This is the first charcoal furnace that I built its purpose melting small amounts of aluminium into ingots or sand castings for small objects. I don't predict a long "shelf life" for my furnace before it will start cracking but I will get a few melts before I scrap it, This is just a prototype and hopefully in the future I will make a bigger and better furnace.
It's easy and quick to make, I will show in this instructable the steps on how to make it using materials some of us have in our house. The only "big" money might be the refractory materials and characoal.

A few details about my furnace: I made it all 2" thick around including the furnace lid. I used Plywood, MDF, cardboard, chipboard all left overs from other jobs.
The furnace tools I made from a 5mm construction rebar, can of beans and old bathrooms hooks.

In the near future I hope to add to this instructable a few images of my first sand casting.

A few Saftey words: Coming into contact with fire and boiling melted metal can be very dangerous so always work safe using proper full body protection including respirator mask against toxic fumes and dust from mixing refractory materials. Never melt in a wet area or near water, if water comes in contact with the boiling aluminium it might "blow" in your direction.

Step 1: Furnace Construction

A bit of form work: check if you have any scrap plywood, MDF etc' to hold the refractory furnace shape.
I used scraps of plywood, MDF, chipboard, soft wood, cardboard all screwed together.
Start with the walls and the base. Optional: making small wooden angles for the corners will make the corners flat preventing them from breaking.
Drill a hole 2" from the base using a core bit for timber. The diamater of the hole should fit the size of the pipe you will use for the blower.
I used a cardboard pipe to form the opening for the blow pipe.

Step 2: Furnace Core

To shape the furnace core I used an empty plastic can of paint. Cut it to the desired diamater leaving space for the charcoal / crucible and at least 2" to fill the refractory for furnace walls. Using masking tape to hold the can diamater shape makes it quick and easy to remove after your refractory is dry. Make a hole for the cardboard pipe or just tape the pipe to the can to prevent blockage of the opening.

Step 3: Furnace Top

I used 10mm MDF cut to fit the plastic can diamater. It is important to cut the MDF into half this will help to take it off in later stages without breaking the fresh refractory. Make another 2 circles (marked red lines in images) and screw them all together.

Step 4: Furnace Lid

Use the left over materials from your cuttings and attach them together. Use any flat scrap for the base. Place the cardboard pipe in the centre of the lid for the vent opening.
Strips of 5mm MDF will do for holding the rebars in place and will be very easy to snap off in the stripping stage. Screw the MDF into place, mark the hole positions and drill the holes.
I bent 5mm rebars to serve as handles for the lid. Place the handles in the holes and leave a 20mm gap from the base.

Step 5: Furnace Refractory

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Refractory: The mix of materials chemically and physically will help hold furnaces high temperature avoiding thermal shock (in plain english, cracking).

In the third image you can see 2 bits of ply attached to the inside walls of the furnace, they will be stripped of later and the gap left in the material will be a good handle grip for lifting the furnace.

Start from the base of the furnace and drill the copper pipe to the base center point, after that using a block of timber etc. compact the refractory and remove of all unwanted air bubbles.
Drill the center screw out and leave the copper pipe in its place. Attach the plastic can with the same screw in the center over the refractory.
Cover all sides around the plastic can with the refractory mix and compact as you go.
When you get near the top place the cover and secure it with timber to the furnace plywood walls.
When all is compacted well and covered use a hammer to tap on the plywood sides to get rid of any remaining air bubbles.

(I will not go into details regarding the mixing ratio of refractory so if anyone wants to make somthing similar that will last longer check other sites for proper refractory materials mixing and exact measuring ratio's).

I just used cement+sand+fireclay and a small amount of water just turning it into a nice paste. It will probably crack at some stage but I'm sure I will get a few nice melting before that.

I also recommend placing a wire mesh between the core and furnace walls that will help against cracks.

Step 6: Removing Furnace Frame

This is a very important thing to remember: PATIENCE let the refractory dry don't be hasty to remove the cover or you will crack and break it, all your hard work will be for nothing.
Start taking the screws out and removing the sides of the furnace, you can use a chisel or such but don't wedge and pull it against the furnace.
The cardboard can be taken out with one finger, once it is wet it wont stick.
All the scrap you can throw away or use to make another furnace.

Step 7: PATIENCE, LET IT DRY!!!

After the refractory is cured it is time to fire it up....

While waiting I did a bit of aluminum experiment to cheak how hot it gets using my multimeter, nut cracker and camping propane cooker. In the images you can't see the flame but look at the temperature (celsius) on the multimeter screen The nut cracker melted in less then 5 min.

Step 8: Furnace Tools

Crucible: Is the container that holds the aluminium parts before melting them and for casting them in sand or just into an aluminium ingot.
The professional ones are made from different materials such as clay graphite, silicon carbide and more. They can be purchased from as little as a few bucks to couple of hundred.
The crucible I used in this instructable is worth about 2$ - such a bargain..he..he.
In retrospect:
I don't recommend using tin cans at all, it can be dangerous, I got a few ingots out of it but one corn can I used broke inside the furnace with big holes.

I made the crucible from a used can of beans v shaping the lip for easy pouring, I drilled 2 holes to the sides of it and placed 2 bolts to act as lifting pins.
On the bottom I tied steel wire with a loop at the end.
The lifting fork and hook I made from 5mm construction rebar that I welded to an unused bathroom hook.
I also bought a 94c muffin tray (another bargain) to make some ingots.

The rebars are long enough to keep a safe distance from the boiling aluminium and your body.

Step 9: On the Menu

I looked for some aluminium parts in my house and found a few handlels and a nut cracker. So I decided its time to "feed" my furnace.

Step 10: My First Aluminium Ingot

Last sunday we had great weather for a BBQ and a good opportunity to try my furnace. After lighting the BBQ coals I took a few and placed them on the base of the furnace and around the crucible. I had everything ready and even a water hose (for characols only not for the melted aluminium) in case of emergency . I used an old vacum cleaner pipe and taped my wifes hair dryer to one end after sneaking it out of the house. Later on she saw her hair dryer was attached to my furnace and lets just say I won't do it again. I placed the door handles and the locks without the nut cracker (no room in crucible) into the crucible coverd the top with the lid and turned the hair dryer on. I think after about 10 min I turned the dryer off, removed the vent block and looked into the lid vent hole and saw the aluminum handles were gone, I just saw dark slag on top, for a moment I tought nothing melted but then I skimmed the scrap from the top and it looked like it was melting. I left it on for a few more minutes and I took off the lid used my diy lifting tools and got all excited to see that liquid shiny as a mirror coming out into the muffin tray and forming into my first aluminium ingot.

Step 11: This Was a Fun Experiment !!! Some Conclusions

As I predicted my furnace started to crack after more meltings. Before it will "die" I hope to get a small object sand casting. And after that I'll scrap this furnace and make a bigger and better one.

I don't recommend using tin cans at all, it can be dangerous, I got a few ingots out of it but one corn can I used broke inside the furnace with big holes.

This was a great experiment for first time aluminium casting.
Hope you enjoyed my instructable.

This is a short video I made, I know its not great and my hand is in the way and a bit of slag fell into the muffin tray but it gives the general idea of ingot making.