besides it will be useful later. Next, we draw a line around the tank to mark where we’ll cut it open and drilled a hole on the top with a hole saw.
The top part of the tank will be the lid, but now we need an hinge. So we got a piece of iron strip, an M8 threaded rod, some washers and nuts, and welded together an hinge on the back.
We made it look like an handle for 2 reasons: first it will serve as an actual handle to move the furnace around; second, the shape of the hinge will serve as a stop to keep the lid vertical on the open position. It’s not the prettiest welder in the world but it works.
Using more iron strips we made 2 more handles on the front side. One on the lid to open and close the furnace and one on the tank to serve as a support to pick up the furnace and carry it around, together with the hinge handle on the back.
Next we made a hole on the side of the tank, closer to back. This will be the firing entrance for the torch. It should be made with a small angle, usually around 10º and 20º, and not towards the center, because the goal is to make a fire spiral inside, this will be more efficient. It should also have a small downwards angle, in case there is an accident with the crucible, the liquid metal won’t come out of there through the firing hole. We also welded a small protection around the hole.
Now it’s time to prepare the thermal insulation of the furnace. For this we used refractory cement. The one we got holds up to 1200ºC, it should be higher, but this will work too. Mix it well with water and then pour it inside the furnace. The cylinder’s top handle we cut before, we welded it now inside the furnace to reinforce the concrete. We used a small 32mm PVC tube to make the space for the firing hole, and a piece of rolled cardboard to make the hole in the middle of the furnace. Disassemble the lid, welded some iron bars also to reinforce the lid, put a plastic cup or a tube on the top hole, and fill the lid with cement too. Then let them dry up for a few days on a warm place, preferably on the sun. After that the furnace is ready to fire.
Next we sanded the cylinder a little bit to remove any imperfections.
And then we painted it black!!
And this is the final result:
We also made a small iron base for the crucible inside, so that the flames can heat up the bottom of the crucible.
By the way, for the crucible, we are using a piece of iron tube welded to a circular iron base to make a cup for melting the metal.
With the furnace ready we have to start working on the blowtorch. For this we bought a cheap one and adapted a longer output tube with some holes and an adapter to increase or decrease the air intake:
We are using a normal gas hose, but it has a maximum pressure of 0.2 Bar which is way below our needs. Usually this stuff requires pressures between 1 and 2 Bar. Still, our blowtorch has a tap and we’ll keep it open all the time, turning on and off the gas, directly on the bottle, this way, no pressure will be put on the hose itself. If you do the same, make sure the blowtorch tap is always open, otherwise the pressure can blow up the hose.
As a power source we have a Repsol propane cylinder with an adjustable pressure connector:
With all this done, it’s time to fire it up! We turned on the gas and lit it up with a barbecue lighter. And that’s it:
Of course we immediately started melting anything we could find. Starting by aluminium (660ºC), moving on to brass (930ºC) and eventually copper (1084ºC).
We used plaster of Paris and sand to make a mold out of 3D printed pieces:
We even managed to make the Hand of the King from Game of Thrones:
Among other random stuff and ingots:
We also presented this project at Lisbon Maker Faire 2016 which it received a “Maker of Merit” award:
And finally, here’s a video of pouring melted brass onto a oven tray for brass ingots:
Don’t forget this is a very dangerous project. There are lots of hazards working with fire, melted metal and gas. You should always wear appropriate safety gear when working with it. High temperature gloves, boots, jacket, pants and mask. If you’re not sure about it, maybe it’s better you don’t try it at all.